Introduction
Parasites represent a substantial component of global biodiversity and play important roles in ecological communities through their effects on host populations, species interactions, food-web structure and ecosystem functioning (Hudson et al., Reference Hudson, Dobson and Lafferty2006; Frainèr et al., Reference Frainer, McKie, Amundsen, Knudsen and Lafferty2018). Despite their ecological importance, parasite biodiversity remains incompletely characterised, and inadequate taxonomic resolution continues to constrain our understanding of parasite ecology, host associations and biogeographical patterns (Smit and Sures, Reference Smit, Sures, Smit and Sures2025). This limitation is particularly important for parasite groups with complex or heteromorphic life cycles, where parasitic and free-living stages may differ substantially in morphology. As a result, these parasites cannot readily be assigned to the same species without robust taxonomic and molecular reference data.
Approximately 14% of the 10 000+ described species of Isopoda are considered commensals or obligate parasites. These parasitic lifestyles occur predominantly within the Cymothoida suborder, with the family Gnathiidae among the most species-rich families (12 genera, 254 species described from 1804 to 2026) (updated from Hartebrodt et al., Reference Hartebrodt, Wilson and Costello2023; Boyko et al., Reference Boyko, Bruce, Campos-Filho, Hadfield, Hughes, Merrin, Ota and Poore2026). Gnathiids have an unusual biphasic life cycle in which the 3 juvenile stages are temporary ectoparasites of fishes, feeding primarily on blood and other body fluids, whereas the adult males and females are free-living and non-feeding (Smit and Davies, Reference Smit and Davies2004; Hadfield et al., Reference Hadfield, Erasmus and Smit2025a, Reference Hadfield, Erasmus, Bruce, Smit, Smit and Sures2025b). Gnathiid juveniles are among the most prevalent ectoparasites of marine fishes and can influence host condition and behavior, contribute substantially to interactions between cleaner fishes and their clients, and potentially participate in the transmission of fish blood parasites (Grutter, Reference Grutter2003; McKiernan et al., Reference McKiernan, Grutter and Davies2005; Sikkel et al., Reference Sikkel, Pagan, Santos, Hendrick, Nicholson and Xavier2020). They therefore occupy an important position at the interface between parasitism, fish ecology and marine food webs. Paradoxically, although the juvenile stages are parasitic, the taxonomy of Gnathiidae remains based predominantly on the morphology of the free-living adult male. The genus Gnathia, currently comprising the most species diverse within the family, with 147 accepted species (Boyko et al., Reference Boyko, Bruce, Campos-Filho, Hadfield, Hughes, Merrin, Ota and Poore2026), exemplifies this taxonomic challenge. Parasitic juveniles frequently lack the diagnostic morphological characters required for reliable species-level identification, making it difficult to associate gnathiids collected from fishes with formally described species. Detailed adult descriptions consequently provide the taxonomic foundation for identifying the parasitic component of the life cycle. When adult morphology is combined with molecular sequence data, integrative taxonomy provides an especially powerful reference framework through which unidentified parasitic juveniles can subsequently be linked to named species, thereby facilitating studies of host specificity, parasite distribution, transmission, ecology and potential vector roles (Grutter et al., Reference Grutter, Morgan and Adlard2000; Jones et al., Reference Jones, Nagel, Hughes, Cribb and Grutter2007; Hadfield et al., Reference Hadfield, Erasmus and Smit2025a).
The Caribbean Panamanian coast, 1288 km in length, includes a high number of recorded fish and invertebrate species, representing a major hotspot for marine biodiversity (De Grave and Anker, Reference De Grave and Anker2017), including species of the order Isopoda. This region is also geographically significant, formed through the tectonic evolution of the Caribbean basin (Harmon, Reference Harmon2005). This historical event had significant effects on marine and terrestrial faunal distributions as well as global oceanic circulation patterns (Wegner et al., Reference Wegner, Wörner, Harmon and Jicha2011) and potentially contributed to the biodiversity hotspot along the Panamanian coast. Nevertheless, the diversity of gnathiid ectoparasites in this region remains incompletely documented.
