Introduction
Species of Gyrodactylidae van Beneden & Hesse, 1863 are small viviparous monogenoids occurring primarily on freshwater and marine fishes and rarely on amphibians and cephalopods. They are characterised by two terminal cephalic lobes bearing head organs and spike sensilla, blind intestinal caeca, a male copulatory organ (MCO) generally with one large spine and one to many smaller spinelets, a haptor armed with 16 (8 pairs) of marginal, hinged hooks, usually a ventral anchor/bar complex comprised of a pair of anchors supported by deep and superficial bars, and by lacking eyespots and a vagina. Hyperviviparity, i.e., the uterus typically containing one or more successive generations of developing embryos, characterises all viviparous species (adapted from Mizelle and Kritsky Reference Mizelle and Kritsky1967; Bakke et al Reference Bakke, Cable and Harris2007).
Harris et al (Reference Harris, Shinn, Cable and Bakke2004) found that 470 nominal species and subspecies of Gyrodactylus von Nordmann, 1832 were recorded in the world literature through 2003, 409 of which were considered taxonomically valid. Gyrodactylus represents the second-largest monogenoidean genus after Dactylogyrus Diesing, 1850, highlighting its taxonomic and economic significance. This extraordinary diversity underscores the necessity for accurate species delimitation, which has traditionally relied on the haptoral and copulatory sclerites (Bakke et al Reference Bakke, Cable and Harris2007). However, delimitation of species of Gyrodactylus is often challenging due to limited variation in haptoral sclerites among closely related species (Rahmouni et al Reference Rahmouni, Seifertová and Šimková2023). Consequently, integrative approaches that combine morphology with molecular markers, particularly the nuclear ITS rDNA region consisting of ITS1-5.8S-ITS2 rDNA and mitochondrial cytochrome c oxidase subunit I (cox1), have become indispensable for reliable species identification (Hansen et al Reference Hansen, Bakke and Bachmann2007; Ziętara et al Reference Ziętara, Huyse, Lumme and Volckaert2002).
Beyond their taxonomic importance, species of Gyrodactylus are consequential for two key reasons. First, under specific conditions such as those often found in intensive aquaculture, gyrodactylids may reach highly elevated densities that often cause or are associated with severe economic loss (King and Cable Reference King and Cable2007; Pinacho-Pinacho et al Reference Pinacho-Pinacho, Calixto-Rojas, García-Vásquez, Guzmán-Valdivieso, Barrios-Gutiérrez and Rubio-Godoy2021). Prominent examples include Gyrodactylus salaris Malmberg, 1957, a major pathogen of Atlantic salmon in Scandinavia (Johnsen and Jensen Reference Johnsen and Jensen1986), and Gyrodactylus cichlidarum Paperna, 1968, which impacts cichlid aquaculture in Africa and Asia (García-Vásquez et al Reference García-Vásquez, Hansen, Christison, Rubio-Godoy, Bron and Shinn2010; Grano-Maldonado et al Reference Grano-Maldonado, Rodríguez-Santiago, García-Vargas, Nieves-Soto and Soares2018). Second, their direct life cycle, short generation time, and comparatively high host-specificity may enable rapid evolutionary responses to host immune pressures, positioning species of Gyrodactylus as excellent models for studying host–parasite coevolutionary dynamics (Hahn et al Reference Hahn, Weiss, Stojanovski and Bachmann2015; Huyse and Volckaert, Reference Huyse and Volckaert2005; Lei et al Reference Lei, Jakovlić, Zhou, Liu, Yan, Jin, Wang, Li, Wang and Zhang2024).
The teleostean order Siluriformes comprises approximately 4,025 valid species of catfishes across 522 genera and 41 families worldwide (Froese and Pauly Reference Froese and Pauly2025). In India, the Bagridae is the second most diverse catfish family after the Sisoridae, where it is represented by 7 genera and 44 valid species (Froese and Pauly Reference Froese and Pauly2025). Within the Bagridae, species of Mystus form a taxonomically diverse assemblage of small-to medium-sized catfishes distributed throughout southern and southeastern Asia, where they are an important component of the subsistence and commercial fisheries. In India, Day’s mystus, Mystus bleekeri (Day), is harvested for domestic consumption and the ornamental trade (Froese and Pauly Reference Froese and Pauly2025), with its domestic demand driven primarily by its superior nutritional profile that often exceeds that of other freshwater species (Mazumder et al Reference Mazumder, Kunda, Iqbal, Basak and Das2021).
To date, seven monogenoidean species have been reported parasitising M. bleekeri, all assigned to Cornudiscoides Kulkarni, 1969 of the Dactylogyridae (Table 1). The present study expands this inventory with the description of two new gyrodactylid species from this host. An integrated taxonomic approach, combining morphometric analysis of haptoral and reproductive sclerites with sequencing of the complete ITS rDNA region (ITS1–5.8S–ITS2) and 28S rDNA, was employed for the descriptions. The findings added to the underestimated diversity of Gyrodactylus species in India and highlighted the need for additional targeted parasitological surveys, coupled with an integrated approach to the taxonomy of the group to ensure accurate species delimitation, while simultaneously providing valuable sequence data from a region with limited baseline information.
Species of Monogenoidea previously reported from Day’s mystus, Mystus bleekeri (Day) (Siluriformes: Bagridae)*
Table 1. Long description
The table is organized into three columns: Species, Locality, and References.
* Row 1: Cornudiscoides bleekerai Agrawal and Vishwakarma, 1966. Locality: Gomti River, Lucknow, Uttar Pradesh, India. Reference: Agrawal and Vishwakarma 1996.
* Row 2: Cornudiscoides gussevi Agrawal and Vishwakarma, 1966. Locality: Gomti River, Lucknow, Uttar Pradesh, India. Reference: Agrawal and Vishwakarma 1996.
* Row 3: Cornudiscoides susanae Agrawal and Vishwakarma, 1966. Locality: Gomti River, Lucknow, Uttar Pradesh, India. Reference: Agrawal and Vishwakarma 1996.
* Row 4: Cornudiscoides tukarami Agrawal and Vishwakarma, 1966. Locality: Gomti River, Lucknow, Uttar Pradesh, India. Reference: Agrawal and Vishwakarma 1996.
* Row 5: Cornudiscoides geminus Gussev, 1976. Locality: Lucknow, Uttar Pradesh, India. Reference: Gussev 1973.
* Row 6: Cornudiscoides agarwali Agrawal and Vishwakarma, 1966. This species has two localities: Lucknow, Uttar Pradesh, India (Reference: Agrawal and Vishwakarma 1996) and Itanagar, Arunachal Pradesh, India (Reference: Tripathi et al 2019).
* Row 7: Cornudiscoides tripathii Verma, Agrawal and Saxena, 2022. Locality: Lucknow and Barabanki, Uttar Pradesh, India. Reference: Verma et al 2022.
A footnote indicates the data is after Lim et al 2001 and Pandey and Agarwal 2008.
* After Lim et al (Reference Lim, Timofeeva and Gibson2001); Pandey and Agarwal (Reference Pandey and Agrawal2008).
Materials and methods
Sample collection and preparation
Between November 2022 and February 2025, 106 specimens of M. bleekeri were purchased live from retail fish markets in Lucknow, northern India. The fishes were identified using Talwar and Jhingran (Reference Talwar and Jhingran1991), the common name was that presented in Froese and Pauly (2025), and the scientific name was verified in Fricke et al (Reference Fricke, Eschmeyer and Van der Laan2025). In the laboratory, fishes were euthanised with an overdose of sodium bicarbonate-buffered tricaine methanesulfonate (MS-222 100 mg/L; Sigma-Aldrich, USA). Gills and fins were excised and immersed in hot water (60°C) for 5 minutes to relax attached parasites. They were then fixed in hot formalin overnight and subsequently preserved in 70% ethanol. Some gyrodactylids were examined live, identified based on morphology, and then preserved in 95% laboratory-grade ethanol and stored at −20°C for subsequent molecular analysis.
Specimens preserved in formalin were screened using a stereomicroscope (EZ4HD, Leica Microsystems, Germany). Retrieved monogenoids were mounted as wholemounts on glass slides following the protocols of Tripathi et al (Reference Tripathi, Matey and Agarwal2022) and examined with a light microscope (DM4B, Leica Microsystems, Germany) equipped with a digital camera (Leica DFC7000 T) and LAS X imaging system for descriptions, measurements, and photographs; measurements, all in micrometres, represented straight-line distances between extreme points, expressed as the range followed by the mean and number (n) of structures measured in parentheses; total body length included that of the haptor; superficial bar length represents the horizontal dimension. The recurve of the anchor point relative to the anchor shaft (referred to as the ‘anchor recurve’) was defined as the angle formed by its vertex located at the point along the distal curve of the anchor shaft most distal from the anterior tip of the superficial anchor root, with the two respective rays originating at the distal (posterior) margin of the anchor knob and at the tip of the anchor point (Figure 1); angle measures (in degrees) were determined via ProgRes CapturePro v2.8.8 software (Jenoptik Gryphax®). Infection parameters, namely prevalence, mean intensity, and mean abundance, were calculated following the respective definitions given by Bush et al (Reference Bush, Lafferty, Lotz and Shostak1997). Type and voucher specimens were deposited in the National Museum of Natural History (USNM), Smithsonian Institution, Washington, D.C., as stated in the taxonomic accounts of the new species and those recorded in the annotated checklist of Indian species of Gyrodactylus.
Diagram illustrating the angle of ‘anchor recurve’ (ar).
