Redescription of Rhabdochona anguillae (Nematoda: Rhabdochonidae), a parasite of eel, Anguilla anguilla, in Europe

FOLIA PARASITOLOGICA 45: 233-238, 1998 Saraiva and Moravec: Redescription of Rhabdochona anguillae Redescription of Rhabdochona anguillae (Nematoda: Rhabdochonidae), a parasite of eel, Anguilla anguilla, in Europe Aurélia M. Saraiva 1 and František Moravec 2 1 Departamento de Zoologia, Faculdade de Ciências, Universidade do Porto, 4050 Porto, Portugal; 2 Institute of Parasitology, Academy of Sciences of the Czech Republic, Branišovská 31, 370 05 České Budějovice, Czech Republic Key words: Rhabdochona anguillae, Nematoda, Anguilla anguilla, Portugal Abstract. The nematode Rhabdochona anguillae Spaul, 1927, a specific intestinal parasite of the European eel, Anguilla anguilla (L.), is redescribed and illustrated from specimens collected from eels of the Sousa River, northern Portugal (prevalence 20%, intensity 2-13). The species is characterized by the presence of 14 anterior teeth in the prostom, small non-bifurcate deirids, absence of lateral preanal papillae, by the length (0.460-0.660 and 0.130-0.150 mm) and the shape of spicules, fairly large (0.041-0.054 0.025-0.030 mm) mature eggs without filaments, and by the bluntly pointed to rounded tip of the tail. Its morphological features are discussed in relation to other congeneric species. This nematode has hitherto been recorded only from eels in southern Europe (Portugal, Spain, Bulgaria). The nematode genus Rhabdochona Railliet, 1916 includes a large number of species that are intestinal parasites of freshwater fishes in Europe, Asia, Africa and North- and South America. Of the nine species of this genus occurring in Europe (Moravec 1994), Rhabdochona anguillae Spaul, 1927, a specific parasite of the European eel, Anguilla anguilla (L.), has remained insufficiently known. Originally it was inadequately described by Spaul (1927), from eels in Spain, on the basis of one male and four females which had been sent to the then British Museum (Natural History) by Dr. Hornyold. The type specimens of R. anguillae, deposited in the Natural History Museum in London, were re-examined by Moravec (1975), but because of their poor condition, many taxonomically important features (number of anterior teeth in the prostom, character of deirids, eggs, etc.) could not be established. This species was reported also by Margaritov (1968) from the Belishka River, a tributary of the Mesta River (Aegean Sea basin), in Bulgaria and by Carvalho-Varela et al. (1981, 1984) from Portugal. Although the latter authors did not give any data on the morphology of R. anguillae, Margaritov (1968) provided a few measurements of one male and one female at his disposal. In 1996, during a parasitological study of the European eel, Anguilla anguilla, from the Sousa River, a tributary of the Douro River, northern Portugal, several specimens of R. anguillae were collected from this fish. A study of these nematodes enabled us to make a detailed redescription of this species, which is the subject of the present paper. MATERIALS AND METHODS The eels, Anguilla anguilla (L.) (body length 10.6-63.8 cm), were collected in May 1996, by electrofishing, from the Sousa River, a tributary of the Douro River, from the locality Senhora do Salto situated about 20 km east of Porto, northern Portugal. The nematodes recovered from infected fish were washed in physiological saline (0.9%) and then they were fixed in hot 70% ethanol and stored again in 70% ethanol. For light microscopical study, the specimens were cleared in glycerol. En face study was carried out according to Anderson s (1958) method utilizing the hanging drop of glycerol-gelatine. Drawings were made with a microscope drawing attachment. Measurements are based on 8 male and 8 female specimens; all measures are in millimetres. For examination by scanning electron microscopy (SEM), the nematodes were postfixed in 1% OsO 4, dehydrated through an ethanol and an acetone series and then subjected to critical point drying. The specimens were coated with gold and examined with a JSM-6300 scanning electron microscope at an accelerating voltage of 15 kv. A few