The present study contributes to resolving this taxonomic impediment by describing Gnathia gunayalaensis n. sp. and providing supplementary taxonomic information for Gnathia magdalenensis Müller, Reference Müller1988 from the Caribbean coast of Panama. By using an integrative taxonomic approach that combines light microscopy, scanning electron microscopy (SEM) and molecular characterization, we establish morphological and molecular reference data for the adult males of both species. The combined morphological and molecular reference data generated here for the free-living males provide the taxonomic framework necessary for future identification and molecular matching of their parasitic juvenile stages, thereby facilitating studies of host associations, parasite ecology and biogeographical patterns.
Materials and methods
Field collection
Specimen collection was conducted at Bocas Del Toro (9°21′03.4″ N; 82°15′26.3″ W) and Punta Galeta (9°24′06.8″ N; 79°51′39.3″ W) (Figure 1) during September and October 2024, using light traps (Artim and Sikkel, Reference Artim and Sikkel2016; Shodipo et al., Reference Shodipo, Lauguico, Stiefel and Sikkel2023) set at night in reef habitat, 3–6 m depth. This technique captures both fed and unfed juvenile-stage gnathiids. To develop them into the free-living adult male stages used for identification and taxonomy, fed juveniles were immediately placed in small plastic containers with fresh seawater and allowed to moult. Unfed juveniles were first allowed to feed on captive wild-caught host fish species following the methods described in Grutter et al. (Reference Grutter, Feeney, Hutson, McClure, Narvaez, Smit, Sun and Sikkel2020). Based on their abundance in the study area, personal observations and known hosts for gnathiids at other Caribbean sites (Coile and Sikkel, Reference Coile and Sikkel2013; Hendrick et al., Reference Hendrick, Nicholson, Pagan, Artim, Dolan and Sikkel2023), the following fish species were used: sergeant major (Abudefduf saxatilis; Pomacentridae), French grunt (Haemulon flavolineatum; Haemulidae), hamlet (Hypoplectrus sp.; Serranidae) and schoolmaster snapper (Lutjanus apodus; Lutjanidae). Following the final moult of stage 3 larvae into adults, males were preserved in 95% molecular-grade ethanol for morphological and molecular analysis.
Study area map indicating known records of Gnathia magdalenensis Müller, Reference Müller1988 (based on Müller, Reference Müller1988; Kensley and Schotte, Reference Kensley and Schotte1989; Monroy-Velázquez et al., Reference Monroy-Velázquez, Rodríguez-Martínez and Alvarez2017), as well as the sampling localities for the present study. Occurrence data were obtained from GBIF.Org (2026).
Morphological characterization
Identification protocols based on Erasmus et al. (Reference Erasmus, Smit, Spitzer, Sikkel, Bruce and Hadfield2025) were followed for light microscopy (Figure 2) and SEM, with the addition of an FEI Quanta 250 FEG Scanning Electron Microscope integrated with an Oxford X-MAX 20 EDX System (Thermo Fisher Scientific, Hillsboro, OR, USA). Adobe Illustrator and DELTA (DEscription Language for TAxonomy) were used to illustrate and describe the species, respectively. Total length of the habitus was measured from the frontal margin (excluding the mandibles) to the posterior point of the pleotelson. Type material and voucher specimens were deposited in the Florida Museum of Natural History (FLMNH).
Dorsal (A) and ventral (B) views of Gnathia gunayalaensis n. sp. male holotype (FLMNH UID:77403); dorsal (C) and ventral (D) views of Gnathia magdalenensis Müller, Reference Müller1988 male voucher specimen (FLMNH UID:77405). Scale bar: 200 µm.
Molecular characterization
DNA was extracted using modified protocols for the PCRBiosystems Rapid DNA Extraction Kit (PCRBiosystems available from Analytical Solutions, Randburg, South Africa) and the Macherey–Nagel NucleoSpin® Tissue extraction kit (GmbH & Co. KG, Sandton, South Africa) with a 23-h incubation period. Modification of the PCRBio protocol included adding only 10 µL of lysis buffer and 5 µL of protease buffer, and diluting with 450 µL molecular-grade water instead of the recommended 900 µL, to ensure a high DNA concentration. The internal transcribed spacer 2 region (ITS2: 3S-forward and ITS2.2-reverse) of the ribosomal gene cluster and a partial fragment of the mitochondrial cytochrome c oxidase subunit I (COI: LCO1490 and HCO2198) genes were amplified using specific polymerase chain reactions (PCR) relevant to each primer after trials with various primer combinations and PCR conditions (see Erasmus et al., Reference Erasmus, Smit, Spitzer, Sikkel, Bruce and Hadfield2025 for primer list). Following visualization by agarose gel electrophoresis, samples were sent to Inqaba Biotechnical Industries (Pty) Ltd. (Pretoria, South Africa) for purification and sequence generation. The sequences obtained were assembled in Geneious v. 11.1.4 bioinformatics software (Biomatters, Auckland, New Zealand). Sequences were compared to published records on GenBank and BOLD, and new sequences were subsequently submitted to GenBank (COI: PZ944974; PZ944975 and ITS2: PZ945823; PZ945824 for G. gunayalaensis and G. magdalenensis, respectively).