DNA extraction, amplification, and sequencing
Genomic DNA was extracted from morphologically identified individuals using the Extracta DNA Prep kit (Quantabio, Beverly, US). Fragments spanning ITS1, the 5.8S rRNA gene, and ITS2 to the 5′ end of the 28S rRNA gene were amplified using the primer pair ITS1A/ITS2 (Matejusová et al., Reference Matejusová, Gelnar, McBeath, Collins and Cunningham2001), whereas the D1–D2 domains of the 28S rRNA gene were amplified using the primer pair C1/D2 (Hassouna et al Reference Hassouna, Michot and Bachellerie1984). The ITS1–5.8S–ITS2 fragment was amplified using a modified PCR profile based on Hahn et al. (Reference Hahn, Bakke, Bachmann, Weiss and Harris2011): 95°C for 3 min; 35 cycles of 95°C for 30 s, 50°C for 30 s, and 72°C for 1 min; followed by a final extension at 72°C for 7 min. Amplification of the 28S rRNA gene followed Šimková et al (Reference Šimková, Matejusová and Cunningham2006). Amplicons were visualised on 1% agarose gels stained with SYBR-Safe (Invitrogen, USA) using a BioVision imaging system (Vilber Lourmat, France), purified with a QIAquick PCR purification kit (Qiagen, USA), and bidirectionally sequenced with the respective PCR primers and additional internal primers ITS3A and ITS4.5 (Matejusová et al Reference Matejusová, Gelnar, McBeath, Collins and Cunningham2001) for the ITS fragment. Chromatograms were edited in SnapGene v5.3 (http://www.snapgene.com), contigs assembled in DNA Baser Sequence Assembler v4.36 (Heracle BioSoft, 2013), and the resulting sequences subjected to BLASTn searches against the NCBI nucleotide database for species-level identification. Sequence similarity was assessed in conjunction with query coverage, alignment length, and E-values, rather than percentage identity alone.
Checklist compilation
The checklist of Gyrodactylus spp. recorded from Indian fishes was compiled through a systematic literature review using peer-reviewed journals, books, bibliographic databases, and electronic databases such as the Web of Science, Google Scholar, Scopus, and the Zoological Record. The literature review was supplemented by personal contacts with parasitologists working on Indian monogenoids. The checklist is divided into two sections, the first of which includes species of Gyrodactylus that are taxonomically valid and meet the requirements for availability outlined in the International Code of Zoological Nomenclature (ICZN or ‘the Code’) (International Commission on Zoological Nomenclature 1999); the second section includes species previously identified or described that are taxonomically invalid or unavailable sensu the Code.
Results
Two species of Monogenoidea were recovered from the skin, fins, and gills of M. bleekeri and identified as undescribed species of Gyrodactylus, based on an integrated evaluation of both morphological and molecular data. Morphometric descriptions and two molecular markers (ITS rDNA and 28S rDNA) of the two species are provided, followed by an annotated checklist of species of Gyrodactylus previously reported from fishes of India.
Class Monogenoidea Bychowsky, 1937
Subclass Polyonchoinea Bychowsky, 1937
Order Gyrodactylidea Bychowsky, 1937
Gyrodactylidae van Beneden & Hesse, 1863
Gyrodactylus octoacanthicus n. sp.
ZooBank registration: urn:lsid:zoobank.org:act:B077262B-8673-4872-8821-6714500AA61A.
Taxonomic summary
Type host and locality: Day’s mystus, Mystus bleekeri (Day) (Siluriformes: Bagridae): Local fish markets along the River Gomti, Lucknow, Uttar Pradesh, India (26°45′–26°50′N, 80°50′–81°05′E).
Site of infection: skin, fins, gill lamellae.
Ecological parameters: Table 2.
Ecological parameters of Gyrodactylus octoacanthicus n. sp. and Gyrodactylus ascia n. sp. parasitising Day’s mystus, Mystus bleekeri (Day), during the present study
Table 2. Long description
The table consists of three columns. The first column lists the Infection parameter, the second column is for Gyrodactylus octoacanthicus n. sp., and the third column is for Gyrodactylus ascia n. sp.
Data rows are as follows.
* Number of fish examined. 106 for both species.
* Number of fish infected. 12 for G. octoacanthicus and 6 for G. ascia.
* Number of parasites recovered. 188 for G. octoacanthicus and 353 for G. ascia.
* Minimum prevalence percentage. 11.3 percent for G. octoacanthicus and 5.7 percent for G. ascia.
* Minimum abundance. 1.8 for G. octoacanthicus and 3.3 for G. ascia.
* Minimum mean intensity plus or minus S D. 15.66 plus or minus 4.33 for G. octoacanthicus and 58.80 plus or minus 40.56 for G. ascia.
Type specimens: Holotype, USNM 1770898; 9 paratypes, USNM 1770899–1770907.
DNA reference sequences: ITS rDNA (PV565488.2, PZ787873, PZ787874); 28S rDNA (PV565490).
Etymology: The specific name (an adjective) is from Latin (octo = eight + acanthus = a thorn or sharply pointed projection, appended to the suffix -icus = pertaining to) and refers to the eight spinelets of the MCO.
Morphological characterisation (Figures 2–3)
Description: Body proper fusiform, often with parallel lateral margins, comprising cephalic region, trunk, peduncle; greatest width usually at level of uterus when containing near-term embryo. Tegument smooth. Cephalic region with two prominent cephalic lobes, each bearing conspicuous head organ and spike sensillum. Two bilateral pairs of cephalic glands located anterolateral and posterolateral to pharynx; each gland composed of several individual cells. Peduncle tapering posteriorly from level of gonads, uniting with dorsal surface of haptor. Mouth indistinct, midventral, anterior to pharynx. Pharynx comprising two tandem sub-hemispherical bulbs; larger posterior bulb glandular, muscular; smaller anterior bulb with eight digitiform rami from which glandular secretions are released. Oesophagus short; intestinal caeca indistinct. Gonads indistinct, apparently located immediately posterior to uterus; genital pore midventral at level of intestinal bifurcation. MCO situated posterior to intestinal bifurcation, armed with anterior spine and broad U-shaped arc of generally eight smaller spinelets posterior to spine; two lateral-most spinelets larger than medial spinelets (Figures 2B and 2B1). Seminal vesicle pyriform, located to left and slightly posterior to MCO; prostatic reservoirs not observed. Uterus with delicate wall, containing up to two generations of developing embryos. Haptor ovate, longer than wide, armed with a single ventral anchor/bar complex, 16 (8 pairs) of ventro-marginal hooks. Anchor delicate, with elongate base, arcing shaft, elongate recurved point; base with long superficial root, ventrolateral fold, dorsomedial knob (Figure 2A); angle of anchor recurve 48° (47°–57°). Superficial bar with short anterolateral rami inserted beneath anchor folds; narrow superficial-bar shield extending posteriorly about 2/3 length of anchor shaft (Figure 2A). Dorsal bar rod-shaped, variably bent into an inverted U, with narrowed ends deeply embedded within anchor knobs (Figure 2A1). Hooks similar, extrahamular (Kritsky and Mizelle Reference Kritsky and Mizelle1968); each having a small hooklet with short straight shaft, evenly curved point, base with rounded globose and protruding heel, toe with short proximal shelf and angular depressed tip; slender shank (Figure 2C); filament hook (FH) loop extending basally from hooklet to about 1/4 to 1/3 length of shank.
Phase contrast micrographs of the haptoral and copulatory sclerites of Gyrodactylus octoacanthicus n. sp. (A) Anchor/bar complex, (A1, inset) showing superficial bar; (B) male copulatory organ (MCO) from two specimens, (B1, inset) showing configuration of the spine and spinelets; (C) haptoral hook mounted in proteinase-K medium, filament loop absent, removed by digestion. Scale bars: A = 25 μm; A1, B, C = 10 μm.
Line drawings of the haptoral and copulatory sclerites of Gyrodactylus octoacanthicus n. sp. (A) Anchor-bar complex; (B) male copulatory organ (MCO); (C) haptoral hook; (D) enlarged view of haptoral hooklet.
Figure 3. Long description
The drawing consists of four panels labeled A through D.
Panel A, top left: Shows the anchor-bar complex. It features two large, symmetrical anchors with curved points facing outward. A ventral bar with a central rectangular plate and a dorsal bar connect the two anchors. A vertical scale bar to the right indicates a length of 50 micrometers.
Panel B, bottom left: Displays the male copulatory organ, or M C O. It is a circular structure containing several small, inward-pointing spines and a larger central hook-like sclerite. A vertical scale bar to the right indicates a length of 15 micrometers.
Panel C, top right: Illustrates a haptoral hook. It consists of a long, thin shaft that curves slightly at the proximal end and terminates in a small hooklet at the distal end. A vertical scale bar to the right indicates a length of 20 micrometers.
Panel D, bottom right: Provides an enlarged view of the haptoral hooklet seen in panel C. The hooklet is solid black, showing a sharp, curved point and a thickened base. A vertical scale bar to the right indicates a length of 6 micrometers.
Measurements: Body 424 (377–518; n = 8) long, greatest width 78 (65–96; n = 7). Posterior pharyngeal bulb 31 (30–32; n = 3) wide; anterior pharyngeal bulb 24 (21–28; n = 3) wide. MCO 15 (14–17; n = 9) in diameter. Haptor 82 (73–91; n = 8) long, 71 (66–77; n = 5) wide. Anchor 62 (59–69; n = 10) long; anchor point/shaft angle 48° (47–57°). Deep bar 21 (18–23; n = 7) long; superficial bar 24 (22–27; n = 9) long (transverse length); superficial-bar shield 19 (17–22; n = 8) long. Hook 34 (32–36; n = 12) long; hooklet 6–7 (n = 12) long.
Molecular characterisation
Sanger sequence data were successfully generated for all target loci. Three complete ITS rDNA sequences, each 1299 bp in length (GenBank: PV565488.2, PZ787873, PZ787874), and one 28S rDNA fragment, 884 bp in length (GenBank: PV565490), were obtained. All sequences were suitable for subsequent species delimitation and comparative analyses.
BLASTn searches against the NCBI nucleotide database (accessed on 03 August 2026) returned no close matches for G. octoacanthicus n. sp. The ITS rDNA sequences (1299 bp) identified G. anguillae Ergens, 1960 from the American eel, Anguilla rostrata (Lesueur) (Anguilliformes: Anguillidae), collected in the USA (GenBank: AB063291), as the top BLAST hit (80.88% sequence identity). The 28S rDNA sequence (884 bp) identified G. salaris Malmberg, 1957, from the Atlantic salmon, Salmo salar Linnaeus (Salmoniformes: Salmonidae), from Norway (GenBank: AJ542394), as the top BLAST hit (83.17% sequence identity).
Differential diagnosis
Gyrodactylus contains over 400 species occurring on phylogenetically disparate teleostean fishes worldwide (Harris et al Reference Harris, Shinn, Cable and Bakke2004). Freshwater catfishes (Siluriformes) are common throughout most biogeographical regions of the world and serve as hosts of Gyrodactylus spp. The sheer numbers of known species of the genus, coupled with congeners occurring on freshwater catfishes from nearly all biogeographical regions, except Australia and Antarctica, render determination of species that closely resemble G. octoacanthicus n. sp. difficult. The only feature of G. octoacanthicus n. sp. that appears to separate it from most congeners is the morphology of the superficial bar of the anchor/bar complex, where the elongate narrow shield originating from the medioposterior margin of the bar proper and extending far posteriorly between the shafts of the two ventral anchors is unusual, if not unique. The BLASTn analyses of both the ITS and 28S rDNA genes failed to indicate close relationships of G. octoacanthicus n. sp. with any species of Gyrodactylus having corresponding sequences recorded in GenBank, further confounding determinations of similar or related species to G. octoacanthicus n. sp.
