Introduction
Among the fishery resources in the Amazon region, Arapaima gigas shows outstanding value compared to other fish species due to its large size and cultural gastronomic value (Imbiriba Reference Imbiriba2001). In this species, parasitic helminths have been recorded in various infection sites. Monogeneans: Dawestrema cycloancistrium Price and Nowlin, 1967 and Dawestrema cycloancistrioides Kritsky, Boeger and Thatcher, 1985 infecting the gills. Nematodes: Goezia spinulosa infecting the stomach; Philometra senticosa and Nilonema senticosum infecting the swim bladder. In the intestine, Hysterothylacium larvae were observed (Andrade-Porto et al Reference Andrade-Porto, Cárdenas, Martins, Oliveira, Pereira, Araújo and Malta2015). Acanthocephalans: Polyacanthorhynchus macrorhynchus and Trematodes: Caballerotrema arapaimense, and C. brasiliense (Marinho et al Reference Marinho, Tavares-Dias, Dias-Grigório, Neves, Yoshioka, Boijink and Takemoto2013; Serrano-Martínez et al Reference Serrano-Martínez, Tantaleán, Leguía, Quispe, Casas and Gina2015; Thatcher, Reference Thatcher1980). Many of these records belong to intensively or semi-intensively farmed species, where the specific handling conditions directly or indirectly alter parasitic infections in these individuals (Delgado et al Reference Delgado, Oliveira-Malta, Ismiño-Orbe, Chu-Koo, Silva-Gomes and Tello-Martín2007).
Gastric nematodes such as Goezia spinulosa of the Raphidascarididae family are frequently reported, causing tissue lesions in Arapaima gigas, particularly severe in farmed fish fry. The other less frequently reported gastric nematode that infects Osteoglossiformes fish Arapaima gigas in natural habitats was initially named Porrocaecum draschei (Bruce et al Reference Bruce, Adlard and Cannon1994) and later reclassified as Terranova serrata (Drasche 1884) of the Anisakidae family, but with limited morphological details. In this work, we present a co-infection by gastric nematodes and, additionally, provide new morphological and morphometric data of Neoterranova serrata (syn=Terranova serrata) in A. gigas individuals from artisanal fishing in natural areas carried out by residents of the Mamirauá Reserve community in the Middle Solimões region.
Material and methods
During the dry season (October-December) of 2023, a total of 60 Arapaima gigas stomachs were collected from a fisheries management program conducted by local residents within the Mamirauá Reserve (2°16′S; 65°41′W) (Category VI in the IUCN classification system), at the confluence of the Solimões and Japurá rivers, in the state of Amazonas, northern Brazil. Subsequently, organs were packaged in polyethylene bags (Zip Lock plastic bags) and transported in insulated boxes containing flake ice (−10°C±1°C) to the Laboratory of the Mamirauá Institute for Sustainable Development in the city of Tefé, Amazonas. For parasitological analysis, each organ was separated in containers, and the parasites were recovered, washed, and fixed in 70% alcohol for analysis by light and scanning electron microscopy. Additionally, stomach samples from of host infected with parasites were fixed in 10% formalin and subjected to routine haematoxylin-eosin staining for histopathological examination. The collected nematodes were examined on temporary slides with 50% Amann’s Lactophenol. Subsequently, 15 specimens of each species were measured, and photographs were taken using a digital camera coupled to a Nikon Eclipse SI optical microscope using Prime Life Science software.
