Rhabdochona angusticaudata Moravec & Nagasawa 2018, sp. n.

Rhabdochona angusticaudata sp. n. Figs. 8–10

ZooBank number for species:

urn:lsid:zoobank.org:act: 32A17EE6-4EFE-4416-BF90-A34F066B94CE

Description. Medium-sized nematodes with finely transversely striated cuticle Fig. 10A,B,F). Lateral alae absent. Mouth roughly hexagonal. Four small submedian cephalic papillae, 2 lateral amphids and 4 submedian sublabia present (Figs. 8E, 9A–E). Prostom funnel-shaped, without basal teeth (Fig. 8B,D); anterior teeth 14–16 in number, 3–4 dorsal, 3–4 ventral and 4 lateral (arranged in 2 pairs) on either side (Figs. 8B–E, 9A–E, 10A). Deirids small, non-bifurcated, with rounded ends, situated slightly asymmetrically approximately at mid-length of vestibule (Figs. 8A–C,F, 9F, 10B). Excretory pore at level of posterior part of muscular oesophagus (Figs. 8A,C, 10A). Tail of both sexes conical, slender, with sharply pointed tip.

Male (2 complete and 1 incomplete specimens; holotype; measurements of paratypes in parentheses): Length of body 8.42 mm (13.6 mm in complete specimen), maximum width 78 (81). Prostom 21 (18–21) long and 15 (15) wide in lateral view. Length of vestibule including prostom 126 (117–126). Muscular oesophagus 231 (204–228) long, 24 (24–27) wide, length of glandular oesophagus 1.18 (1.35–1.40) mm, width 45 (48–60). Nerve ring, excretory pore and deirids 207 (177–207), 297 (270–312) and 80 (63–66), respectively, from anterior extremity. Preanal papillae: 9 subventral and 1 lateral pairs present; lateral pair situated at level of third subventral pair (counting from cloacal opening). Of 6 postanal pairs of papillae, second pair situated more laterally, remaining pairs subventral; lateral pair just posterior to first subventral pair (Figs. 8L, 10C). Minute phasmids situated laterally short distance posterior to level of last pair of postanal papillae (Figs. 8L, 10D). Precloacal ventral cuticular ornamentations (area rugosa) absent. Large (left) spicule well sclerotised, 384 (300) long; length of its shaft 183 (144), representing 48% (48%) of whole spicule length; distal tip moderately widened, lanceolate (Fig. 8G,H,L). Small (right) spicule boat-shaped, 87 (81) long, with distinct dorsal barb at distal tip (Fig. 8K, L). Length ratio of spicules 1: 4.4 (1: 3.7). Length of tail 300 (255).

Female (1 gravid specimen with mature eggs, allotype): Length of body 19.1 mm, maximum width 165. Prostom 24 long and 18 wide. Length of vestibule including prostom 123. Muscular oesophagus 306 long, 27 wide, length of glandular oesophagus 1.63 mm, width 63. Nerve ring, excretory pore and deirids 219, 300 and 90, respectively, from anterior extremity. Vulva equatorial, 9.63 mm from anterior extremity (at 50% of body length) (Fig. 10F); vagina muscular, 66 long, directed posteriorly from vulva. Uterus filled with many eggs. Fully developed eggs larvated, oval, thick-walled, size 39–42 × 21–24, with smooth surface, without filaments; thickness of egg wall 4 (Fig. 8I). Tail conical, slender, 333 long, ending in sharp cuticular spike (Figs. 8J,M, 10E).

Type host: Japanese eel, Anguilla japonica (Anguillidae, Anguilliformes).

Site of infection: Intestine.

Type locality: Renjoji River (brackish-water area and freshwater middle-reaches), Ainan, Ehime Prefecture, Shikoku, Japan (collected 16 and 25 June 2007).

Total prevalence and intensity: 19% (3 fish infected/16 fish examined); 9 nematodes. Brackish-water area near the river mouth: 2 fish infected, 46.5 and 49.4 cm TL/5 fish examined, 32.0– 49.4 cm TL; 1 and 4 nematodes. Freshwater middle-reaches of the river: 9% (1 fish infected, 47.5 cm TL/11 fish examined, 35.2–53.2 cm TL); 4 nematodes.

Deposition of type specimens: IPCAS N-1157 (mounted on SEM stubs).

Etymology: The specific name angusticaudata (= narrow-tailed) is the Latin adjective, relating to the characteristic feature of this species, i.e., a markedly narrow female tail.

Remarks. At present the genus Rhabdochona Railliet, 1916 comprises about 100 species parasitising freshwater fishes in all zoogeographical regions (Moravec 2010). The general morphology of R. angusticaudata sp. n., especially the number of prostomal teeth and no ornamentations at the tip of the tail, indicates that this species belongs to the nominotypic subgenus Rhabdochona, as defined by Moravec (1975).