Results
Systematics
Family Gnathiidae Leach, 1814
Genus Gnathia Leach, 1814
Type species. Gnathia termitoides Leach, 1814
(= Gnathia maxillaris (Montagu, Reference Montagu1804)); type by original designation (Cohen and Poore Reference Cohen and Poore1994)
Gnathia gunayalaensis n. sp.
Zoobank #: urn:lsid:zoobank.org:pub:7499ECDC-DDC6-4C7A-986F-4369F7108C3F
GenBank accession numbers: COI: PZ944974 (609 bp); ITS2: PZ945823 (644 bp)
Type locality. – Bocas Del Toro (9°21′03.4″N; 82°15′26.3″ W).
Known distribution. – Bocas Del Toro (9°21′03.4″ N; 82°15′26.3″ W) and Punta Galeta (9°24′06.8″ N; 79°51′39.3″ W), Panama.
Habitat. – Depth range of 3–6 m; in reef habitat.
Host. – Larvae were fed on captive Abudefduf saxatilis, Haemulon flavolineatum, Hypoplectrus sp. and Lutjanus apodus.
Etymology. – The species name, Gnathia gunayalaensis, is derived from the Guna Yala region along the Caribbean coast of Panama, the type locality of the species, with the Latin suffix -ensis denoting origin or place.
Type material. – Male holotype: 1 adult male (2·4 mm TL; FLMNH UID:77403) collected and reared from various host species listed above from Bocas Del Toro (9°21′03.4″ N; 82°15′26.3″ W) and Punta Galeta (9°24′06.8″ N; 79°51′39.3″ W), Panama. Male paratype: 1 adult male (FLMNH UID:77404), same information as the holotype.
Other material. – 1 adult male, dissected and stained with lignin pink; 1 adult male used for SEM; and 2 adult males were used for DNA, same information as the holotype.
Description of adult male. – Body (Figure 3A) 3·3 times as long as greatest width, widest at pereonite 3; dorsal surfaces with tubercles anteriorly, sparsely setose.
Gnathia gunayalaensis n. sp. (FLMNH UID:77403), male holotype (2·4 mm) and dissected specimen. (A) Dorsal habitus; (B) dorsal cephalosome; (C) dorsal mandible; (D) maxilliped; (E) pylopod; (F) pylopod articles 2 and 3; (G) antennula; (H) antenna. Scale bar: A–E, G, H: 100 µm; F: 50 µm.
Cephalosome (Figure 3B) quadrate, 0·9 times as long as wide, lateral margins subparallel, posterior margin medially concave; dorsal surface with numerous granules; dorsal sulcus wide, shallow, extended; para-ocular ornamentation weakly developed, posterior median tubercle present. Frontolateral processes present. Frontal margin (Figure 6A,B) slightly produced, median point with processes. External scissura (Figure 6C) present, wide, shallow. Mediofrontal process (Figure 6A) present, strong, bifid, with a pair of simple setae. Superior frontolateral process present, single, weak, conical, with 2 pairs of long simple setae. Inferior frontolateral process absent. Mesioventral margin (Figure 6B) straight, granulated and setose, anterior tip dorsally visible. Supraocular lobe pronounced, pointed; accessory supraocular lobe not pronounced. Eyes present, elongate, 0·2 times as long as cephalosome length, contiguous with head surface, ommatidia arranged in rows.