nematode specimens have been deposited in the helminthological collection of the Institute of Parasitology, Academy of Sciences of the Czech republic, České Budějovice, Czech Republic (Cat. No. N - 689). RESULTS Of a total of 25 Anguilla anguilla examined, 5 (20 %) proved to be infected with Rhabdochona anguillae, with the intensity 2-13 (mean 7.8) nematodes per fish. Although the nematodes occurred throughout the host s intestine, they evidently preferred its middle third. Most nematodes were mature, including gravid females with Address for correspondence: A.M. Saraiva, Departamento de Zoología, Faculdade de Ciências, Universidade do Porto, Plaza Gomes Teixeira, 4050 Porto, Portugal. Phone: ++351 2 2050290; Fax: ++351 2 2004777. 233

Fig. 1. Rhabdochona anguillae Spaul, 1927. A - anterior end of body; B,C,D - cephalic end, lateral, dorsoventral and apical views; E - small spicule; F - distal tip of large spicule; G,H - caudal end of male, lateral and ventral views; I - female tail; J,K - tail tip of male; L - mature egg; M,N - tail tip of female. 234

Saraiva and Moravec: Redescription of Rhabdochona anguillae Fig. 2. Rhabdochona anguillae Spaul, 1927, light microscopical micrographs. A - anterior end, lateral view (scale bar = 25 µm); B - posterior end of male, lateral view (scale bar = 50 µm). fully mature (larvated) eggs in uteri. This shows that the parasite s oviposition takes place in May in this locality. Other endohelminths recorded from A. anguilla in this locality were the nematodes Spinitectus inermis (Zeder, 1800) (prevalence 16%, intensity 2-23) and Paraquimperia tenerrima (Linstow, 1878) (24%, 1-2), and the trematode Nicolla sp. (5%, 5). Redescription of Rhabdochona anguillae Spaul, 1927 Figs. 1-3 Medium sized nematodes with finely transversely striated cuticle (Fig. 3C,D). Lateral alae absent. Mouth roughly hexagonal (Figs. 1D, 3B). Two lateral amphids and four small, submedian cephalic papillae present. Prostom funnel-shaped, thick-walled, without basal teeth (Figs. 1B,C, 2A); anterior teeth 14 in number, 3 dorsal, 3 ventral and 4 lateral (arranged in two pairs) on either side (Fig. 1D); exceptionally teeth of one lateral pair fused together (Fig. 3B). Deirids small, nonbifurcated (Fig. 3C), situated slightly posterior to posterior end of prostom (Fig. 1B,C). Excretory pore (Fig. 3D) at level of posterior part of muscular oesophagus. Tail of both sexes conical, slender, with bluntly rounded tip (Figs. 1J,K,M,N). Male (8 specimens): Length of body 9.81-12.86, maximum width 0.218-0.245. Prostom 0.030-0.041 long and 0.021-0.026 wide in lateral view. Vestibule (stoma) length including prostom 0.120-0.174. Length of muscular oesophagus 0.330-0.420, of glandular oesophagus 2.00-2.28. Nerve ring, excretory pore and deirids 0.198-0.271, 0.318-0.360 and 0.054-0.057, respectively, from anterior extremity. Preanal papillae: 8, sometimes 9, subventral pairs present, of which second pair (counting from cloacal opening) formed by slightly smaller and more laterally situated papillae. Of 6 postanal pairs of papillae, second pair situated more laterally, remaining pairs subventral. Precloacal ventral cuticular ornamentations (area rugosa) absent. Larger (left) spicule well sclerotized (Fig. 2B), 0.460-0.660 long; length of its shaft 0.280-0.369, representing 5660% of whole spicule length; distal tip moderately widened, lanceolate. Smaller (right) spicule 0.130-0.150 long, without distinct dorsal barb at distal tip. Length ratio of spicules 1 : 4.3-4.4. Length of tail 0.250-0.326. Female (8 specimens): Length of body of gravid specimens with mature eggs 15.80-20.85, maximum width 0.313-0.370. Prostom 0.032-0.046 long and 0.024-0.028 wide in lateral view. Vestibule length including prostom 0.099-0.187. Length of muscular oesophagus 0.300-0.474, of glandular oesophagus 2.182.54. Nerve ring and excretory pore 0.183-0.281 and 0.381-0.454, respectively, from anterior extremity; 235