Most species of this subgenus possess bifurcate deirids and only a few of them, e.g. Rhabdochona acuminata (Molin, 1860), Rhabdochona marcusenii Moravec et Jirkû, 2014 or Rhabdochona fabianae Ramallo, 2005, have simple, rod-like deirids (Cremonte et al. 2002, Moravec and Jirkû 2014, Ramallo 2005) or the deirids are hammer-shaped (Rhabdochona papuanensis Moravec, ŘÍha et Kuchta, 2008) or oval, leaf-like (Rhabdochona pseudomysti Moravec et Yooyen, 2011) (see Moravec et al. 2008, Moravec and Yooyen 2011). However, the study of the exact shape of deirids in Rhabdochona usually requires SEM examination, but this method has not yet been used for many species.

The non-bifurcated deirids with rounded distal tips, the character of anterior prostomal teeth, absence of basal teeth, a slender female tail and non-filamented eggs of R. angusticaudata sp. n. are also found in two species of Rhabdochona that are specific parasites of freshwater eels: Rhabdochona anguillae Spaul, 1927 infecting A. anguilla in Europe and Rhabdochona keralaensis Moravec, Sheeba et Kumar, 2012 in Anguilla bengalensis (Gray) in India (Spaul 1927, Moravec et al. 2012b). Of these, R. keralaensis markedly differs from the new species in the size of body (body length of male and female 12.5–17.1 and 22.3–27.8 mm, respectively, vs 8.4–13.6 mm and 19.1 mm), length of the left spicule (585–636 µm vs 300–384 µm), more numerous pairs of subventral preanal papillae (11–13 vs 9), presence (vs absence) of the bifurcated cuticular membrane forming a distinct dorsal outgrowth on the distal tip of the left spicule and a somewhat different arrangement of anterior prostomal teeth.

Rhabdochona anguillae, as redescribed by Moravec (1975) and Saraiva and Moravec (1998), differs from the new species mainly in the lengths of spicules (left and right spicule 460–660 and 130–150 µm long, respectively, vs 300–384 and 81–87µm), in the relative length of shaft of the left spicule representing 56–60% (vs 48%) of the total spicule length, the dorsal barb on the right spicule is indistinct (vs right spicule with a conspicuous dorsal barb) and its fully developed eggs are larger (41–54 × 25–30 µm vs 39–42 ×21–24 µm).

An inadequately described species, Rhabdochona minjiangensis Wang, 1976, was reported from Anguilla japonica in China (see PRLFTU 1976, Peng et al. 2011). Although it parasitises the same host species as R. angusticaudata sp. n., it can be easily distinguished from it by the length of the left spicule (640–700 µm vs 300–384 µm), the absence (vs presence) of a dorsal barb on the right spicule and by more numerous (11 vs 9) pairs of subventral preanal papillae. In these features, R. minjiangensis is more similar to R. anguillae and R. keralaensis. Unfortunately, some taxonomically important morphological features, such as the shape of deirids or the detailed structure of the left spicule, were not provided in the description of R. minjiangensis. Moreover, the number of reported anterior prostomal teeth (4 pairs) is probably misleading, because these were not studied in apical view, and lateral preanal and postanal papillae were apparently overlooked. Therefore, a redescription of this species based on LM and SEM examinations is needed.

To date, six valid species of Rhabdochona have been reported from fishes in Japan: Rhabdochona coronacauda Belous, 1965, Rhabdochona denudata honshuensis Moravec et Nagasawa, 1989, Rhabdochona japonica Moravec, 1975, Rhabdochona oncorhynchi (Fujita, 1921), Rhabdochona tridentigeris Yamaguti, 1941 and Rhabdochona zacconis Yamaguti, 1935 (see Nagasawa 2017). Consequently, R. angusticaudata sp. n. is the seventh species of this genus known to occur in Japan. By the general morphology including the length of the left spicule, the new species resembles R. denudata honshuensis, a parasite of cyprinids, but these species can be easily distinguished from each other by the shape of deirids, which are simple in R. angusticaudata sp. n. and bifurcate in R. denudata honshuensis.

In this study, several individuals of A. japonica from the brackish-water area near the mouth of the Renjoji River were found to be infected by R. angusticaudata sp. n. (see “Total prevalence and intensity”), which, however, does not indicate that the nematode is a brackish-water parasite. Like other members of the genus, R. angusticaudata sp. n. is a freshwater parasite and considered to infect A. japonica in the freshwater environment (e.g. middle-reaches of rivers). Some infected eels may have moved downstream to the brackish-water region.