Past surveys of the gyrodactylids infecting the catfishes of India are limited, with only two siluriform hosts having been found infected thus far. These hosts include the stinging catfish, Heteropneustes fossilis (Bloch) infected by Gyrodactylus gussevi Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990 (now Gyrodactylus fossilisii nom. nov.; see Remarks under G. fossilisii) (for which a sequence identified as Gyrodactylus sp. g VT-2014 (GenBank KJ461316) was assigned to Rysavyius by Janulewicz et al Reference Janulewicz, Pietkiewicz and Ziętara2024) and Gyrodactylus neonephrotus malmbergi Singh, Agrawal & Sharma, 2006 (an unavailable name); and the striped dwarf catfish, Mystus vittatus (Bloch) parasitised by Gyrodactylus mizellei Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990 (now Gyrodactylus pandeyi nom. nov.; see Remarks under G. pandeyi) (Dubey et al Reference Dubey, Gupta and Agarwal1990; Singh and Agarwal 1994). The original descriptions of all three previously described species of Gyrodactylus from Indian catfishes are inadequate. Nonetheless, the figures of the superficial bars of G. gussevi, G. neonephrotus malmbergi, and G. mizellei presented in the respective original descriptions may separate them from G. octoacanthicus n. sp. The similar morphology of the anchors of the three former species also suggests that the Indian species, including G. octoacanthicus n. sp., may comprise a subgroup of gyrodactylids infecting these fishes in India. However, new collections and adequate descriptions of the gyrodactylids from the above hosts, as well as from other catfishes occurring in India, would be required to establish such a relationship and to determine possible synonymy among them.
Gyrodactylus ascia n. sp.
ZooBank registration: urn:lsid:zoobank.org:act:7E2A5C1C-A03E-4B58-92EE-357160961EEE.
Taxonomic summary
Type host and locality: Day’s mystus, Mystus bleekeri (Day) (Siluriformes: Bagridae): Local fish markets along the River Gomti, Lucknow, Uttar Pradesh, India (26°45′–26°50′N, 80°50′–81°05′E).
Additional Indian record: Striped dwarf catfish, Mystus vittatus (Bloch) (Siluriformes: Bagridae); Local fish markets along the River Gomti, Lucknow, Uttar Pradesh, India (26°45′–26°50′N, 80°50′–81°05′E).
Site of infection: skin, gills, fins, barbels, eyes.
Ecological parameters: Table 2.
Type specimens: Holotype, USNM 1770908; 9 paratypes, USNM 1770909–1770917.
DNA reference sequences: ITS rDNA (GenBank PV565491.3, PZ785137–PZ785138); ITS2 rDNA (PV565492); 28S rDNA (GenBank PV599773).
Etymology: The specific epithet (a noun) is derived from Latin (ascia = a carpenter’s adze or axe) and refers to the shield of the superficial bar resembling the blade of a carpenter’s adze.
Morphological characterisation (Figures 4–5)
Description: Body proper fusiform, comprising cephalic region, trunk, peduncle. Tegument smooth. Cephalic region with two prominent cephalic lobes bearing conspicuous head organs and spike sensillum. Bilaterally paired cephalic glands lying posterolateral to pharynx; prepharyngeal glands not observed. Peduncle short, tapering posteriorly to unite with dorsal surface of haptor. Mouth indistinct; oesophagus short. Pharyngeal bulbs sub-hemispherical; posterior pharyngeal bulb glandular, muscular, larger than anterior pharyngeal bulb; anterior bulb with eight digitiform processes. Intestinal caeca terminating blindly near body midline posterior to uterus. Gonads indistinct; genital pore midventral near oesophageal bifurcation. MCO ventral to oesophageal bifurcation, armed with large anterior spine and three broadly U-shaped concentric rows of smaller spinelets; each row with varying number but less than 10 spinelets (Figures 4B and 4B1–B2); seminal vesicle pyriform, situated to left and posterior to MCO (Figure 4B); vas deferens not observed. Uterus delicate, containing up to two generations of embryos. Haptor subcircular to elongate ovate, often with digitated margin; each haptoral digit containing one hook. Anchor with base having elongate arcing superficial root, indistinct ventral fold, flattened knob; gently curved shaft; sharply recurved elongate point (Figure 4A); angle of anchor recurve 65° (61°–78°). Ventral bar subquadrate, lacking anterolateral processes; anchor-bar shield shaped as the blade of a carpenter’s adze, extending posteriorly nearly to level of union of anchor shaft and point (Figure 4A). Dorsal bar short, often appearing bipartite (Figure 4A1). Hook distribution extrahamular (Kritsky and Mizelle Reference Kritsky and Mizelle1968); each hook with small hooklet, long shank tapered proximally; hooklet with short somewhat blunt point, ventrally leaning shaft, small slightly upturned toe, globose heel; FH loop extending posteriorly from hooklet to about ¾ shank length (Figure 4C). Two reniform vesicular-like structures lateral to the anchor-bar complex (Figure 4A).
Phase contrast micrographs of the haptoral and copulatory sclerites of Gyrodactylus ascia n. sp. (A) Anchor/bar complex; inset (A1) showing deep bar; (B) male copulatory organ (MCO), seminal vesicle (SV), and prostatic reservoir (PR), (B1–B2, insets) close-up views of spines and spinelets of the male copulatory organ; (C) haptoral hook. Scale bars: A = 30 μm; B = 50 μm; B1–B2, C = 10 μm.
Figure 4. Long description
The image consists of six panels.
Panel A, at the top, shows the anchor and bar complex at a 30 mu m scale. It features two large, symmetrical curved anchors connected by a central bar. Inset A 1 at the top left provides a high-magnification view of the deep bar.
Panel B, at the bottom left, shows the reproductive organs at a 50 mu m scale. From top to bottom, labels indicate the M C O (male copulatory organ), the P R (prostatic reservoir), and the S V (seminal vesicle).
Panels B 1 and B 2, located in the bottom center, are close-up views of the M C O spines and spinelets at a 10 mu m scale, showing fan-like arrangements of small, sharp structures.
Panel C, at the bottom right, displays a single haptoral hook at a 10 mu m scale, characterized by a long, thin shaft and a curved, bifid tip.
Line drawings of the haptoral and copulatory sclerites of Gyrodactylus ascia n. sp. (A) Anchor-bar complex; (B) male copulatory organ (MCO) and prostatic reservoir (PR); (C) haptoral hook; (D) enlarged view of haptoral hooklet.
Measurements: Body 562 (497–649; n = 8) long, greatest width 129 (113–140; n = 7). Posterior pharyngeal bulb 46 (41–52; n = 6) wide; anterior pharyngeal bulb 41 (39–45; n = 4) wide. MCO 13 (11–16; n = 9) in diameter. Haptor 135 (117–154; n = 8) long, 121 (104–129; n = 4) wide. Anchor 114 (104–127; n = 10) long; anchor point/shaft angle 65° (61–78°). Superficial bar 25 (22–28; n = 6) long (transverse length); superficial-bar shield 44 (39–48; n = 9) long. Hook 28 (27–29; n = 6) long; hooklet 5 (4–6; n = 3) long.
Molecular characterisation
Sanger sequence data were successfully generated for all target loci. Three identical, complete ITS rDNA sequences, each 1014 bp in length (GenBank: PV565491.3, PZ785137–PZ785138), and one 28S rDNA fragment, 780 bp in length (GenBank: PV599773), were obtained from M. bleekeri.
An additional partial ITS2 sequence, 475 bp in length (GenBank: PV565492.2), was obtained from M. vittatus. All sequences were retained for downstream species-level resolution.
BLASTn searches against the NCBI GenBank nucleotide database (accessed on 03 August 2026) returned no close matches for G. ascia n. sp. The ITS rDNA sequences (1014 bp) yielded Gyrodactylus sp. HSS-2009 from Trichogaster fasciata Bloch & Schneider, 1801 (formerly Colisa fasciatus), collected in India (GenBank: KM434259), as the top BLAST hit (81.93% sequence identity). The 28S rDNA fragment (780 bp) had G. macracanthus Hukuda, Reference Hukuda1940, from Misgurnus anguillicaudatus (Cantor, 1842) (Cypriniformes: Cobitidae), collected in Australia (GenBank: FJ971995), as the top BLAST hit (91.21% sequence identity).
Differential diagnosis
Gyrodactylus ascia is distinguished from congeners by having (1) anchors with elongate arcing superficial roots; (2) a superficial‑bar shield shaped as the blade of a carpenter’s adze; (3) haptoral hooks with hooklets having a globose heel, upturned toe, ventrally sloping shaft, and short blunt point; and (4) an MCO armed with a large spine and as many as three U-shaped concentric rows of spinelets having fewer than 10 spinelets each. Based on a comparison of the morphology of the superficial bars, this species may be a member of a subgroup of species of Gyrodactylus infecting siluriform fishes from southern Asia, including G. octoacanthicus n. sp., G. fossilisii nom. nov. (syn. G. gussevi Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990), and G. pandeyi nom. nov. (syn. G. mizellei Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990). Gyrodactylus ascia is easily separated from the latter species by the morphology of the superficial-bar shield (shield of the latter three species subrectangular).
Annotated checklist of previously reported gyrodactylid species in India
Gyrodactylus bullatarudis Turnbull, Reference Turnbull1956
Type host and locality: Guppy, Lebistes reticulatus Peters (now Poecilia reticulata Peters) (Cyprinodontiformes: Poeciliidae): type locality unknown.
Location on host: Not mentioned by Turnbull (Reference Turnbull1956); apparently external surface.
Type specimens: Holotype not designated by Turnbull (Reference Turnbull1956), although museum records for USNPC 38154 (now USNM 1338753) indicate that Turnbull’s specimens included a holotype and paratype.
Indian record: Gills of P. reticulata; geographic locality in India not recorded by Tripathi et al. (Reference Tripathi, Matey, Buchmann and Hahn2025).