Fig. 3. Rhabdochona anguillae Spaul, 1927, SEM micrographs. A - cephalic end, subapical view; B - cephalic end, apical view (specimen with one lateral pair of teeth fused together); C - deirid; D - excretory pore. deirids not seen. Tail conical, slender, 0.281-0.490 long. Vulva postequatorial, 6.80-7.34 from posterior end of body. Muscular vagina directed posteriorly from vulva. Eggs oval, size 0.041-0.054 x 0.025-0.030, with smooth surface; thickness of egg wall 0.006. DISCUSSION The present study shows some morphological features characteristic of Rhabdochona anguillae, which were not previously reported but which are very important for distinguishing this species from other congeners. For the first time are described the deirids and the number and arrangement of anterior teeth in the prostom; while the latter proved to be identical with those found in most European species of Rhabdochona, the deirids are very different (non-bifurcated). R. anguillae is the only European species in which the small (right) spicule is without a dorsal barb; this barb is moderately developed and rarely may be even absent in 236 R. phoxini Moravec, 1968, but this species differs from R. anguillae in the sharply pointed tip of the tail, in addition to other distinguishing features (e.g. length of the left spicule, shape of the male tail). Moravec (1994) used the length of the larger (left) spicule (longer than 0.6 mm) as the diagnostic feature of R. anguillae in the key to species of Rhabdochona from European fishes. This was based on data on the spicule length given by Spaul (1927) (0.654 mm) and Margaritov (1968) (0.604 mm). However, the present study shows that there is a certain intraspecific variability in the length of this spicule, this being within the limits 0.460-0.660 mm. The distal tip of this spicule and the shape of the prostom were found to be similar to those described and illustrated by Spaul (1927). Of the European species of Rhabdochona with smooth eggs, only R. oncorhynchi (Fujita, 1921), a parasite of salmonids, resembles R. anguillae in the length of the left spicule (0.426-0.630 mm); however, the former

Saraiva and Moravec: Redescription of Rhabdochona anguillae differs in having the small spicule with a well developed dorsal barb, its prostom is with distinct basal teeth, the female tail is broad and the distal tip of the left spicule is provided with a ventral tooth-like process (Moravec 1994). Spaul (1927) reported eight pairs of preanal papillae in R. anguillae, but Moravec (1975) found nine papillae on the right side and eight papillae on the left side in the same type specimen. Margaritov (1968) reported only six pairs of preanals in the single available male from Bulgaria. The males in the present material had eight to nine pairs of preanal papillae, indicating a certain intraspecific variability in number of preanal papillae in R. anguillae. Such variability is known also in other congeneric species (Moravec 1972a). Surprisingly, no lateral preanal papillae were found in R. anguillae, in which it differs from all congeneric species occurring in Europe (Moravec 1994) as well as from most other adequately described species. Usually one to five pairs of lateral preanal papillae are present in different Rhabdochona species (Moravec 1972a). The number (6 pairs) and arrangement of postanal papillae in R. anguillae are the same as in the majority of Rhabdochona species in which, however, the second pair is usually situated more laterally. A characteristic feature of R. anguillae already mentioned by Spaul (1927) is the elongated conical tail with a bluntly pointed to rounded tip. By this and some other morphological features, R. anguillae shows affinities to some species occurring in Asia and Europe (R. ergensi Moravec, 1968, R. glyptothoracis Karve et Naik, 1951, R. japonica Moravec, 1975, R. longicauda Dzhalilov, 1964, R. similis Moravec, Ali et Abul-Eis, 1991) and North America (R. cotti Gustafson, 1949); all the named species possess eggs provided with polar filaments and, therefore, Moravec (1975, 1994) assumed that the eggs of R. anguillae might be filamented too. But the present study shows, for the first time, that the filaments are lacking in the mature eggs of this species. Spaul (1927) described the eggs to be oval, clear, smooth and thin-walled, size 39 x 25 µ ; having reexamined two of the four type females, Moravec (1975) found only immature eggs