For scanning electron microscopy (SEM), fixed specimens were post-fixed in 1% Osmium Tetroxide, dehydrated through a graded ethanol series, and subjected to critical-point drying. Subsequently, samples were sputter-coated with gold and examined using a JEOL JSM IT500-HR scanning electron microscope operated at an accelerating voltage of 10 kV. Parasite identification was performed according to Costa et al (Reference Costa, Guimarães, Cabral and Mundim1995), Moravec (Reference Moravec1998), Thatcher (Reference Thatcher2006) and Luque et al (Reference Luque, Aguiar, Vieira, Gibson and Santos2011) . The ecological descriptors of the parasitic community, such as prevalence, intensity, and average abundance, were calculated according to Bush et al (Reference Bush, Lafferty, Lotz and Shostak1997). All measurements are provided in millimetres unless otherwise specified and followed by the range in parentheses. The diagnostic characters used to differentiate our specimens from other similar ascarids (Porrocaecum, Pseudoterranova, Pulchrascaris, Euterranova and Neoterranova) were based on the taxonomic keys proposed by Moravec and Justine (Reference Moravec and Justine2020), and for G. spinulosa were based on the paper of Travassos et al (Reference Travassos, Artigas and Pereira1928) and Santos et al (Reference Santos, Vicente and Jardim1979). The appropriate license (SISBIO) was obtained by the team to carry out fish sampling in all units advised by the Mamirauá Institute for Sustainable Development (Number: 77176-7) and (SIGED Process No. 01.01.030101.000583/2022-78) to work in the reserves of SEMA (State Secretariat for the Environment) and the CEP (Research Ethics Committee) of IDSM (CAAE: 55899222.7.0000.8117).
Results
Of the 60 gastrointestinal organs of A. gigas examined, 85% (51/60) showed gastric parasitic infection with at least one nematode species. Of these, 5 hosts presented monoparasitism by G. spinulosa, 7 hosts by N. serrata, and 39 hosts with biparasitism (G. spinulosa and N. serrata). Of the total, 73.3% (44/60) of the fish examined were parasitised by G. spinulosa and 76.7% (46/60) by N. serrata.
Gastric lesions
16% (10/60) of the infected fish presented gastric mucosal ulcers in the middle third of the cardiac stomach, with sizes ranging from 1.5 to 2.4 mm in length and a depth of 3.1 to 3.5 mm. In 9% (7/10) of these ulcers, specimens of G. spinulosa were removed, and in 5% (3/10), specimens of N. serrata were recovered from within these ulcers. The macroscopic lesions consisted of ulcerative-type gastric mucosal inflammation at the sites of nematode aggregation (Figure 1A, D). After removal of stomach contents, a crater-shaped lesion with thick edges was observed in the mucosa (Figure 1B, E). Microscopically, segments of nematodes with cuticular spines were observed in the gastric lumen (Figure 1C). Anterior regions of the nematodes were deeply embedded in the ulcerated mucosa and submucosa without reaching the muscularis mucosae (Figure 1F), and bacterial nests were present in the ulcerated area. Furthermore, necrosis was observed in the gastric glands and a severe and diffuse inflammatory reaction composed of eosinophils (predominant), lymphocytes, and rare macrophages in the mucosa and submucosa of the stomach.
Macroscopic lesion and histology of pyloric region of Arapaima gigas showing ulcerative gastric and parasited with Goezia spinulosa (A, B, and C) and Neoterranova serrata (D, E, and F): (A and D) Clusters of specimens nematodes burrowed into the mucosa. An ulcer (yellow circle). (B and E) After removal of stomach contents an ulcer can be seen surrounded by thickening of the mucosa (yellow circle). Bar = 0.5 mm. (C and F) Sections of nematodes in the gastric lumen (n); extensively ulcerated area and thickened (ua), with scarce gastric glands and inflammatory reaction composed of eosinophils, lymphocytes, and macrophages; non-ulcerated area containing epithelium and gastric glands (nu). Bar = 100 μm.
Figure 1. Long description
The composite consists of two rows of three panels each.
Top Row (Goezia spinulosa infection):
Panel A is a macroscopic photo of the pyloric mucosa with dark stomach contents at the top. A yellow circle in the center highlights a cluster of nematodes burrowed into the tissue.
Panel B shows the same tissue after cleaning. The yellow circle highlights a distinct ulcerated area surrounded by thickened, pale mucosa.