Indian specimens: None curated; photographic voucher in Tripathi et al. (Reference Tripathi, Matey, Buchmann and Hahn2025).
Status in India: Exotic; currently known only within the ornamental fish trade.
GenBank sequences (Indian origin): ITS2 (OR405269).
Taxonomic status: Valid.
Remarks
The name Gyrodactylus bullatarudis Turnbull, Reference Turnbull1956 is available under the ICZN, and the species is currently taxonomically valid. The Indian record is supported by morpho-molecular congruence, a photographic voucher, and an ITS2 sequence (Tripathi et al Reference Tripathi, Matey, Buchmann and Hahn2025). However, the absence of voucher specimens or hologenophore supporting the Indian record impedes verification of the occurrence of the species in India. Further validation of the species in India through direct morphological comparison with available type specimens and supplementary molecular sequences is recommended.
Gyrodactylus chauhani Agrawal & Bhatnagar, Reference Agrawal and Bhatnagar1997
Type host and locality: Banded gourami, Colisa fasciatus (Bloch & Schneider) (now Trichogaster fasciata Bloch & Schneider) (Anabantiformes: Osphronemidae): water bodies of Lucknow, Uttar Pradesh, India.
Location on host: Not indicated by Agrawal and Bhatnagar (Reference Agrawal and Bhatnagar1997).
Type specimens: Holotype not designated; type specimens apparently lost.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Valid.
Remarks
Although a holotype was not designated by Agrawal and Bhatnagar (Reference Agrawal and Bhatnagar1997) and the type specimens have apparently been lost, Gyrodactylus chauhani Agrawal & Bhatnagar, Reference Agrawal and Bhatnagar1997 is currently taxonomically valid, and its name is available under Articles 11.9 and 13.1 of the ICZN. A taxonomic redescription of the species with designation of a neotype and molecular characterisation is highly recommended. Further study may indicate that this species is a senior synonym of Gyrodactylus colisai Chaudhary & Singh, Reference Chaudhary and Singh2011, a species that also occurs on the banded gourami in the freshwaters of Uttar Pradesh.
Gyrodactylus colisai Chaudhary & Singh, Reference Chaudhary and Singh2011
Type host and locality: Banded gourami, Colisa fasciata (Bloch & Schneider) (now Trichogaster fasciata Bloch & Schneider) (Anabantiformes: Osphronemidae): Hastinapur, Meerut, Uttar Pradesh, India (28°50′N; 77°45′E); the coordinates (28°50′N, 77°45′E) indicate that the type locality lies approximately 30 mi southwest of Hastinapur and about 6 mi south of Meerut along highway 34; no nearby waterways are indicated at this location according to Google Maps.
Location on host: Body surface (skin).
Type specimens: Holotype and paratype (slides) deposited in the Museum of the Department of Zoology, Ch. Charan Singh University, Meerut, and identified as ‘voucher number HS/Monogenea/2009/07’.
GenBank sequences (Indian origin): 28S rDNA (GQ925912).
Status in India: Native.
Taxonomic status: Valid.
Remarks
The original description of G. colisai by Chaudhary and Singh (Reference Chaudhary and Singh2011) is insufficient in that accurate and precise details of the male copulatory organ are lacking, and that the superficial bar without a posterior shield and the haptoral hooks with bifurcated toes require confirmation. Further, the overlap in host and geographical proximity of G. colisai and G. chauhani raises the possibilities of synonymy or, less likely, a species complex. Thus, a formal redescription of the species, including integrative morphological and molecular comparisons with related taxa, particularly G. chauhani, is required.
Gyrodactylus elegans indicus Tripathi, 1959
Type hosts and locality: Type host was not specified by Tripathi (Reference Tripathi1957) but the helminths were first found infecting fry of the roho labeo or roho, Labeo rohita (Hamilton) held in an aquarium with fry of the mrigal, Cirrhina mrigala (Hamilton) (now Cirrhinus mrigala), the reba carp or bata, Cirrhina reba (Hamilton) (now Cirrhinus reba), and catla, Catla catla (Hamilton) (all Cypriniformes: Cyprinidae), all of which were later observed infected with the gyrodactylid: Fish fry purchased from a local fish market (precise location unknown), India.
Location on host: Mainly skin, fins, and behind the opercular opening; few on gills.
Type specimens: Holotype not designated; type specimens apparently lost or destroyed.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Species inquirenda.
Remarks
The original description of Gyrodactylus elegans von Nordmann, 1832 was apparently based primarily on specimens infecting Abramis brama (Linnaeus) (Cypriniformes: Leuciscidae) (originally reported as Cyprinus brama Linnaeus), as suggested by von Nordmann’s (1832) legend to Plate X, Figure 1. Tripathi (Reference Tripathi1957) described G. elegans indicus, apparently based on specimens collected from fry of one or more of four other cyprinid hosts obtained from a local fish market in India (locality unknown). A definitive type host of the subspecies was not identified, as Tripathi (Reference Tripathi1957) initially found three specimens of the subspecies on fry of L. rohita and later on fry of all four of the cyprinid hosts held together in an aquarium in his laboratory. Whether he only utilised the three specimens from the fry of L. rohita, those naturally infecting L. rohita in local ponds, or those from the fry of all four cyprinids held in his aquarium for the description of the subspecies is unclear.
In the absence of a designated type specimen and host, molecular data, and a complete description of G. elegans indicus, the taxonomic identity of the subspecies is uncertain and is herein regarded as species inquirenda. A comprehensive redescription based on fresh material from L. rohita and/or from the other cyprinid hosts found in the local ponds, probably of southern India near Tripathi’s home base, Tungabhadra Dam, Karnataka, India, along with molecular characterisation, is needed to assess the validity of the subspecies and perhaps its elevation to the rank of species.
Gyrodactylus eutheraponsis Venkatanarsaiah & Kulkarni, Reference Venkatanarsaiah and Kulkarni1980
Type host and locality: Largescaled terapon, Eutherapon theraps (Cuvier) (now Terapon theraps Cuvier) (Centrarchiformes: Terapontidae): Kakinada and Machilipatnam, Bay of Bengal, India.
Location on host: Gills.
Type specimens: Holotype not designated; type specimens deposited in the Museum of the Zoology Department, Osmania University, Hyderabad, India (Venkatanarsaiah & Kulkarni, Reference Venkatanarsaiah and Kulkarni1980).
Other Indian records: Fingerlings of the flathead grey mullet, Mugil cephalus (Linnaeus) and the largescale mullet, Liza macrolepis (Smith) [now Planiliza macrolepis (Smith)] (both Mugiliformes: Mugilidae), and the jarbua terapon, Terapon jarbua (Forsskål) [now Terapon jarbua (Fabricius)] (Centrarchiformes: Terapontidae): Gosthani Estuary near Bheemunipatnam (suburb of Visakhapatnam), Andhra Pradesh, India (Madhavi, Reference Madhavi2000).
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Valid.
Remarks
The specimens of G. eutheraponsis parasitising T. jarbua and the two phylogenetically unrelated hosts, M. cephalus and P. macrolepis, were not preserved by Madhavi (Reference Madhavi2000), and molecular data are lacking. Absence of this information and the fact that species of Gyrodactylus generally exhibit a comparatively high host specificity, suggest that the identifications of G. eutheraponsis on the three-host species by Madhavi (Reference Madhavi2000) may all or in part be erroneous. New morphological analyses of gyrodactylids parasitising Madhavi’s hosts as well as a comparison of molecular data from gyrodactylids infecting these hosts with those from Terapon theraps, type host of G. eutheraponsis, should be conducted for confirmation of all host records published subsequent to the original description.
Gyrodactylus fossilisii (Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990) nom. nov.
Syn. Gyrodactylus gussevi Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990 (nec Gyrodactylus gussevi Ling, Reference Ling1962; Gyrodactylus gussevi Najdenova, Reference Najdenova1966); Rysavyius gussevi (Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990) Janulewicz, Pietkiewicz & Ziętara, Reference Janulewicz, Pietkiewicz and Ziętara2024.
Type host and locality: Stinging catfish, Heteropneustes fossilis (Bloch) (Siluriformes: Heteropneustidae): Local ponds near Raipur, Madhya Pradesh, India.
Location on host: Gills.
Type specimens: Holotype not designated by Dubey et al. (Reference Dubey, Gupta and Agarwal1990), apparently lost or destroyed.
GenBank sequences (Indian origin): ITS1-5.8-ITS2 (KJ461316; currently the only available molecular reference attributed to G. gussevi Dubey et al., Reference Dubey, Gupta and Agarwal1990).
Status in India: Native.
Taxonomic status: Valid.
Remarks
The naming of this species as G. gussevi from H. fossilis by Dubey et al. (Reference Dubey, Gupta and Agarwal1990) resulted in the name immediately becoming a junior primary homonym of two previously described species of Gyrodactylus: Gyrodactylus gussevi Ling, Reference Ling1962 from Soldatov’s catfish, Silurus soldatovi Nikolskii & Soin; the Amur catfish, Silurus asotus Linnaeus [originally reported as Parasilurus asotus (Linnaeus)]; and their hybrids (all Siluriformes; Siluridae) in China; and Gyrodactylus proterorhini Ergens, 1967 (originally reported as Gyrodactylus gussevi Najdenova, Reference Najdenova1966) from four species of Gobius (Gobiiformes, Gobiidae) in the Black Sea off Sevastopol, Ukraine (Ling, Reference Ling1962; Najdenova, Reference Najdenova1966; Dmitrieva & Prokhorova, Reference Dmitrieva and Prokhorova2025). Articles 57.2, 60.1, and 60.3 of the ICZN indicate that a junior primary homonym is permanently invalid and must be replaced with a new name (nomen novum), for which Gyrodactylus fossilisii nom. nov. is proposed as the replacement for Gyrodactylus gussevi Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990. Should the transfer of this species to Rysavyius proposed by Janulewicz et al (Reference Janulewicz, Pietkiewicz and Ziętara2024) ultimately be confirmed, the replacement epithet established herein would apply equally to that taxon, which is currently represented molecularly by GenBank accession KJ461316.