Pereon (Figure 3A) lateral margins subparallel, with few setae; anteriorly with numerous fine granules. Pereonite 1 partially fused dorsally with cephalosome; dorsolateral margins partly obscured by cephalosome. Pereonite 2 wider than pereonite 1. Pereonite 4 with anterior constriction, median groove present. Areae laterales present on pereonite 5, dorsal sulcus wide. Pereonite 6 with strongly developed lobi laterals, lobuii absent. Pereonite 7 narrow, overlapping with pleonite 1. Pleon covered in pectinate scales and epimera not dorsally visible on all pleonites; lateral margins with 3 pairs of simple setae, with 1 pair of simple setae medially.
Pleotelson (Figures 4A and 6F) 1·1 times as long as anterior width, covered in pectinate scales; lateral margins sparsely crenulate, anterolateral margins weakly convex, with 3 pairs of submarginal setae; posterolateral margin weakly concave, with 1 pair of submarginal setae; mid-dorsal surface with 1 pair of sub-median setae, apex with 2 setae.
Gnathia gunayalaensis n. sp. dissected specimen. (A) Dorsal pleotelson and uropods; (B) pleopod 2 with no appendix masculina. Scale bars: 100 μm.
Gnathia gunayalaensis n. sp. dissected specimen. (A–E) Pereopods 2–6, respectively. Scale bars: 100 μm.
Gnathia gunayalaensis n. sp. male, detailed scanning electron microscope images. (A) Dorsal view of frontal margin and mandibles; (B) ventral view of frontal margin and mandibles; (C) ventral view of the base of both antennae with lamina dentata; (D) pylopod article 2 and article 3 minute; (E) propodus of pereopod with robust setae and serrated setae; (F) pleotelson with uropods. Scale bars: A–C, F: 100 μm; D, E: 50 μm. D, dentate blade; ES, external scissura; FM, frontal margin; MT, mandibular tooth (carina); MVM, mesioventral margin; MVR, mesioventral ridge; RS, robust seta; SP, superior frontolateral process; SS, serrated seta.
Antennula (Figure 3G) 0·7 times shorter than antenna; peduncle with 3 articles, article 1 without tubercles, article 1 1·9 times as long as wide, 1·4 times as long as article 2, with 2 simple setae, article 2 0·7 times as long as article 1, 1·4 times as long as wide, 0·4 times as long as article 3, with 4 simple setae; article 3 2·3 times as long as article 2, 3·6 times as long as wide, with 6 simple setae. Flagellum 1·1 times as long as article 3, with 5 articles; article 2 with 2 simple setae; article 3 with 1 aesthetasc seta and 1 simple seta; article 4 with 1 aesthetasc seta and no simple seta; article 5 terminating with 1 aesthetasc and 1 simple seta. Antenna (Figure 3H) peduncle with 4 articles; article 3 3·2 times as long as wide, 2·1 times as long as article 2, with 1 penicillate seta and 7 simple setae; article 4 1·4 times as long as article 3, 4·7 times as long as wide, with no penicillate setae and with 14 simple setae. Flagellum with 7 articles, 1·6 times as long as article 4, 2·2 times as long as article 3, terminating with 2 simple setae.
Mandible (Figures 3C and 6A,B) 0·6 as long as the length of cephalosome, 0·5 times as long as the width of cephalosome, triangular, evenly weak; mandible apex 20% total length; mandibular seta present. Carina present, smooth, along proximal half. Incisor knob-like, distal denticulation present. Blade present, dentate, unevenly convex, midventrally convex, dentate along 83% of margin. Pseudoblade, internal lobe and dorsal lobe absent. Basal neck long; erisma present; lamina dentata visible dorsally. Maxilliped (Figure 3D) 5-articled; article 1 lateral margin with continuous marginal scale-setae; article 2 lateral margin with 5 plumose setae; article 3 lateral margin with 6 plumose setae; article 4 lateral margin with 5 plumose setae; article 5 with 6 plumose setae, and 3 simple setae; endite extending to mid-margin of article 3. Pylopod (Figure 3E) article 1 1·6 times as long as wide, without areolae and distolateral lobe; posterior and lateral margins forming a rounded curve; lateral margin with 32 large plumose setae; mesial margin with continuous scale-setae; ventral surface 2 simple setae present; distal margin with 6 simple setae. Article 2 (Figure 6D) 1·1 times as long as wide; with 6 simple setae; article 3 minute.