Panel C is a micrograph of a tissue section stained in pink and purple. A large central void represents the ulcerated area, labeled u a. Arrows labeled n point to cross-sections of nematodes within the gastric lumen. The label n u at the top right indicates a non-ulcerated area with intact epithelium.
Bottom Row (Neoterranova serrata infection):
Panel D is a macroscopic photo showing a cluster of larger nematodes burrowed into the mucosa, highlighted by a yellow circle.
Panel E shows the cleaned mucosa where the yellow circle identifies a deep, red-centered ulcer.
Panel F is a micrograph showing a highly irregular and deeply eroded tissue surface. Multiple u a labels indicate the extensively ulcerated areas. Arrows labeled n point to nematode sections near the top. The label n u at the top left indicates the remaining non-ulcerated epithelium.
Infection by Goezia spinulosa
Morphologically, G. spinulosa had an elongated, medium-sized, whitish body, a thick cuticle containing transverse rows of posteriorly directed spines. One dorsal lip and two ventrolateral lips. The oesophagus was clavate, with a nerve ring encircling the anterior third of the oesophagus, an excretory pore slightly posterior to the level of the nerve ring, deirids located halfway along the oesophagus, a short ventricle, and a narrow, long ventricular appendage. The intestinal cecum was wide and short, slightly extending beyond the base of the oesophagus, with a conical tail (Figure 2).
Light microscopy of Goezia spinulosa. (A) Male, lateral view, showing location of lips, junction oesophagus/ventriculus, final portion of ventricular appendix, and spicules. Bar =1 mm. (B) Female, showing location of lips, junction oesophagus/ventriculus, final portion of ventricular appendix, vulvar aperture, and digitiform process. Bar =1 mm.
Figure 2. Long description
Panel A, located on the left, shows a male nematode in a lateral view. Starting at the anterior end, an arrow points to the Lips. Moving down the body, the next label indicates the Junction esophagus-ventriculus. Further down the curved body, an arrow points to the Final portion of ventricular appendix. Near the coiled posterior end, an arrow identifies the Position of the spicules. A scale bar at the bottom left equals 1 mm.
Panel B, located on the right, shows a larger female nematode. At the anterior tip, an arrow points to the Lips. Moving distally, labels identify the Junction esophagus-ventriculus and the Final portion of ventricular appendix. Near the mid-body, an arrow points to the Position of the vulvar aperture. At the tapered posterior tip, an arrow points to the Digitiform process. A scale bar at the bottom right equals 1 mm.
Male (15 specimens): Length of body (12.3) 8.0–15.2, maximum width (0.5) 0.3–0.6. Oesophagus (0.7) 0.57–0.96 long (4.0–8.1% of body length). Nerve ring, excretory pore and deirids (0.20) 0.13–0.29; (0.30) 0.20–0.39 and (0.40) 0.31–0.49, respectively, from anterior extremity. Ventriculus (0.8) 0.5–1.0 × (1.0) 0.81–1.2, length of ventricular appendix (2.5) 1.7–3.5. Intestinal caecum (0.20) 0.13–0.17 long. Spicules (0.60) 0.33–0.69. long (3.6–5.6% of body length). Preanal pairs 16, unpaired median preanal papilla. One pair of small adanal papillae present. Postanal pairs 4. Tail short including digitiform process (0.10) 0.10–0.18 long (Figure 2A).