Janulewicz et al (Reference Janulewicz, Pietkiewicz and Ziętara2024), failing to provide the appropriate binomial for the species, transferred G. gussevi Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990 to the newly proposed Rysavyius Janulewicz, Pietkiewicz & Ziętara, Reference Janulewicz, Pietkiewicz and Ziętara2024 as Gyrodactylus gussevi (Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990), on the basis of a phylogenetic analysis of ITS rDNA sequences obtained from specimens of Gyrodactylus spp. Whether the actions of Janulewicz et al (Reference Janulewicz, Pietkiewicz and Ziętara2024) regarding the proposal of Rysavyius and the transfer of G. gussevi to that genus will eventually be shown to be taxonomically valid or premature is unknown, but at present, their taxonomic acts appear to be minimally supported. Notably, the molecular sequence representing G. gussevi (GenBank KJ461316; submitted by Dr. Vipin Tomar [India]), lacked shoring of a voucher or hologenophore for species’ verification; the complete ITS rDNA region, while highly informative for species delimitation and the recognition of major lineages within Gyrodactylus, has more limited utility for resolving deeper phylogenetic relationships (Ziętara et al Reference Ziętara, Huyse, Lumme and Volckaert2002); taxon sampling in their study was minimal; and morphological support for the transfer of the species to Rysavyius was generally lacking, and what was provided, contradicted their proposed generic diagnosis. As a result, acceptance of Rysavyius and of the transfer of G. gussevi to that genus is withheld until further comprehensive analyses, both at the organismal and molecular levels, are conducted. Further, a redescription of the species and verification of the molecular sequence pertaining to G. gussevi remain necessary.
Gyrodactylus gurleyi Price, Reference Price1937
Type host and locality: Goldfish, Carassius auratus (Linnaeus) (Cypriniformes: Cyprinidae): Texas, U.S.A.
Location on host: Fins.
Type specimens: Type, USNM 1340037 (formerly USNPC 39584).
Indian record: Gills of C. auratus: Specific locality in India not recorded by Tripathi et al. (Reference Tripathi, Matey, Buchmann and Hahn2025).
Indian specimens: 5 voucher specimens, USNM 1770921–1770925.
GenBank sequence (Indian origin): ITS2 (MZ356513).
Status in India: Exotic.
Taxonomic status: Valid.
Remarks
The Indian record is based on specimens recovered from C. auratus and is supported by a photographic voucher and an ITS2 sequence (Tripathi et al Reference Tripathi, Matey, Buchmann and Hahn2025). As part of the present study, five voucher specimens from the Indian material were deposited in the Smithsonian Institution. Although no hologenophore was preserved for the sequenced specimen, preventing direct linkage between the ITS2 sequence and a voucher specimen, the deposited material enables future morphological verification of the record.
Gyrodactylus hyderabadensis Venkatanarsaiah, Reference Venkatanarsaiah1979
Syn. Gyrodactylus paranephrotus hyderabadensis Singh, Vibha, Agarwal & Kumar, 1999 (erroneous combination unavailable sensu the ICZN)
Type host and locality: Channa sp. (Anabantiformes: Channidae): Hyderabad, Andhra Pradesh, India.
Location on host: Gills.
Type specimens: Holotype not designated; ‘type specimen’ deposited in the Museum of the Zoology Department, Osmania University, Hyderabad.
Other Indian record: Skin and gills of spotted snakehead, Channa punctata (Bloch, 1793) [originally reported as C. punctatus (Bloch)] (Anabantiformes: Channidae): Meerut, Uttar Pradesh, India (Singh et al., Reference Singh, Vibha and Kumar1999, as Gyrodactylus paranephrotus hyderabadensis (an erroneous combination) & G. hyderabadensis).
Other Indian specimens: Singh et al. (Reference Singh, Vibha and Kumar1999) stated, ‘The slides (of their helminth specimens) will be deposited in Helminthological Section of Zoological Survey of India, Calcutta’ (parentheses ours). However, no accession numbers were provided, and we were unable to verify whether the material was actually deposited.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Valid.
Remarks
The name Gyrodactylus hyderabadensis Venkatanarsaiah, Reference Venkatanarsaiah1979 is available under Articles 11.9 and 13.1 of the ICZN. Although the original description lacks important diagnostic detail, the species is considered taxonomically valid while requiring further study for reliable identification. The subsequent record of G. hyderabadensis (as Gyrodactylus paranephrotus hyderabadensis) from C. punctata in Uttar Pradesh by Singh et al (Reference Singh, Vibha and Kumar1999) also lacks morpho-molecular support precluding reliable differentiation of the species from congeners parasitising Channa spp. in India, such as G. presidencyus Jha, Ray & Homechaudhuri, Reference Jha, Ray and Homechaudhuri2002 and G. raipurensis Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990, suggesting one or more potential synonymies.
Singh et al (Reference Singh, Vibha and Kumar1999) introduced the trinomial, Gyrodactylus paranephrotus hyderabadensis, an erroneous combination because the epithet ‘paranephrotus’ has not been previously proposed as a species group within Gyrodactylus and its use by these authors did not meet the requirements for availability sensu the ICZN. Singh et al (Reference Singh, Vibha and Kumar1999) may have intended the epithet as a subgenus of Gyrodactylus, as it previously had been proposed by Malmberg (Reference Malmberg, Ergens and Rysavý1964), but their usage of ‘paranephrotus’ as the specific name in the trinomial does not suggest that intention.
Gyrodactylus kobayashii Hukuda, Reference Hukuda1940
Syns. Gyrodactylus kobayashii Hukuda, Reference Hukuda1940; Gyrodactylus elegans of Yamaguti (1940); Gyrodactylus elegans yamagutii Yin & Sproston, 1948; Gyrodactylus yamagutii Yin & Sproston, 1948; Gyrodactylus baueri Ergens & Yukhimenko, Reference Ergens and Yukhimenko1975
Type host and locality: Goldfish, Carassius auratus Linnaeus (Cypriniformes: Cyprinidae): Seoul, South Korea (originally reported by [Hukuda, Reference Hukuda1940] as Keijo, Chosen, Japan [see Remarks below].
Location on host: Skin and fins.
Type specimens: Location of the type specimens of G. kobayashii was not recorded by Hukuda (Reference Hukuda1940); these specimens are likely no longer extant. Holotype of G. baueri, Institute of Parasitology, Czech Academy of Sciences, Prague (no. 368).
Indian records: Gills of C. auratus: Fish farms at Howrah (23°00′N, 88°25′E), Kolkata (22°52′N, 88°35′E), North 24 Parganas (23°12′N, 88°40′E), South 24 Parganas (22°45′N, 88°42′E), West Bengal (Das et al., Reference Das, Dhar, Kumar, Mistri, Parida, Bisai, Devnath, Jana and Behera2025); commercial aquaria in Lucknow, Uttar Pradesh, New Delhi (National Capital Territory of India), and Kolkata, West Bengal, all India (Tripathi et al., Reference Tripathi, Matey, Buchmann and Hahn2025).
Indian specimens: Five voucher specimens, USNM 1770926–1770930.
GenBank sequences (Indian origin): 18S rDNA (PP922389); ITS2 (MZ356536).
Status in India: Exotic; known in India only from commercial fish farms and aquaria.
Taxonomic status: Valid.
Remarks
The type localities for the three species of Gyrodactylus described by Hukuda (Reference Hukuda1940) (Gyrodactylus kobayashii, Gyrodactylus macracantus Hukuda, Reference Hukuda1940, and Gyrodactylus micracanthus Hukuda, Reference Hukuda1940) were both listed in the respective taxonomic summaries as ‘Keijo, Chosen, Japan’, which were the Japanese names for Seoul (Keijo) and the Korean Peninsula (Chosen) during the time of control of the Korean Peninsula by the Empire of Japan (1910 to 1945). The following statement was graciously provided by Dr. Kazuo Ogawa, Meguro Parasitological Museum: ‘Hukuda wrote that he bought goldfish (type host of G. kobayashii) in Keijo, present Seoul, when Japan had occupied Korea. He also wrote that goldfish was brought there from Japan and sold … as an ornamental fish, while Misgurnus anguillicaudatus (type host for G. macracantus Hukuda, Reference Hukuda1940 and G. micracanthus Hukuda, Reference Hukuda1940) was sold at a fish market for food.…’ (parentheses ours).
The Indian records of G. kobayashii from Tripathi et al (Reference Tripathi, Matey, Buchmann and Hahn2025) are supported by a photographic voucher and an ITS2 sequence, and those of Das et al (Reference Das, Dhar, Kumar, Mistri, Parida, Bisai, Devnath, Jana and Behera2025) by an 18S rDNA sequence. Neither Tripathi et al (Reference Tripathi, Matey, Buchmann and Hahn2025) nor Das et al (Reference Das, Dhar, Kumar, Mistri, Parida, Bisai, Devnath, Jana and Behera2025) provided a morphological voucher or hologenophore, which limited independent verification of their respective identifications. As part of the present study, five voucher specimens from the Indian material were deposited in the National Museum of Natural History, Smithsonian Institution (USNM 1770926–1770930), thereby enabling future morphological verification of the Indian material.
Ergens and Yukhimenko (Reference Ergens and Yukhimenko1975) designated the Amur ide, Leuciscus waleckii (Dybowski) (Cypriniformes: Cyprinidae) the type host of G. baueri and also recorded the parasite from the Prussian carp, Carassius gibelio (Bloch) (Cypriniformes: Cyprinidae) [originally reported as Carassius auratus gibelio (Bloch)]. The fact that species of Gyrodactylus generally exhibit a comparatively high host specificity suggests that the helminth reported from C. auratus from eastern Asia, G. kobayashii, may not be conspecific with G. baueri as indicated by Pugachev et al. (Reference Pugachev, Gerasev, Gussev, Ergens and Khotenowsky2010), the investigators responsible for the proposal of synonymy of the two species.
Carassius auratus is an invasive species in Indian freshwater ecosystems. Although G. kobayashii has not been reported from wild populations of C. auratus in India, its presence in commercial aquaria and fish farms (Tripathi et al Reference Tripathi, Matey, Buchmann and Hahn2025) raises concerns regarding potential escape of its monogenoidean parasites resulting in spill-over infections to the native fish fauna.
Gyrodactylus medius Kathariner, Reference Kathariner1895
Type hosts and locality: Weatherfish, Misgurnus fossilis (Linnaeus) (Cypriniformes: Cobitidae) (originally reported as Cobitis fossilis Linnaeus) and carp Cyprinus carpio Linnaeus (Cypriniformes: Cyprinidae). In the original description of G. medius, Kathariner (Reference Kathariner1895) listed the weatherfish first, followed by the carp as hosts of the parasite, but did not specifically designate either species as the ‘type host’: Europe.