Pereopods 2–6 (Figure 5) with long simple setae and randomly covered in pectinate scales; propodus distal robust setae slightly longer than proximal robust setae; inferior margins with weak tubercles. Pereopod 2 (Figure 5A) with tubercles on ischium to carpus; basis 3·8 times as long as greatest width, superior margin with 2 setae, inferior margin with 10 setae; ischium 0·6 times as long as basis, 2·5 times as long as wide, superior margin with 4 setae, inferior margin with 7 setae; merus 0·6 times as long as ischium, 1·9 times as long as wide, superior margin with 3 setae and with bulbous protrusion; inferior margin with 5 setae; carpus 0·7 times as long as ischium, 3·1 times as long as wide, superior margin with no setae, inferior margin with 5 setae; propodus (Figure 6E) 0·8 times as long as ischium, 4·4 times as long as wide, superior margin with 3 simple setae and 1 penicillate seta, inferior margin with 2 simple setae and 2 robust setae; dactylus 0·7 times as long as propodus. Pereopods 3 and 4 (Figure 5B,C) not similar to pereopod 2; pereopod 3 with few tubercles on ischium to carpus; pereopod 4 with tubercles from basis to carpus; pereopod 5 (Figure 5D) differing from pereopod 2 with dense patch of scale-setae on inferior margin; similar to pereopod 6. Pereopod 6 (Figure 5E) with tubercles on carpus; basis 3·2 times as long as greatest width, superior margin with 1 seta and 3 penicillate setae, inferior margin with 7 setae; ischium 0·6 times as long as basis, 2·7 times as long as greatest width, superior margin with 4 setae, inferior margin with 7 setae; merus 0·6 times as long as ischium, 1·9 times as long as wide, superior margin with 2 setae, inferior margin with 4 setae and dense patch of scale-setae; carpus 0·6 times as long as ischium, 3·2 times as long as wide, superior margin with 1 seta, inferior margin with 3 setae; propodus 0·8 times as long as ischium, 4·2 times as long as wide, superior margin with 4 setae, inferior margin with 4 simple setae and 2 robust setae; dactylus 0·8 times as long as propodus.
Penial process not apparent.
Pleopod 2 (Figure 4B) peduncle 1·3 times as wide as long, mesial margin with 2 coupling setae, lateral margin with 1 simple seta; exopod 2·2 times as long as wide, distally broadly rounded, with 9 plumose setae; endopod 2·2 times as long as wide, distally narrowly rounded, with 8 plumose setae; appendix masculina absent.
Uropod (Figures 4A and 6F) rami extending to pleotelson apex and slightly beyond pleotelson, apices narrowly rounded. Peduncle with 1 dorsal seta. Uropod endopod 2·9 times as long as greatest width, dorsally with 5 penicillate setae; lateral margin weakly sinuate, lateral margin with 2 simple setae; proximomesial margin strongly convex, with 6 long plumose setae. Uropod exopod not extending to end of endopod, 4·5 times as long as greatest width; lateral margin weakly sinuate, with 4 simple setae; proximomesial margin straight, distally convex, with 6 long plumose setae.
Diagnosis
Gnathia gunayalaensis n. sp. can be identified by the distinctly pointed superior, inferior and produced frontal margin, as well as the falciform shape of the mandibles with a finely serrated inner margin on the blade.
Remarks
Gnathia gunayalaensis n. sp. is morphologically most similar to Gnathia lignophila Müller, Reference Müller1993, Gnathia notostigma Cohen and Poore, Reference Cohen and Poore1994 and Gnathia scabra Ota, Reference Ota2012, bearing similar frontal margins in that the mediofrontal process is always shorter than the frontolateral processes, sometimes granular along the distal margin. Gnathia gunayalaensis n. sp. differs from both G. notostigma and G. scabra in pereonite 1 lateral margins not being visible dorsally (Cohen and Poore, Reference Cohen and Poore1994; Ota, Reference Ota2012). Para-ocular ornamentation is very pronounced in G. lignophila and G. notostigma, which is not the case in G. gunayalaensis n. sp.; the arrangement of plumose setae on the maxilliped differs in all the above-mentioned species compared to 5, 6, 5, 6 noted in G. gunayalaensis n. sp. (G. lignophila [6, 7, 5, 7]; G. notostigma [4, 8, 5, 6] and G. scabra [3, 7, 4, 1]). In addition, the shape and size of the mandibles differ in G. notosigma in having a pseudoblade dorsally; G. lignophila is 0·7 times the length of the mandible (compared to 0·6 times) (Müller, Reference Müller1993); and G. scabra is the most similar in size and shape. The new species bears no areolae on the pylopod, while G. notostigma and G. scabra bear 3 large areolae on article 1 of the pylopod. A distinct pigmentation pattern was noted on the dorsal habitus in all the specimens of this new species, appearing as darker areas with lighter patches in between, as seen in the preserved specimens photographed (Figure 2A, B).