Female (15 specimens): Length of body (16.5) 11.3–23.4 and (0.5) 0.4–0.7 maximum width. Spination of cuticle throughout body length. Oesophagus (0.80) 0.72–0.98 long (3.5–7.0% of body length). Nerve ring, excretory pore and deirids (0.20) 0.21–0.24; (0.30) 0.29–0.33 and (0.40) 0.40–0.47, respectively, from anterior extremity. Ventriculus (0.80) 0.66–0.96 × (1.1) 1.1–1.3, ventricular appendix (2.4) 1.9–3.1. Intestinal caecum (0.20) 0.21–0.29 long. Vulva opening from anterior extremity (8.4) 5.9–10.0 and 42.7–62.3% of body length. Eggs spherical (42.3) 39.5 × 49.8 μm. Tail short conical with digitiform process (0.1) 0.12–0.17 long. A total of 1055 adult specimens were recovered from the stomachs. The mean intensity was 24.0 (±39.7), and the mean abundance was 17.6 (±39.7), with a range of 1 to 201 nematodes per host (Figure 2B).
Redescription of Neoterranova serrata
Anisakidae Railliet & Henry, Reference Railliet and Henry1912
Neoterranova Moravec & Justine, Reference Moravec and Justine2020
Neoterranova serrata (Drasche, 1884) (Based on light microscopy and scanning electron microscopy: Table 1, Figures 3–5)
Morphometric comparison of Neoterranova serrata collected from Arapaima gigas in Brazil with Terranova serrata (Syn: Porrocaecum draschei) by Baylis (Reference Baylis1927). Measurements in millimetres
Table 1. Long description
The table compares morphometric characterisation across four columns: Neoterranova serrata (Male and Female from this study) and Terranova serrata (Male and Female from Baylis, Reference Baylis1927). All measurements are in millimetres.
* Length of body: Male 18.2 (14.5 to 26.5); Female 27.3 (20.2 to 35.0); Baylis Male 25; Baylis Female 28.
* Maximum width: Male 0.52 (0.44 to 0.65); Female 0.65 (0.44 to 0.85); Baylis Male 0.62; Baylis Female 0.73.
* Length of lips: Male 0.06 (0.05 to 0.07); Female 0.07 (0.07 to 0.08).
* Total oesophagus: Male 1.83 (1.5 to 2.5); Female 2.2 (1.7 to 2.6); Baylis Male 2.65; Baylis Female 2.9.
* Oesophagus width: Male 0.18 (0.13 to 0.26); Female 0.18 (0.15 to 0.23).
* Nerve-ring: Male 0.38 (0.30 to 0.51); Female 0.44 (0.30 to 0.52); Baylis (unspecified sex) 0.47.
* Deirids: Male 0.47 (0.35 to 0.65); Female 0.48 (0.36 to 0.58); Baylis (unspecified sex) 0.65 to 0.70.
* Ventriculus length: Male 0.35 (0.26 to 0.39); Female 0.45 (0.34 to 0.58); Baylis (unspecified sex) 0.45 to 0.50.
* Ventriculus width: Male 0.15 (0.12 to 0.19); Female 0.16 (0.12 to 0.22).
* Caecum: Male 0.62 (0.36 to 0.76); Female 0.88 (0.67 to 1.1); Baylis (unspecified sex) 0.70.
* Tail: Male 0.17 (0.14 to 0.21); Female 0.28 (0.22 to 0.39); Baylis Male 0.16; Baylis Female 0.23.
* Length spicules: Male 1.4 (1.1 to 1.7); Baylis Male 1.5.
* Vulva: Female 14.3 (10.0 to 20.4); Baylis Female 12.5.
* Eggs: Female 0.042 to 0.050; Baylis Female 0.045 to 0.050.
* Oesophagus all over T L percentage: Male 10.1 percent (8.3 to 11.7); Female 8.2 percent (6.6 to 10.2); Baylis Male 10.6 percent; Baylis Female 10.4 percent.
* Caecum all over L body percentage: Male 3.4 percent (1.9 to 4.6).
* Spicules all over L body percentage: Male 9.4 percent (6.3 to 9.4); Baylis Male 6 percent.
* Vulva all over L body percentage: Female 52.4 percent (40.0 to 69.5); Baylis Female 44.6 percent.
* Number of Specimens: Male 15; Female 15; Baylis not mentioned.