Location on host: External surface.
Type specimens: Specimens used in the original description apparently have not survived.
Indian record: Labeo rohita (Hamilton), Cyprinus carpio Linnaeus, Carassius auratus (Linnaeus) (all Cypriniformes: Cyprinidae): Karnataka, India (Devaraj et al., Reference Devaraj, Subramanya and Manissery1977).
Indian specimens: Unknown.
GenBank sequences (Indian origin): None.
Status in India: Exotic.
Taxonomic status: Valid.
Remarks
The name G. medius is available under Articles 11.9 and 13.1 of the ICZN, but precise identity of the parasite as intended by Kathariner (Reference Kathariner1895) remains unclear. Harris et al. (Reference Harris, Shinn, Cable and Bakke2004) noted that G. medius is among the most problematic species of Gyrodactylus, with the name having been applied to morphologically similar parasites from a wide range of hosts. Current usage often associates the species with the carp, C. carpio, although the weatherfish was listed first by Kathariner (Reference Kathariner1895) as one of the two hosts of the parasite. The Indian records of G. medius from three cyprinid hosts (L. rohita, C. carpio, and C. auratus) (Devaraj et al Reference Devaraj, Subramanya and Manissery1977) lack representative illustrations and molecular data, and the survival and/or location of voucher specimens are unknown, all risking misidentification. Furthermore, G. medius has been reported from fishes assigned to many other families, including the Cobitidae, Gasterosteidae, Centrarchidae, Gaidropsaridae, Sciaenidae, Zoarcidae, and Acipenseridae, among others (Yamaguti Reference Yamaguti1963), almost all of which strongly suggest that misidentifications of the parasite are more than common. Thus, a redescription with new topotypic material and an integrative analysis of the Indian forms of G. medius are needed to confirm the status of the parasite in the country.
Gyrodactylus pandeyi nom. nov.
Syn. Gyrodactylus mizellei Dubey, Gupta & Agarwal Reference Dubey, Gupta and Agarwal1990
Type host and locality: Striped dwarf catfish, Mystus vittatus (Bloch) (Siluriformes: Bagridae): Raipur, Madhya Pradesh, India.
Location on host: Gills.
Type specimens: Holotype not designated, apparently lost or destroyed.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Valid as G. pandeyi.
Etymology: The new specific name is proposed in honour of the late Prof. K.C. Pandey in recognition of his significant contributions to the knowledge of the helminths of India.
Remarks
Gyrodactylus mizellei Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990 is a junior subjective homonym of Gyrodactylus mizellei Kritsky & Leiby, 1971 described from the Sauger Sander canadensis (Griffith & Smith), and is therefore invalid under Article 57.2 of the ICZN. Gyrodactylus pandeyi nom. nov. is proposed as a replacement name to preserve nomenclatural continuity. The original description lacks a holotype and molecular data, leaving the taxonomic identity of the species poorly defined. A redescription based on new topotypic specimens from M. vittatus, along with an integrative analysis, is needed to confirm the status of the parasite in India.
Gyrodactylus presidencyus Jha, Ray & Homechaudhuri, Reference Jha, Ray and Homechaudhuri2002
Type host and locality: Spotted snakehead, Channa punctata (Bloch) (Anabantiformes: Channidae): Port Canning, South 24-Parganas, West Bengal, India.
Location on host: Skin, fins, gills, and brachiostegal area.
Type specimens: Type slides stated to be deposited in the National Collection of the Zoological Survey of India (ZSI), Kolkata (accession number not provided); a holotype was not specifically identified by Jha et al. (Reference Jha, Ray and Homechaudhuri2002). However, the use of ‘type slides’ by Jha et al. (Reference Jha, Ray and Homechaudhuri2002) is presumed to indicate that the specimens mounted thereon represent syntypes upon which the description of G. presidencyus was based, thus meeting the requirements of Article 72.3 of the ICZN.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Valid.
Remarks
Four named species of Gyrodactylus have been recorded from snakeheads in India: G. presidencyus, G. hyderabadensis, Gyrodactylus raipurensis Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990 and the invalid G. medius orientalis Singh, Rastogi & Rastogi, Reference Singh, Rastogi and Rastogi2006 (Dubey et al Reference Dubey, Gupta and Agarwal1990; Jha et al Reference Jha, Ray and Homechaudhuri2002; Singh et al Reference Singh, Vibha and Kumar1999; 2006). The comparatively high host specificity observed among species of Gyrodactylus suggests potential conspecificity among the specimens reported in the four papers. A comprehensive study of the gyrodactylid parasites of species of Channa spp., particularly those infecting C. punctata from throughout its distribution in India, would likely be necessary to determine the final taxonomic status of each of the above-named gyrodactylids.
Gyrodactylus raipurensis Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990
Type hosts and locality: Red seam snakehead, Channa gachua (Hamilton) and spotted snakehead, C. punctata (both Anabantiformes: Channidae); Raipur, Madhya Pradesh, India. A type host, chosen from the two species of Channa from which the parasite was collected, was not designated. Although both species are, therefore, considered type hosts of the parasite, C. gachua was listed first in the taxonomic summary of the species.
Location on hosts: Gills.
Type specimens: Holotype not designated; type specimens apparently lost or destroyed.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Valid.
Remarks
Gyrodactylus raipurensis is the fourth species of Gyrodactylus described from species of Channa in India: other named species include G. hyderabadensis, Gyrodactylus medius orientalis Singh, Rastogi & Rastogi, Reference Singh, Rastogi and Rastogi2006 and G. presidencyus Jha, Ray & Homechaudhuri, Reference Jha, Ray and Homechaudhuri2002. The morphology of the atypical haptoral sclerites of G. raipurensis, particularly the anchors, supports it as being distinct from the latter species (see Figures 2 and 3 in Dubey et al Reference Dubey, Gupta and Agarwal1990). A redescription with new topotypic specimens, along with molecular data, is recommended to further confirm its taxonomic status and relationship with related congenerics.
Gyrodactylus recurvensis Rukmini & Madhavi, Reference Rukmini and Madhavi1989
Syn. Gyrodactylus sp. of Madhavi (Reference Madhavi1980)
Type hosts and localities: Blue panchax, Aplocheilus panchax (Hamilton) (Cyprinodontiformes: Aplocheilidae): freshwater stream near Visakhapatnam and a freshwater lake near Kondakaria, both Andhra Pradesh, India. Green panchax, Aplocheilus blochi Arnold (Cyprinodontiformes: Aplocheilidae): Chilka Lake, Odisha, India.
Additional records: A. panchax: 5 km from Andhra University, Waltair, Andhra Pradesh, India (as Gyrodactylus sp.) (Madhavi, Reference Madhavi1980). Aplocheilus melastigma (McClelland) [now Oryzias melastigma (McClelland)] (Cyprinodontiformes: Adrianichthyidae): 5 km from Andhra University, Waltair, Andhra Pradesh, India (as Gyrodactylus sp.) (Madhavi, Reference Madhavi1980).
Location on host: Gills, scales and fins.
Type specimens: Holotype, Zoological Survey of India (ZSI), Kolkata (accession number not specified).
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Valid.
Remarks
Gyrodactylus recurvensis is the only species of Gyrodactylus reported from aplocheilids and adrianichthyids in India. The original description of G. recurvensis included sufficient morphological detail, including illustrations of the haptoral structures. Although the accession number for the holotype was not published, its deposition in the collection of the Zoological Survey of India, Kolkata, was mentioned by Rukmini and Madhavi (Reference Rukmini and Madhavi1989). Given the original description and compliance with provisions of the ICZN at the time of publication, the name is available and the species is taxonomically valid. Fresh material from the various aplocheilid and adrianichthyid hosts occurring in India should be examined and sequenced to determine the parasite’s host preferences and phylogenetic relationships.
Gyrodactylus sprostonae Ling, Reference Ling1962
Type hosts and locality: Goldfish, Carassius auratus (Linnaeus) and carp, Cyprinus carpio Linnaeus (both Cypriniformes: Cyprinidae): Yingkou (formerly Newchwang) and Liao Ho (a district of Yingkou), China.
Location on host: Gills.
Type specimens: Holotype not designated; location of type specimens unknown.
Indian record: Gills of C. auratus: Geographic locality in India not specified by Tripathi et al. (Reference Tripathi, Matey, Buchmann and Hahn2025).
Indian specimens: Two voucher specimens, USNM 1770919-1770920.
GenBank sequences (Indian origin): ITS2 (MZ356537).
Status in India: Exotic; introduced via ornamental fish trade, currently confined to commercial aquaria.
Taxonomic status: Valid.
Remarks
The Indian record of G. sprostonae is supported by ITS2 sequence data, although neither a voucher specimen nor a hologenophore was preserved (Tripathi et al Reference Tripathi, Matey, Buchmann and Hahn2025). As a result, confirmation of the species in India is questionable and the corresponding geographic record reported by Tripathi et al (Reference Tripathi, Matey, Buchmann and Hahn2025) is provisional. Nevertheless, surveillance of G. sprostonae (if present) and other gyrodactylid species infecting the invasive C. auratus in India should be conducted for potential spill-over infections to native freshwater-fish populations.
Indian species of Gyrodactylus lacking compliance with the ICZN
With the exception of the undescribed Gyrodactylus sp. of Tripathi (2025), the remaining species mentioned below are considered ‘unavailable names’ because the authors failed to comply with the current ICZN.
Gyrodactylus baughi Agrawal, Pandey & Tripathi, Reference Agrawal, Pandey and Tripathi2004
Type host and locality: Ticto barb, Puntius ticto (Hamilton) [now Pethia ticto (Hamilton)] (Cypriniformes: Cyprinidae); River Gomti, Lucknow, Uttar Pradesh, India.
Location on host: Body surface.
Type specimens: Holotype not designated; location of type specimens unknown, although voucher specimens were apparently deposited in the collection of the Zoological Survey of India, Kolkata.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Unavailable (ICZN Article 16.4).
Remarks
The name Gyrodactylus baughi Agrawal, Pandey & Tripathi, 2004 is regarded as unavailable because it was published after 1999 without the designation and deposition of a holotype, contravening Articles 16.4 and 72.3 of the ICZN. A compliant description and appropriate designation and deposition of type material of the species occurring on the ticto barb would be required for establishment of an available name of the species.
Gyrodactylus medius exotica Singh, Rastogi & Rastogi, Reference Singh, Rastogi and Rastogi2006
Type host and locality: Carassius sp. (Cypriniformes: Cyprinidae): Meerut, Uttar Pradesh, India.