Gnathia magdalenensis Müller, Reference Müller1988
Gnathia magdalenensis Müller, Reference Müller1988 male voucher specimens (FLMNH UID:77405), dorsal view habitus. Scale bar: 100 μm.
Gnathia magdalenensis Müller, Reference Müller1988 male, detailed scanning electron microscope images. (A) Dorsal view of cephalosome including mandibles; (B) ventral view of frontal margin; (C) lateral view of cephalosome with excavated dorsal sulcus, posterior median tubercle, and pereonite 1 visible; (D) pylopod article 2; (E) distinct serrated setae on pereopod 2; (F) broad penial process. Scale bars: A, C: 200 μm; B: 100 μm; D, E: 50 μm. DS, dorsal sulcus; FS, fringe setae; IL, internal lobe; MC, mandibular carina; MFP, medial frontal process; OT, ocular tubercles; P1, pereonite 1; PP, penial process; 2nd Pr, secondary process; PT, posterior tubercle; SLM, serrated lateral margin; SS, serrated setae.
(urn:lsid:marinespecies.org:taxname:257191)
GenBank accession numbers: COI:PZ944975 (672 bp); ITS2: PZ945824 (613 bp)
Type locality. – Santa Marta, Colombia (Müller, Reference Müller1988).
Known distribution. – Carrie Bow Cay, Belize (Kensley and Schotte, Reference Kensley and Schotte1989); Puerto Morelos, Mexico (Monroy-Velázquez et al., Reference Monroy-Velázquez, Rodríguez-Martínez and Alvarez2017); Bocas Del Toro (9°21′03.4″ N; 82°15′26.3″ W) and Punta Galeta (9°24′06.8″ N; 79°51′39.3″ W), Panama (present study).
Habitat. – Depth range of 3–30 m, coral rubble (Müller, Reference Müller1988); reef habitat (present study).
Host. – Larvae were fed on captive Abudefduf saxatilis, Haemulon flavolineatum, Hypoplectrus sp. and Lutjanus apodus (present study).
Material examined. – Male voucher: 2 adult males (∼2·7 mm TL; FLMNH UID:77405; UID:77406) collected and reared from various host species listed above, from Bocas Del Toro (9°21′03.4″ N; 82°15′26.3″ W) and Punta Galeta (9°24′06.8″ N; 79°51′39.3″ W), Panama.
Other material. – 3 adult males used for SEM and 3 adult males used for DNA, same information as the voucher.
Diagnosis
The Panama specimens conform to the original description by Müller (Reference Müller1988), as well as the diagnosis provided by Kensley and Schotte (Reference Kensley and Schotte1989), in that the frontal margin has 3 processes (projections) with the medio-frontal process slightly shorter than the superior (submedial) pair, and an internal lobe on the mandible. From the detailed SEM micrographs, an additional character – a secondary process on top of the medio-frontal process at the base of the setae – was documented. In addition to the new character defined in the present study, other features were also highlighted in the SEM micrographs, such as a row of fringe setae (Figure 8B, FS) anterior to the buccal cavity on the ventral view, as well as the broad, produced penial openings separated by 83% of sternal width – 0·3 times as long as the basal width – penial process (Figure 8F). Supporting the original description by Müller (Reference Müller1988), a faintly serrated outer margin of the lateral projections on the frontal margin can be seen in Figure 8A (serrated lateral margin), and the tuberculation above the eyes can be seen in Figure 8C (ocular tubercles). However, small differences were noted in the anteriomedial part (dorsal sulcus) of the head, which is reported to be slightly excavated, and pereonite 1 (P1) is not visible dorsally, however, a deep excavation is visible from the lateral view of the cephalosome in Figure 8C; and it is clear from Figures 7 and 8C that P1 is visible dorsally. A distinct pigmentation pattern was also noted on the dorsal habitus of this species, often appearing as darker striped areas with lighter block areas in between, as seen in the preserved specimens photographed (Figure 2C,D).