The values of the total oesophagus of Baylis (Reference Baylis1927) include the ventricle length.
* The values mentioned do not specify whether they are for male or female specimens.
Neoterranova serrata, stomach parasite of Arapaima gigas (A) Female anterior end, showing nerve ring (nr), oesophagus (e), ventriculus (v), and intestinal caecum (ic). Bar = 300 μm. (B) Preequatorial vulvar aperture (va). Bar = 300 μm. (C) Female posterior end, showing anal aperture (a). Bar = 300 μm. (D) Larval fourth stage, showing oesophagus (e), ventriculus (v), intestinal caecum (ic), intestine (in), and anal aperture (a). Bar = 500 μm. (E) Male anterior end, showing nerve ring (nr), oesophagus (e), ventriculus (v), and intestinal caecum (ic). Bar = 200 μm. (F) Male, lateral view, showing oesophagus (e), ventriculus (v), intestine (in), and spicules (s). Bar = 500 μm. (G) Male anterior end, showing lips (li), nerve ring (nr), and deirids (d). Bar = 50 μm. (H) Posterior end of male, lateral view, showing cloacal aperture (c) and spicules (s). Bar = 100 μm.
Figure 3. Long description
The figure consists of eight panels labeled A through H.
Panel A shows the female anterior end with a scale bar of 300 micrometers. Arrows point to the nerve ring n r near the tip, the long oesophagus e, the ventriculus v, and the intestinal caecum i c at the base of the oesophagus.
Panel B displays the preequatorial vulvar aperture v a on the side of the body. Scale bar is 300 micrometers.
Panel C shows the female posterior end tapering to a point, with an arrow indicating the anal aperture a. Scale bar is 300 micrometers.
Panel D presents the fourth stage larva in a curved posture. Scale bar is 500 micrometers. Labels identify the oesophagus e, intestinal caecum i c, ventriculus v, intestine i n, and the anal aperture a at the tail.
Panel E shows the male anterior end with a scale bar of 200 micrometers. It highlights the nerve ring n r, oesophagus e, ventriculus v, and intestinal caecum i c.
Panel F provides a lateral view of the male. Scale bar is 500 micrometers. It shows the oesophagus e, ventriculus v, intestine i n, and the coiled posterior containing spicules s.
Panel G is a high-magnification view of the male anterior end. Scale bar is 50 micrometers. Arrows point to the lips l i at the very tip, the nerve ring n r, and the deirids d on the lateral margin.
Panel H shows the male posterior end in lateral view. Scale bar is 100 micrometers. Arrows indicate the cloacal aperture c and the internal spicules s.
Morphologically, nematodes are quite large with a transversely striated cuticle. Dorsal lip with 2 double papillae, each subventral lip with 1 double papilla, lateral amphids, and excretory pore located at the base of the subventral lips. Each lip has a short row of uniformly sized denticles extending from the central portion of the inner margin of the lips (Figure 5A, B); lateral wings absent, deirids clearly visible near the level of the nerve ring. Oesophagus long and narrow, elongated ventricle (Figure 3A, E), without a ventricular appendage. Intestinal cecum present, extending only beyond the ventricle (Figure 2E). Spicules similar, approximately the same length. Gubernaculum absent. Numerous genital papillae. Vulva anterior to the middle of the body (Figure 3B). Conical tail and without ornamentation. L4 larvae were also recovered, exhibiting three lips, an intestinal cecum, a ventriculus, and a conical tail, with no reproductive structures observed. In total, 451 L4 larvae and 2163 adults of the species Neoterranova serrata were recovered. The mean intensity was 56.8 (±62.7), and the mean abundance was 43.6 (±62.4), with a range of 1 to 229 nematodes per host.