Location on host: Skin.
Type specimens: Holotype (2002 P/3) and paratypes (2002 P/2 (2-7)) were identified by Singh et al. (Reference Singh, Rastogi and Rastogi2006), but the institution in which they were deposited was not specified (presumed to be either in the personal collections of the authors or that of the Department of Zoology, Ch. Charan Singh University, Meerut, Uttar Pradesh).
GenBank sequences (Indian origin): None.
Status in India: Likely exotic.
Taxonomic status: Unavailable (ICZN Article 16.4).
Remarks
The trinomial Gyrodactylus medius exotica is unavailable under Article 16.4 and 72.3 of the ICZN, as it was published after 1999 without a designated depository for the holotype. Several species of Gyrodactylus have been reported from goldfish in India and from its native habitats in eastern Asia and Japan, and it is highly likely that some synonymy exists among them. However, if G. m. exotica represents a taxonomically valid taxon, it should likely be elevated to a species-level rank after a compliant description sensu the Code is developed. The invasive nature of Carassius spp., in general, also raises concerns about potential spill-over infections of the parasite to native fishes, which warrants continued surveillance in India.
Gyrodactylus medius malmbergi Singh, Rastogi & Rastogi, Reference Singh, Rastogi and Rastogi2006
Type host and locality: Carassius sp. (Cypriniformes: Cyprinidae); skin; Meerut, Uttar Pradesh, India.
Location on host: Skin.
Type specimens: Holotype (2002 P/2) and paratypes (2002 P/2 (2-7)) were identified, but the institution in which the type specimens were deposited was not specified (presumed to be in the personal collections of the authors or that of the Department of Zoology, Ch. Charan Singh University, Meerut).
GenBank sequences (Indian origin): None.
Status in India: Likely exotic.
Taxonomic status: Unavailable (ICZN Article 16.4).
Remarks
The trinomial, Gyrodactylus medius malmbergi, is unavailable under Article 16.4 and 72.3 of the ICZN, as an explicit designation of the name and location of the collection in which the holotype was deposited was not provided. Thus, Gyrodactylus medius malmbergi Singh, Rastogi & Rastogi, Reference Singh, Rastogi and Rastogi2006 is considered an unavailable name. The trinomial also fails as a valid taxonomic name, as it represents a junior subjective homonym of Gyrodactylus malmbergi Ergens, 1961, based on the Principle of Coordination of the ICZN (Article 46).
Gyrodactylus medius orientalis Singh, Rastogi & Rastogi, Reference Singh, Rastogi and Rastogi2006
Type host and locality: Type host not designated between Channa punctata (Bloch) and Channa striata (Bloch) (both Anabantiformes: Channidae); Meerut, Uttar Pradesh, India.
Location on host: Gills and skin.
Type specimens: Holotype (2002 P/1) and paratypes (2002 P/2 (2-19)) were identified, but the institution in which the type specimens were deposited was not specified (presumed to be in either the personal collections of the authors or that of the Department of Zoology, Ch. Charan Singh University, Meerut).
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Unavailable (ICZN Article 16.4).
Remarks
Gyrodactylus medius orientalis, the fourth species-group of Gyrodactylus described from C. punctata after G. hyderabadensis, G. raipurensis, and G. presidencyus, is unavailable under Article 16.4 of the ICZN. Although the holotype and 18 paratypes were mentioned as the basis for the trinomen, Singh et al (Reference Singh, Rastogi and Rastogi2006) failed to explicitly identify the institution and its location where the specimens were deposited. Therefore, the name G. medius orientalis is an unavailable name sensu the Code.
Gyrodactylus neonephrotus malmbergi Singh & Agrawal, Reference Singh and Agrawal1994
Type host and locality: Stinging catfish, Heteropneustes fossilis (Bloch) (Siluriformes: Heteropneustidae): Meerut, Uttar Pradesh, India.
Location on host: Skin.
Type specimens: Holotype and paratypes intended to be deposited in the collection of the Zoological Survey of India, Calcutta, India; collection numbers not available.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Unavailable (ICZN Article 16.4).
Remarks
It is unclear whether Singh and Agrawal (Reference Singh and Agrawal1994) considered the name ‘neonephrotus’ as a generic-group or species-group name, because it was used in their paper both as the specific name in Gyrodactylus neonephrotus malmbergi and as a subgenus of Gyrodactylus (as Gyrodactylus (Neonephrotus)). Nonetheless, the trinomen, Gyrodactylus neonephrotus malmbergi, is unavailable sensu the ICZN, as the parent species-group epithet ‘malmbergi’ is preoccupied by Gyrodactylus malmbergi Ergens, 1961, rendering the trinomial permanently invalid as a junior homonym under the Principle of Coordination and Article 57.2 of the ICZN. As the name fails to meet the requirements of the above-mentioned section of the ICZN, it is regarded as unavailable.
Gyrodactylus punti Agrawal, Pandey & Tripathi, Reference Agrawal, Pandey and Tripathi2004
Type hosts and locality: Pool barb, Puntius sophore (Hamilton), and ticto barb, Pethia ticto (Hamilton) [formerly Puntius ticto (Hamilton)] (Cypriniformes: Cyprinidae): River Gomti, Lucknow, Uttar Pradesh, India.
Location on host: Body surface and gills.
Type specimens: Holotype or syntype series not designated, although voucher specimens were indicated to be deposited in the collection of Zoological Survey of India, Kolkata. A single specimen of G. punti, a putative syntype, was found in the collections of the authors and is now deposited in the USNM (1770918).
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Unavailable (ICZN Article 16.4).
Remarks
The name Gyrodactylus punti is unavailable under Articles 16.4 and 72.3 of the ICZN, as a designated holotype and depository were not provided with the original description. Although the specimen (USNM 1770918) is presumed to be a syntype of G. punti, no information indicating such is provided on the original slide label. As a result, this specimen should be considered with caution as representing a part of the original series used for the description of this species.
Gyrodactylus punti is the second species of the genus recorded from the pool barb in India following G. vivekanensis Shukla, Reference Shukla2001. Although G. punti and G. vivekanensis share the same host species and have type localities only approximately 100 km apart, Agrawal et al (Reference Agrawal, Pandey and Tripathi2004), apparently unaware of Shukla’s earlier description, did not distinguish or compare the two species. That the two species (both unavailable names, see Remarks for G. vivekanensis Shukla, Reference Shukla2001 below) share the same host and proximate geographical localities suggests that they may be synonymous and represent a species now in need of a descriptive account that fulfils the requirements of the ICZN.
Gyrodactylus vivekanensis Shukla, Reference Shukla2001
Type host and locality: Pool barb, Puntius sophore (Hamilton) (Cypriniformes: Cyprinidae): Vivek Nagar fish market, Sultanpur, Uttar Pradesh, India.
Location on host: Gills.
Type specimens: Holotype not designated; type specimens apparently lost or destroyed.
GenBank sequences (Indian origin): None.
Status in India: Native.
Taxonomic status: Unavailable (ICZN Article 16.4).
Remarks
The name Gyrodactylus vivekanensis Shukla, Reference Shukla2001 is unavailable under Article 16.4 and 72.3 of the ICZN. The name was published after 1999 without designation of a holotype or a syntype series and an indication of an institutional depository. The specimens described by Shukla (Reference Shukla2001) likely represent an undescribed species of Gyrodactylidae, which requires a formal description and publication compliant with the ICZN. A determination of its relationship with specimens identified as Gyrodactylus punti Agrawal, Pandey & Tripathi, 2004 is also desirable.
Gyrodactylus sp. of Tripathi et al. (Reference Tripathi, Matey, Buchmann and Hahn2025)
Host and locality: Common carp, Cyprinus carpio (Linnaeus): Lucknow, Uttar Pradesh.
Location on host: Gills.
Specimens: None curated; photographic voucher (Tripathi et al., Reference Tripathi, Matey, Buchmann and Hahn2025)
GenBank sequences (Indian origin): ITS2 (MZ358889).
Status in India: Likely exotic and introduced via ornamental fish trade, confined to commercial aquaria in India.
Taxonomic status: Apparently an undescribed species.
Remarks
This species was determined to be a genetically characterised but morphologically unidentified species of Gyrodactylus collected from the gills of the common carp in India by Tripathi et al (Reference Tripathi, Matey, Buchmann and Hahn2025). The identification is supported by a photographic voucher and an ITS2 sequence, which shows significant divergence from described congeners (Tripathi et al Reference Tripathi, Matey, Buchmann and Hahn2025). Pending formal morphological description, the species is treated as Gyrodactylus sp. of Tripathi et al (Reference Tripathi, Matey, Buchmann and Hahn2025). Its occurrence on an exotic host underscores the need for surveillance for potential spill-over of parasite infections to native fish.
Discussion
Two novel species, Gyrodactylus octoacanthicus and G. ascia, were described from Day’s mystus, Mystus bleekeri, in northern India, using an integrated taxonomic approach. Both species were morphologically distinct, setting them apart from all known congeners. Molecular analyses substantiated their specific status, with sequence divergence from the top BLAST hit in GenBank of 19.12% in G. octoacanthicus n. sp. and 18.07% in G. ascia n. sp. at the ITS loci, values that are well above the 1% threshold proposed for interspecific delineation within the genus (Huyse et al Reference Huyse, Malmberg and Volckaert2004; Ziętara and Lumme, Reference Ziętara and Lumme2002). The sequence divergence from the top BLAST hit for the 28S rDNA gene was also high (16.83% in G. octoacanthicus n. sp. and 8.79% in G. ascia n. sp.), providing additional evidence for the distinctiveness of the two species. Further, the discovery of the two species on a previously sampled host underscores the understudied diversity of species of Gyrodactylus in India and highlights the critical role of integrative taxonomy in uncovering cryptic species within the genus. The generation of ITS and 28S rDNA sequence data for both species also contributes important genetic resources for future taxonomic, phylogenetic, and diagnostic studies of Indian gyrodactylids.