Remarks
Based on previously published records, Gnathia magdalenensis is most similar to Gnathia johanna Monod, 1926, in having 3 projections on the frontal margin, but can be distinguished by a different shaped inner lobe on the mandible of G. magdalenensis. Additionally, Gnathia serrula Kenlsey, Schotte & Poore, 2009 is also similar in having the internal lobe on the mandible, but can be distinguished by the more excavated frontal margin of G. serrula. Gnathia bermudensis Hadfield, Schizas, Chatterjee & Smit, 2019 also features 3 projections on the frontal margin; however, differ in having slightly pointed supraocular lobes, a weak bifid mediofrontal process, and no internal lobe present on the mandible (Hadfield et al., Reference Hadfield, Schizas, Chatterjee and Smit2019). None of these congeners reports having an additional secondary process on top of the medio-frontal process at the base of the setae.
Phylogenetic results
The newly generated sequences for ITS2 of both Gnathia gunayalaensis n. sp. (474–644 bp) and Gnathia magdalenensis (597–613 bp) include 2 isolates each. The resulting alignment for published records of ITS2 sequences within the family Gnathiidae (including 3 genera) was generated using MUSCLE (Edgar, Reference Edgar2004), with the Isopoda Asellus (Asellus) aquaticus (Linnaeus, 1758) [MT001465] selected as the outgroup. The closest available relative within Gnathia differs by 21·4%, while Elaphognathia differs by 27·3% and Thaumastognathia differs by 48% from the new species, and 26·4%, 32·2% and 48·8% from G. magdalenensis, respectively (Table 1).
Genetic distance matrix of the ITS2 alignment with percentage similarity above the diagonal division and nucleotide p-distance below the diagonal division
Notes: Column headings correspond to the representative row titles.
References: G. jimmybuffetti (Erasmus et al., Reference Erasmus, Hadfield, Sikkel and Smit2023); G. tridens (Erasmus et al., Reference Erasmus, Smit, Spitzer, Sikkel, Bruce and Hadfield2025); E. alcalai (Yumul et al., Reference Yumul, Smit, Hadfield, Shodipo, Sikkel and Erasmus2026); E. cornigera, E. discolor, E. nunomura, E. rangifer (Ota et al., Reference Ota, Tanaka, Hirose and Hirose2010); T. bicorniger (Ota et al., Reference Ota, Kurashima and Horie2022); A. aquaticus (Sworobowicz et al., Reference Sworobowicz, Mamos, Grabowski and Wysocka2020).
The best nucleotide substitution model was determined using MEGA (Tamura et al., Reference Tamura, Stecher and Kumar2021), and the phylogenetic analysis was subjected to the JC + G model (Jukes and Cantor, Reference Jukes, Cantor and Munro1969). The ITS2 maximum likelihood phylogenetic tree topology supports the species and genus variation (Figure 9), with the newly generated sequences clustering together in the genus Gnathia, separate from the other 2 sequenced genera within the family Gnathiidae.
Maximum likelihood consensus tree for representative sequences of Gnathiidae, based on the ITS2 dataset. The novel sequences are highlighted in bold, while each genus within Gnathiidae is blocked in a different colour. Nodal support is indicated at the nodes; the scale bar indicates the expected number of substitutions per site.
In addition, 3 novel COI sequences were obtained for G. gunayalaensis n. sp. (604–609 bp) and G. magdalenensis (672 bp). No reliable phylogenetic analysis could be performed for COI due to low nodal support. With only a few published sequences available, phylogenetic assumptions about Gnathiidae are incomplete and warrant greater attention in future studies.
