| Parasite | Ergasilus zacconis |
|---|---|
| Taxonomy | Arthropoda, Crustacea, Copopoda, Poecilostomatoida |
| Hosts | Ayu (Plecoglossus altivelis), freshwater minnow (Zacco platypus) |
| Infection site | Gill |
| Clinical signs | Ergasilus zacconis with egg sacs is observed in the gill (Fig. 1). The gill of heavily infected fish is anaemic. |
| Parasitology | Ergasilus zacconis is an ectoparasitic crustacean. It is composed of cephalosome, metasome, genital somite, 2 abdominal somites, anal somite and caudal ramus (Kim and Nagasawa, 2006). Only females (0.7-1.7 mm) are visually observed (Fig. 2). A nauplius larva, which is hatched from eggs, becomes a copepodite larva, which attaches to hostfs gill filament by a pair of craw-shaped antennae and feeds on the proliferated gill tissues. This parasite, which had been originally named as Pseudergasilus zaconnis, was transferred to the genus Ergasilus (Kim and Choi, 2003) and redescribed by Kim and Nagasawa (2006) (Nagasawa et al., 2007). |
| Pathology | Pathological changes (haemorrhage, congestion, proliferation of the epithelia, aedema, mucus secretion, etc.) are observed in the gill (Nakajima and Egusa, 1974). Heavily infected fish exhibit growth retardation resulting from anorexia. |
| Health hazard | Since this parasite is not infectious to human, it is harmless in food hygiene. |
| Diagnosis | Check the morphology of an adult female of the parasite. Ergasilus zacconis can be differentiated from E. parasiluri, a parasite of catfish, by the size at various points of the body (Nakajima and Egusa, 1973). Ergasilid copepods in Japan can be classified according to the keys given by Nagasawa et al. (2007). |
| Other information | In 1970s, outbreaks of this disease occurred in seedlings of the ayu introduced from the Lake Biwa. |
| References | Kim, I.-H. and S.-K. Choi (2003): Copepod parasites (Crustacea) of freshwater
fishes in Korea. Korean J. Syst. Zool., 19, 57-93. Kim, I.-H. and K. Nagasawa (2006) Redescription of Ergasilus zacconis (Copepoda: Poecilostomatoida: Ergasilidae) parasitic on the freshwater fish Zacco platypus from Japan. Korean J. Syst. Zool., 22, 121-125. Nakajima, K. and S. Egusa (1973): Parasitic copepode, Pseudergasilus zacconis YAMAGUTI, found on the gills of cultured ayu, Plecoglossus altivelis ? I. Morphology. Fish Pathol., 8, 106-110. (In Japanese) Nakajima, K. and S. Egusa (1973): Parasitic copepode, Pseudergasilus zacconis YAMAGUTI, found on the gills of cultured ayu, Plecoglossus altivelis ? II. Histological damage and bathing effects of some chemicals. Fish Pathol., 9, 95-99. (In Japanese) Nagasawa, K., T. Umino, D. Uyeno and S. Ohtsuka (2007): A checklist of ergasilid copepods (Crustacea) occurring as fish parasites or plankton in Japan (1895-2007). Bull. Biogeogr. Soc. Japan, 62, in press. (In Japanese) |
Fig. 2. Female adlt of E. zacconis
Fig. 1. Ergasilus hanging the gill filament.
(Photos by S. Egusa)