Male (based on 15 specimens): Length of body (18.2) 14.5–26.5 (Figure 3F); maximum width of body at level ventriculus (0.52) 0.44–0.65. Length of lips (0.06) 0.05–0.07. Length of oesophagus (1.83) 1.5–2.5, representing 8.3–11.7% (10.1%) of body length; maximum width of oesophagus (0.18) 0.13–0.26. Nerve ring and deirids (0.38) 0.30–0.51 and (0.47) 0.35–0.65, respectively, from anterior extremity (Figure 3G). Length of ventriculus (0.35) 0.26–0.39; width of ventriculus (0.15) 0.12–0.19. Length of Intestinal caecum (0.62) 0.36–0.76 representing 1.9–4.6% (3.4%) of body length (Figure 3E). Posterior end of body curved ventrally (Figure 3H). Length of spicules (1.4) 1.1–1.7, representing 6.3–9.4% (9.4%) of body length (Figure 3H, 4G). Gubernaculum absent. Total caudal papillae 36–40 pairs of preanals and 4 postanals present; first pair of postanal papillae doubled (Figure 4A, B, D, E, F); phasmids situated short distance anterior to level of posterior pair of lateral postanal papillae (Figure 4C). Tail pointed (0.17) 0.14–0.21 long (Figure 3H).
Scanning electron microscopy of male Neoterranova serrata, stomach parasite of Arapaima gigas. (A) Ventrodorsal view of the male’s tail with the tip of the spicule (s) partially exposed (arrowhead). Bar = 20 μm; (B) Ventral view of the posterior end; one can observe the distribution of the caudal papillae (yellow asterisk). Bar = 100 μm. (C) Tip of tail (t), ventrolateral view of the postcloacal papillae and phasmid (ph). Bar = 100 μm. (D) Posterior end demonstrating cloacal papillae (yellow asterisk) and spicule (s). Bar = 50 μm; (E) Detail of pedunculate caudal papillae. Bar = 50 μm. (F) Detail of sessile caudal papillae. Bar = 50 μm. (G) Detail of tip spicules. Bar = 50 μm.
Figure 4. Long description
A collection of seven S E M micrographs labeled A through G.
Panel A shows a ventrodorsal view of the curved tail. A yellow arrow points to the spicule s, and a white arrowhead indicates its partially exposed tip. A line of yellow asterisks marks the caudal papillae. Scale bar equals 20 micrometers.
Panel B provides a ventral view of the posterior end. A dense row of yellow asterisks follows the outer edge of the tail, leading to the cloaca cl. Scale bar equals 100 micrometers.
Panel C is a ventrolateral view of the tail tip t. Yellow asterisks mark postcloacal papillae, and a yellow arrow points to the phasmid ph. Scale bar equals 100 micrometers.
Panel D shows the posterior end in profile. A yellow arrow points to the curved spicule s and the tail tip t. A series of yellow asterisks tracks the distribution of cloacal papillae along the ventral curve. Scale bar equals 50 micrometers.
Panel E is a high-magnification detail of a single pedunculate caudal papilla, appearing as a raised, stalk-like structure. Scale bar equals 50 micrometers.
Panel F shows three sessile caudal papillae arranged vertically, appearing as circular, flattened discs on the striated cuticle. Scale bar equals 50 micrometers.
Panel G provides a close-up of the distal tips of two spicules, showing their sharp, curved morphology. Scale bar equals 50 micrometers.