Discussion
Following the description of the new parasitic gnathiid species from Panama, 16 of the 148 recognized Gnathia species occur in the Tropical Northwestern Atlantic marine province of the Tropical Atlantic realm. The region is known for various marine environments consisting of coral reefs, rocky shores, sandy bays, as well as seagrass and mangrove beds, which provide essential habitats for gnathiid isopods (and other cryptic species) (Monroy-Velázquez et al., Reference Monroy-Velázquez, Rodríguez-Martínez and Alvarez2017). Given the high abundance of reef fish, 316 species reported in a recent study along the Colombian region (Estape et al., Reference Estape, Estape, Robertson, Pena and Dunlap-Smith2025), a multitude of endless parasitic opportunities are present. Considering the state of natural environments over the last few decades, it is crucial to understand the diversity of these cryptofauna, which can aid in understanding how they respond to various environmental parameters, how they are distributed within the region, and how their behavior is affected within ecosystems (Monroy-Velázquez et al., Reference Monroy-Velázquez, Rodríguez-Martínez and Alvarez2017). In this case, the study confirms the distribution of Gnathia magdalenensis along the southwestern coast of the Caribbean, from Mexico to Colombia (see Figure 1), while focused ecological studies would better define factors influencing the distribution and host specificity of these cryptofauna in the Caribbean region.
As taxonomists, contributing to biodiversity knowledge through the description of new species is the main goal; however, using new approaches to better define known diversity is just as rewarding. As such, the present study not only adds a new species but also highlights the use of integrative taxonomy to contribute additional knowledge of known species. Although compared to others, the original description of G. magdalenensis is sufficient for its time, there are still features to highlight with the aid of modern equipment (i.e., the secondary process on the mediofrontal process; the excavated dorsal sulcus; visibility of P1; fringe setae along the buccal cavity; the penial process). Considering the number of species known from this region, it is advisable to investigate whether additional information could be obtained from newly collected material or by applying new approaches to type and voucher material held in museums.
In the present study, the different genera within Gnathiidae clustered together in well-supported clades, reinforcing the morphological differentiation at family level. While this preliminary ITS2 phylogeny indicates that species with available sequences from the 3 different genera clade together, more molecular data are needed to resolve species- and genus-level variation. In particular, COI, as well as mitochondrial genome and whole-genome characterization within the Gnathiidae. Although phylogenetic relationships remain uncertain due to limited sequence availability, our aim in generating molecular data for adult males is to facilitate the identification of females and larvae, particularly in cases where multiple species occur at the same locality or on the same hosts.
Data availability statement
Data will be made available on request.
Acknowledgements
The authors would like to thank Isabella Cunio for assisting with the collection of gnathiid specimens. We also thank the Smithsonian Tropical Research Institute (STRI) and D. Ross Robertson for facilitating our work in Panama. We thank Anja Vermaak from the North-West University Water Research Group (NWU-WRG) for assistance with molecular characterization, as well as Anine Jordaan (NWU-CRB) and Willie Landman (NWU-UESM) for assistance with scanning electron microscopy. The Nippon Foundation-Nekton Ocean Census Program (https://oceancensus.org/) for supporting the description of this species (Ocean Census Species Number OC-SP-0002061). This is contribution number 1070 from the NWU-WRG.
Author contributions
A.E.: Investigation, data curation, microscopy, illustrations, writing, editing, visualization, M.D.N.: Field collection, rearing and development of specimens, investigation, data curation, writing – editing; G.C.H.: field collection, rearing and development of specimens, writing – editing; K.A.H.: Conceptualization, supervision, resources, investigation, writing – editing; P.C.S.: Conceptualization, field collection, permits, rearing and development of specimens, supervision, funding, writing – editing; N.J.S.: Conceptualization, supervision, resources, investigation, writing – editing.
Financial support
Funding was in part supported by the NWU Postdoctoral Program (for A.E.), the National Research Foundation (NRF) of South Africa (Grant No.: CPRR240430216732; NJ Smit, PI), and US National Science Foundation (NSF DEB 2231250, PC Sikkel, PI). Opinions, findings, conclusions, and recommendations expressed in this publication are those of the authors, and the NWU, NRF and NSF accept no liability in this regard.
Competing interests
The authors acknowledge that N.J.S. is a Parasitology Editorial Board member but was excluded from the review process for this paper.
Ethical standards
Collection of samples was permitted by the Ministero de Ambiente del Republico de Panama (5458), and all methods and protocols were performed as approved by the University of Miami animal ethics committee (23-004 LF). Preserved specimens were shipped to North-West University (NWU) as a donation for further identification, where all methods and protocols were performed as approved by the NWU AnimCare ethics committee (NWU-00784-24-A5). Artificial intelligence was not employed in creating the subject matter of this write-up. Language editing software was applied in some instances to improve the readability of the manuscript.
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