Of the 25 nominal species, subspecies, and unidentified species of Gyrodactylus, now known to infect 20 native and 4 exotic fish hosts in India, 17 (as of January 2026) are listed in the World Register of Marine Species (WoRMS, 2026) and 7 are represented in GenBank with DNA sequences (Table 3). In the present study, only 17 valid Indian species sensu the Code were identified, which included the 2 new species and 2 species considered junior homonyms and renamed herein (Gyrodactylus mizellei Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990, renamed herein as Gyrodactylus pandeyi nom. nov. and Gyrodactylus gussevi Dubey, Gupta & Agarwal, Reference Dubey, Gupta and Agarwal1990 renamed herein as Gyrodactylus fossilisii nom. nov.). This tally almost certainly underestimates the diversity of Gyrodactylus species in Indian waters, largely because past surveys have focused on the gills as the site of infection, while overlooking other key infection sites such as the fins and skin. The annotated checklist compiled herein provides an updated baseline for future biodiversity surveys, taxonomic revisions, and monitoring of gyrodactylid parasites in Indian freshwater fishes.
Parasite-host checklist of the species of Gyrodactylus reported from freshwater fishes of India
Table 3. Long description
The table consists of four columns: Species, Host, Reference, and Validity.
* G y r o d a c t y l u s ascia n. sp. (Native) is found on Mystus bleekeri and Mystus vittatus (Bagridae), referenced as ex nobis, with a Validity of V.
* G y r o d a c t y l u s baughi (Native) is found on Pethia ticto (Cyprinidae), referenced to Agrawal et al (2004), with a Validity of U.
* G y r o d a c t y l u s bullatarudis (Exotic) is found on Poecilia reticulata (Poeciliidae), referenced to Tripathi et al (2025), with a Validity of V.
* G y r o d a c t y l u s chauhani (Native) and G y r o d a c t y l u s colisai (Native) are both found on Trichogaster fasciata (Osphronemidae), with Validity of V.
* G y r o d a c t y l u s elegans indicus (Native) is found on Labeo catla, Cirrhinus reba, Cirrhinus mrigala, and Labeo rohita (Cyprinidae), referenced to Tripathi (1957), with a Validity of V.
* G y r o d a c t y l u s eutheraponsis (Native) is found on Terapon theraps, Mugil cephalus, Planiliza macrolepis, and Terapon jarbua, with a Validity of V.
* G y r o d a c t y l u s fossilisii (Native) is found on Heteropneustes fossilis (Heteropneustidae), with a Validity of V.
* G y r o d a c t y l u s gurleyi (Exotic) and G y r o d a c t y l u s kobayashii (Exotic) are found on Carassius auratus (Cyprinidae), with a Validity of V.
* G y r o d a c t y l u s medius (Exotic) is found on Carassius auratus, Cyprinus carpio, and Labeo rohita, with a Validity of V.
* Several subspecies of G y r o d a c t y l u s medius (exotica, malmbergi, orientalis) are listed with Validity U.
* G y r o d a c t y l u s recurvensis (Native) is found on Aplocheilus blockii and Aplocheilus panchax (Aplocheilidae), with a Validity of V.
Footnotes indicate: E = Exotic, N = Native, U = Unknown/Invalid, V = Valid. Host family codes include A for Aplocheilidae, B for Bagridae, C for Cyprinidae, and Ch for Channidae.
* E = Exotic; N = Native to India; U = unknown.
** Letters in parentheses indicate the families of the respective fish hosts: A = Aplocheilidae; Ad = Adrianichthyidae; B = Bagridae; C = Cyprinidae; Ch = Channidae; H= Heteropneustidae; M = Mugilidae; O = Osphronemidae; P = Poeciliidae; T = Terapontidae.
*** V = Taxon considered valid; U = Taxon considered invalid (unavailable or undescribed taxon).
Several nominal species of Gyrodactylus reported from India were found to be unavailable under the ICZN because their original descriptions did not satisfy the requirements for availability, particularly those relating to the fixation and deposition of name-bearing types. Nevertheless, the nomenclatural status of these taxa should not be interpreted as evidence that the corresponding parasite populations do not exist. Rather, these records likely represent genuine gyrodactylid species that remain in need of formal taxonomic treatment compliant with the Code. Their occurrence on economically and ecologically important hosts highlights the need for renewed collections, integrative morphological and molecular analyses, and the designation of appropriate type material to stabilise their nomenclature and clarify their taxonomic identities.
Taxonomic studies on the species of Gyrodactylus occurring in India have often been impeded by the reliance on ambiguous or erroneous characters, such as (1) the shape and size of soft-body organs, which often depend on procedures used for mounting specimens on microscope slides; (2) presence of an egg, where the developing embryo is referred to as an egg; (3) confluent vs. nonconfluent (blind) caeca; and (4) presence of an armed vagina, where the MCO is referred to as the vagina. Moreover, many descriptive accounts are inadequate, inaccurate, or with overly schematic illustrations. Compounding these issues is the inaccessibility of type and voucher material, with specimens often retained in personal or university collections rather than placed in recognised and stable national or international repositories where access is available to other investigators. In fact, only 2 of the 25 nominal species, subspecies, and undescribed species of Gyrodactylus known from India have had reference material deposited in the Zoological Survey of India, a national repository. Molecular data, critical for resolving cryptic species, are also severely limited. These limitations underscore the need for comprehensive parasitological surveys that integrate both morphological and molecular approaches to elucidate the diversity of species of Gyrodactylus occurring within the subcontinent.
Several species of Gyrodactylus are capable of causing significant disease outbreaks in both wild and cultured fishes. The best-known example is G. salaris, a parasite of salmonids that has caused severe declines of Atlantic salmon populations following its introduction into Norwegian rivers (Johnsen and Jensen Reference Johnsen and Jensen1986; Johnsen et al Reference Johnsen, Møkkelgjerd and Jensen1999; Mo Reference Mo, Woo, Leong and Buchmann2020), leading to one of the most severe wildlife disease disasters ever recorded in Norway (Forseth et al Reference Forseth, Barlaup, Finstad, Fiske, Gjøsæter, Falkegård, Hindar, Mo, Rikardsen, Thorstad, Vøllestad and Wennevik2017). Molecular studies have revealed substantial genetic diversity within G. salaris (Hansen et al Reference Hansen, Bachmann and Bakke2003; Reference Hansen, Bakke and Bachmann2007; Mieszkowska et al Reference Mieszkowska, Górniak, Jurczak-Kurek and Ziętara2018), and different strains have been reported to vary in host specificity, infectivity and pathogenicity (Bakke et al Reference Bakke, Cable and Harris2007; Jørgensen et al Reference Jørgensen, Larsen, Jørgensen, Bresciani, Kania and Buchmann2007). Consequently, accurate morphological and molecular identification is essential for assessing the ecological and economic risks posed by gyrodactylid parasites. Regrettably, in India, the paucity of reliable studies on species of Gyrodactylus hinders accurate assessment of their ecological and economic impacts on native fish populations. Comprehensive investigations on the Indian species of Gyrodactylus, leveraging molecular markers such as 18S rDNA, ITS rDNA, 28S rDNA, and cox1, are urgently needed to resolve taxonomic ambiguities, uncover cryptic species, reveal the extent of the undescribed diversity in this region, and evaluate the potential risks posed by them.
Conclusion
Two new species, Gyrodactylus octoacanthicus n. sp. and G. ascia n. sp. (Monogenoidea: Gyrodactylidae) parasitic on Mystus bleekeri (Bagridae) in northern India, are described, marking only the second record of species of Gyrodactylus from bagrid catfishes in India. Phase-contrast photomicrographs and new molecular sequences complement the original descriptions of both species. These findings highlight the minimal level of current knowledge on the diversity of gyrodactylid species in India, where little is known about their ecology, host specificity, and population dynamics. An annotated checklist of the species of Gyrodactylus recorded from India is provided, which further underscores the need for comprehensive nomenclatural and taxonomic revision of the Indian species of Gyrodactylus to document regional diversity of this genus and to resolve ambiguities.
Future research should expand surveillance to gyrodactylids infecting exotic and invasive fishes, especially tilapias. A notable concern is Gyrodactylus cichlidarum Paperna, 1968, a globally widespread and pathogenic species associated with cultured tilapia (García-Vásquez et al Reference García-Vásquez, Hansen, Christison, Rubio-Godoy, Bron and Shinn2010). With the rapid expansion of tilapia aquaculture in India, especially involving Oreochromis niloticus (Linnaeus) (Department of Fisheries 2020), and the establishment of self-sustaining feral populations in multiple river systems (Singh and Lakra Reference Singh and Lakra2011), the risk of introduction and dissemination of their monogenoids through fish translocation is increasing. Several monogenoidean species have already been disseminated worldwide via tilapia aquaculture (Shinn et al Reference Shinn, Avenant-Oldewage, Bondad-Reantaso, Cruz-Laufer, García-Vásquez, Hernández-Orts, Kuchta, Longshaw, Metselaar, Pariselle, Pérez-Ponce de León, Pradhan, Rubio-Godoy, Sood, Vanhove and Deveney2023), and G. cichlidarum has demonstrated the ability to spill over from aquaculture into wild populations, infecting the native and non-cichlid fishes (García-Vásquez et al Reference García-Vásquez, Pinacho-Pinacho, Rubio-Godoy, Sereno-Uribe, Razo-Mendivil and Pérez-Ponce de León2021). Systematic surveillance of both cultured and wild fish populations is therefore critical to assess and mitigate the potential ecological and economic impacts of gyrodactylids in Indian freshwater ecosystems.
Data availability statement
All data are included in the article; further inquiries can be directed to the corresponding author.
Acknowledgements
We are grateful to Dr. Rejani Chandran (ICAR-National Bureau of Fish Genetic Resources, Lucknow, India) for confirming the identification of Mystus bleekeri and to Prof. Nirupama Agrawal (University of Lucknow, India) for helping with the procurement of older taxonomic literature.
Author contribution
SS: Host collection, slide preparation, and species description; DCK: Taxonomic inputs, manuscript preparation, and revision; CH: Data analysis and manuscript preparation; SP: Slide preparation and species description; AT: Supervision, data analysis, figure preparation, manuscript preparation, and revision.
All authors reviewed and approved the final version of the manuscript for publication.
Financial support
AT acknowledges use of laboratory facilities established through research funding from the Science and Engineering Research Board (SERB), Government of India (SERB–EMR/2017/003232). CH was supported by a stand-alone grant of the Austrian Science Fund (FWF) [https://doi.org/10.55776/P32691].
Competing interests
The authors declare no commercial or financial relationships that could be construed as a potential conflict of interest.
Ethical standards
This study was approved by the Institutional Animal Ethical Committee, University of Lucknow, under protocol number (21/I/2024/IAEC/LU).
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