Female (based on 15 specimens). Length of body (27.3) 20.2–35.0, maximum width at level of vulva (0.65) 0.44–0.85 long. Lips (0.07) 0.07–0.08 long. Length of oesophagus (2.2) 1.7–2.6, maximum width (0.18) 0.15–0.23 long (Figure 3A), representing 6.6–10.2% of body length. Nerve ring and deirids 0.30–0.52 (0.44) and 0.36–0.58 (0.48), respectively, from anterior extremity. Ventriculus (0.45) 0.34–0.58 long; maximum width (0.16) 0.12–0.22. Intestinal caecum (0.88) 0.67–1.1 long. Vulva situated (14.3) 10.0–20.4 from anterior extremity, representing 40.0–60.5% of body length (Figure 5D). Eggs in uterus spherical, 0.042–0.050 in diameter. Tail (0.28) 0.22–039 long (Figure 5C) with a pair of lateral phasmids near the posterior end. Fourth stage larva: (based on 15 specimens): Length of body (11.2) 8.8–13.7, maximum width (0.38) 0.20–0.48. Length of oesophagus (1.4) 1.0–1.7. representing 9.3–15.7% of body length. Nerve ring and deirids (0.30) 0.24–0.38 and (0.57) 0.42–0.67, respectively, from anterior extremity. Ventriculus (0.31) 0.24–0.39 long. Intestinal caecum (0.6) 0.42–0.67 long, representing (5.2) 3.4–6.8% of body length; rectum (0.07) 0.05–0.08 long. Tail (0.16) 0.12–0.22 long (Figure 2D).
Scanning electron microscopy of female Neoterranova serrata, stomach parasite of Arapaima gigas. (A) At the anterior end, we can observe the dorsal lip with two large papillae (cp), ventrolateral lip (vl) with amphid (am) and excretore pore (ep). Bar = 20 μm. (B) Apical view of the dorsal lip, winged, showing dentigerous crest (dr). Bar = 10 μm. (C) Conical tail (t) and anal aperture (an). Bar = 50 μm. (D) Vulvar region (va). Bar = 20 μm.
Figure 5. Long description
A four-panel S E M micrograph set labeled A through D.
Panel A shows the anterior end of the parasite. At the top center is the dorsal lip labeled d l. Below it are two ventrolateral lips labeled v l. The right ventrolateral lip features an amphid labeled a m and a large papilla labeled c p. Below the lips is the excretory pore labeled e p. A vertical scale bar represents 20 micrometers.
Panel B provides a high-magnification apical view of the dorsal lip. At the top right, a large papilla is labeled c p. In the center, the winged dorsal lip displays a serrated dentigerous crest labeled d r. A vertical scale bar represents 10 micrometers.
Panel C shows the posterior end of the parasite. The body tapers into a conical tail labeled t. Adjacent to the base of the tail is the anal aperture labeled a n. A vertical scale bar represents 50 micrometers.
Panel D shows a mid-section of the body with prominent horizontal cuticular ridges. In the center of this section is the vulvar region labeled v a, appearing as a small horizontal slit. A vertical scale bar represents 20 micrometers.
Discussion
Recently, the genus Terranova Leiper et Atkinson, Reference Leiper and Atkinson1914 was separated into two valid genera (Moravec and Justine, Reference Moravec and Justine2020). The genus Euterranova groups nematodes parasitic on sharks, and the genus Neoterranova groups parasites on elasmobranch fish, teleosts, crocodilians, and snakes. Morphologically, this separation is evidenced by the shape of the dentigerous crests. The specimens in this study presented a short row of central denticles on the inner edge of the lips and no prominent denticles, as observed by scanning electron microscopy. This characteristic is compatible with the new classification as Neoterranova serrata (Syn: Terranova serrata), reported as a parasite of the stomach of A. gigas (Araújo et al Reference Araújo, Gomes, Tavares-Dias, Andrade, Belém-Costa, Borges, Queiroz and Barbosa2009; Luque et al Reference Luque, Aguiar, Vieira, Gibson and Santos2011). Additionally, details of the cephalic papillae, excretory pore, amphids, cloacal papillae, spicule shapes, vulvar region, and L4 larval stage morphology of N. serrata are provided for the first time in this study.
