Introduction

Cymothoid isopods (Crustacea: Isopoda: Cymothoidae) are parasitic in the skin, oral cavity, branchial cavity, or body cavity of fish (Brusca 1981). About 330 species of cymothoid isopods are recognized world wide (Trilles 1994). Most occur in the marine environment, but a few freshwater species are known. In Russia, two cymothoid species, Ichthyoxenus amurensis and Mothocya taurica, have been recorded in land water (Bauer 1987). During an anatomical survey on the Amur bitterling, Rhodeus sericeus, cymothoid isopods were found in the body cavity of fish collected in Primorsky, Russia. Rhodeus sericeus is a freshwater fish belonging to cyprinid subfamily Acheilognathinae, a typical group that uses the gills of freshwater unionid mussels as a spawning substratum. Rhodeus sericeus is distributed in the Amur River drainage from its headwaters to its estuary, where it debouches into Peter-the-Great Bay (the Sea of Japan) and the Sea of Okhotsk up to Uda River and Sakhalin Island (Bogutskaya et al. 2008). This paper presents the first report on the infection of R. sericeus by cymothoid isopods.

Materials and methods

All specimens of R. sericeus were collected from the Spassovka River at Zelenovka, in the Lake Khanka Basin of the Amur River drainage, 44°31′27″N, 133°00′99″E (Fig. 1) on 19 August 2003 by Dr. Alexander M. Naseka (Russian Academy of Sciences), and the specimens were offered to Kinki University. The river width at the collecting site was <2 m. The river was partitioned off an artificial dam, and the small pond was formed. The pond was lentic water, the bottom of which was mud. Rhodeus sericeus at this site are abundant and occur with other fish species, including Soldatov’s gudgeon Gobio soldatovi, the topmouth gudgeon Pseudorasbora parva, the Khanka spiny bitterling Acheilognathus chankaensis, and Nikolski’s loach Misgurnus nikolskyi. Among them, R. sericeus were caught for observation by using nets or electroshockers and fixed in a 10% formalin solution.

Fig. 1
figure 1

Map showing the collection site of Rhodeus sericeus (solid circle) in Russia

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In the laboratory, 29 specimens of R. sericeus were dissected from the lateral side of the body and examined for parasites. The presence of an isopod in the body cavity was noted. If the isopod was found, the appearance of infection was observed and recorded under a microscope. The isopod was removed from the body cavity and its body length was measured (the anterior margin of the cephalon to the apex of the pleotelson). Identification of the isopod follows Kussakin (1979) and Bruce (1990). Illustrations were adapted from sketches obtained via a camera lucida. In addition, the standard length of infected and uninfected fish was measured and compared to examine the effect of infection. All specimens examined are deposited at the Faculty of Agriculture, Kinki University, Nara.

Results

The cymothoid isopods observed in this study (Fig. 2) were all identified as genus Ichthyoxenus based on the following characteristics: (1) body symmetrical, vaulted, strongly ovate, nearly circular in dorsal view; (2) cephalon not deeply immersed in pereonite 1; (3) pleon narrow (<0.5 times as wide as pereon), pleonite 1 partly overlapped by pereonite 7; (4) antenna 2 shorter than antenna 1, bases set apart; and (5) pereopods with short, rounded coxa, ischium to carpus flattened, widest distally, and dactylus short, flattened (Bruce 1990). The majority of members of Ichthyoxenus are distributed in China and the far East, and the taxonomic status of many species is open to question (Bruce 1990). In this article, we describe this species as I. amurensis because the structures of mouthparts and pereopods are quite similar to those of I. amurensis in Kussakin (1979), and the cymothoid isopod species have never been reported from the Amur River Basin except for I. amurensis.

Fig. 2
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Female Ichthyoxenus amurensis, habitus, dorsal view. Scale bar 5 mm

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A total of 13 specimens (four females and nine males) of cymothoid isopods were found in nine of 29 R. sericeus specimens examined (prevalence = 31.0%). Among the nine infected fish, four contained a pair (male and female) of isopods whereas five hosted a single parasite. The body size of male isopods [mean ± standard deviation (SD) 8.5 ± 1.9 mm] was much smaller than that of females (11.3 ± 1.3 mm). The infected fish were easily distinguished by the presence of an external orifice near the base of the pectoral fin (Fig. 3a). The orifice ranged from 2.0 to 3.5 mm in diameter. A pair of I. amurensis was orientated with the posterior of their bodies toward the orifice, and some females had their posterior region jutting out of the orifice. Ichthyoxenus amurensis was found within a thin-walled membranous sac in the body cavity of R. sericeus (Fig. 3b). When a male and female I. amurensis cohabited in the body cavity of R. sericeus, the female was positioned in the lower anterior of the body cavity with its head pointed in the direction of the posterior of the host fish, whereas the male was found near the heart of the fish with its head in the direction of the anterior region of the host.

Fig. 3
figure 3

Preserved specimens of Rhodeus sericeus infected (a–c) and uninfected (d) by Ichthyoxenus amurensis. Scale bars 5 mm. An orifice near the posteroventral region of the pectoral fin of R. sericeus (a). Female I. amurensis in the body cavity of R. sericeus (b). Intestine of an uninfected R. sericeus (c). Intestine of an infected R. sericeus (d). Ichthyoxenus amurensis was removed (ab air bladder; ia Ichthyoxenus amurensis; in intestine, li liver, or orifice)

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In this study, the body size of infected and uninfected fish was compared. The infected fish ranged from 46.1 to 60.4 mm standard length (SL) (mean ± SD 50.8 ± 4.5 mm) and the uninfected ranged from 43.3 to 56.3 mm SL (49.2 ± 2.8 mm). The difference in body size between the two groups was not significant (t test, p < 0.05), although the intestine of infected fish was malformed. In general, the intestinal coiling pattern of uninfected R. sericeus appeared as a loop that appeared oval (Fig. 3c). In infected fish, I. amurensis were residing in the anteroinferior region of the body cavity. Hence, the intestine was compressed to the dorsal side of the body cavity and noticeably misshapen (Fig. 3d).

Discussion

Ichthyoxenus amurensis is distributed in the Amur River Basin and two rivers in Premorye, Russia, and is parasitic in the body cavity of freshwater fish (Verigin and Sisoeva 1952; Bauer 1987). The previously recognized hosts of I. amurensis are seven cyprinid species (Trilles 1994): Cyprinus carpio, Hypophthalmichthys molitrix, Elopichthys bambusa, Leuciscus waleckii, Parabramis pekinensis, Carassius gibelio, and Ctenopharyngodon idella. Hence, R. sericeus represents a new host for I. amurensis. In this study, 31.0% of R. sericeus examined were infected. The appearance of infection by I. amurensis matched well the description by Achmerov (1941), who observed I. amurensis in 13 of 50 specimens of L. waleckii collected from the Amur River, a prevalence of 26.0%. Achmerov (1941) and Krykhtin (1951) reported that I. amurensis caused deleterious effects on their hosts, such as decreases in mean weight and growth because they feed on the blood of the host fish. The latter reported that 1397 of 10759 individuals (13%) of edible L. waleckii die before maturing as a result of infection. In this study, there was no significant difference in SL between the infected and uninfected fish. However, the intestines of infected fish were malformed as a result of infection by I. amurensis. Ichthyoxenus amurensis may exert a negative influence on the visceral function of its host.