in them. Since Spaul (1927) mentioned the variations in the measurements in the four available females to be small and because, in contrast to his data, we found the eggs of R. anguillae to be usually larger and rather thick-walled, it is highly probable that Spaul (1927) observed only immature eggs. Margaritov (1968) did not give any data concerning the eggs of this species. The present study shows that the mature eggs of R. anguillae are usually larger than in other congeneric species. The results of this study confirm that R. anguillae is a valid, well defined species, which can easily be differentiated from other congeners by morphological features. The hitherto records of this species and its frequent occurrence in eels of the Sousa River in Portugal indicate that the European eel, Anguilla anguilla, is the true definitive host of this nematode. In contrast to R. anguillae, some other Rhabdochona species can probably be acquired by eels while feeding on forage fishes serving as their definitive hosts. For example, Moravec (1972b, 1974) recorded Rhabdochona congolensis Campana-Rouget, 1961, principally a parasite of African siluroids and characids, from A. anguilla in Egypt. In such cases, eels are not definitive but only postcyclic hosts (Moravec 1994). In the Portugese locality in question (Sousa R.), the only other species of Rhabdochona is R. hellichi (Šrámek, 1901), a parasite of barbel, Barbus bocagei, with filamented eggs. It is worth mentioning that although the values of prevalence and intensity of R. anguillae and Paraquimperia tenerrima were relatively high (20%, 2-13 and 24%, 1-2, respectively) in the locality studied, we never detected any eels infected with both nematode species. It is not clear whether this phenomenon can be ascribed to an interspecific competition or is a result of the host s individual food preferences. The records of R. anguillae so far indicate that this parasite is distributed only in southern Europe (Portugal, Spain, Bulgaria). Hitherto nothing is known about its biology and its possible pathogenicity to its fish host, the eel. REFERENCES ANDERSON R.C. 1958: Méthode pour l examen des nématodes en vue apicale. Ann. Parasitol. Hum. Comp. 33: 171-172. CARVALHO-VARELA M., CUNHA FERREIRA V., CRUZ E SILVA M.P., GRAZINA-FREITAS M.S. 1984: Sobre a parasitofauna da enguia europeia (Anguilla anguilla (L.)) em Portugal. Rep. Trab. L.N.I.V. 16: 143-150. CARVALHO-VARELA M., CUNHA FERREIRA V., CRUZ E SILVA M.P., MONTEIRO M.T., GRAZINA-FREITAS M.S. 1981: Parasites and parasitoses in fish culture in Portugal. J. World Maricult. Soc. 12: 9-14. MARGARITOV N. 1968: On the distribution of Rhabdochona anguillae Spaul, 1927 (Nematoda, Spirurata). Godishnik na Sofiysk. univ., Biol. fak., 1965/1966, 60: 103-105. (In Bulgarian, German summary.) MORAVEC F. 1972a: General characterization of the nematode genus Rhabdochona with a revision of the South American species. Acta Soc. Zool. Bohemoslov. 36: 29-46. MORAVEC F. 1972b: A revision of African species of the nematode genus Rhabdochona Railliet, 1916. Acta Soc. Zool. Bohemoslov. 36: 196-208. MORAVEC F. 1974: On some nematodes from Egyptian freshwater fishes. Acta Soc. Zool. Bohemoslov. 38: 32-51. 237

MORAVEC F. 1975: Reconstruction of the Nematode Genus Rhabdochona Railliet, 1916 with a Review of the Species Parasitic in Fishes of Europe and Asia. Studie ČSAV, No. 8, 104 pp. MORAVEC F. 1994: Parasitic Nematodes of Freshwater Fishes of Europe. Academia and Kluwer Acad. Publishers,Praha and Dordrecht, Boston, London, 473 pp. SPAUL E.A. 1927: On a new species of the nematode genus Rhabdochona. Ann. Mag. Nat. Hist., Ser. 9, 19: 636-641. Received 3 July 1997 Accepted 8 September 1997 FIFTH INTERNATIONAL SYMPOSIUM ON FISH PARASITES 9-13 AUGUST 1999 ČESKÉ BUDĚJOVICE, CZECH REPUBLIC Organized by the Institute of Parasitology, Academy of Sciences of the Czech Republic Contact: Dr. František Moravec or Dr. Jiří Lom Institute of Parasitology Academy of Sciences of the Czech Republic Branišovská 31, 370 05 České Budějovice CZECH REPUBLIC Phone: ++420 38 7775432; Fax: ++420 38 47743 E-mail: moravec@paru.cas.cz, or lom@paru.cas.cz http://www.paru.cas.cz/sympos.htm 238