Among the nematodes that parasitise A. gigas, the nematode Goezia spinulosa is observed to be the most pathogenic due to the tissue lesions caused by the nematode’s cuticular spines, causing extensive gastric ulcers that can compromise the organ’s functionality, especially in fry (Menezes et al Reference Menezes, Dos Santos, Ceccarelli, Tavares, Tortelly and Luque2011). In this study, in addition to the lesions caused by G. spinulosa, ulcerative lesions caused by N. serrata were also observed in the analysed material, although fewer in number and less extensive in the gastric mucosa. Although these lesions may be self-limiting and not compromise the life of the host, cases of parasitic hyperinfections and/or co-infections in altered environments, whether due to climatic factors or human disturbance, can facilitate the entry of secondary bacterial infections and cause disease with the consequence of weight loss and performance in the hosts (Lagrue and Poulin Reference Lagrue and Poulin2015; Sures Reference Sures2006).
Parasite co-infections are the simultaneous presence of two or more parasite species within a single host. These interactions depend on factors like host behaviour, parasite type, host immunity, nutrition, and a healthy environment (Vaumourin et al Reference Vaumourin, Vourc’h, Gasqui and Vayssier-Taussat2015). Generally, a parasite species occupies a specific organ and uses of the niche limits competition. In aquatic ecosystems, fish are more susceptible to co-infections due to the biodiversity of parasites that are found in present in the environment and the presence of multiple factors associated with parasitism. According to observations made by Telfer et al (Reference Telfer, Lambin, Birtles, Beldomenico, Burthe, Paterson and Begon2010) and Rodgers and Bolnick (Reference Rodgers and Bolnick2024) indicate that co-infection by more than one parasite species is favoured by the presence of the first parasite, which increases susceptibility to being infected by a second species in the same location (Kotob et al Reference Kotob, Menanteau-Ledouble, Kumar, Abdelzaher and ElMatbouli2017; Souza et al Reference Souza, Santos and Chagas2019). This hypothesis requires further studies to understand the phenomenon of parasitic co-infection, especially in freshwater ecosystems.
Environmental changes, such as mercury and heavy metal pollution reported in the Juruá river basin, a tributary of the Amazon river (Costa et al Reference Costa, Rego, Brabo, Santos, Jesus, Almeida and Martins2006), and periods of increased water temperature and extreme drought in the Amazon reported in 2023 in the study region (Santos et al Reference Santos, Papa, Hamilton, Melack, Forsberg, Val, Collischonn, Laipelt, Brusso Rossi, Comini de Andrade, Mendel, Alves, Bandeira, Custódio, Gomes, Hymans, Keppe, Mendes, Nascimento, Dos Santos Silva, Vieira, Xavier, Zumak, Ruhoff, Zhou, MacIntyre, Martins, Filizola, Marinho, Severo, Frias, Alquezar, Lauretto, Gravena, Coelho, Chávez-Pérez, Braz-Mota, Chamy, Medeiros Moreira, Guedes Santos, Pacheco Peleja and Marmontel2025), could alter parasitic dynamics, decreasing the richness of parasites or increasing the parasitic load of more pathogenic species and therefore have negative consequences for the health of hosts (Gautam et al Reference Gautam, Misra and Saxena2018; Lizama et al Reference Lizama, Takemoto and Pavanelli2006). Future studies monitoring the parasitism of the species studied here will be necessary to determine the effects on the weight gain and growth of A. gigas in natural habitats, particularly in areas of management and conservation of this species.
Acknowledgements
Thank the Instituto de Desenvolvimento Sustentável Mamirauá and Centro Multiusuário para Análise de Fenómenos Biomédicos (UEA-Manaus) for support in logistics and technical support. Thanks to the National Council for Scientific and Technological Development – CNPq for the research grants (PIBIC Junior to Fabio Batalha da Silva and CMABio FAPERJ for the postdoctoral grant to Makoto E. Caracciolo, under number 200.786-2026).
Author contribution
DFC, FBS, MEC, WAP, and GCO performed the sampling process, morphological and morphometric analyses, and wrote most parts of the manuscript. DFC, MAE, and LM designed the study, organised the fieldwork, and assisted in analysing and interpreting histopathological results.
Competing interests
The authors declare that they have no competing interests.
