The biology of a landlocked form of the normally catadromous salmoniform fish Galaxias maculatus (Jenyns). IV. Effects of cestode and nematode parasites

Abstract

Two helminth parasites, a cestode and a nematode, were found to infect the landlocked form of Galaxias maculatus inhabiting Lake Modewarre, a slightly saline lake in south-western Victoria. The cestode, from the host's coelomic cavity, was identified as the plerocercoid stage of the pseudophyllidean genus Ligula, the occurrence of which in the Australasian region constitutes a new zoo- geographic regional record, and in G. maculatus a new host-family record. The mean percentage weight of plerocercoids per infected fish was approximately 9%, and the maximum almost 50% of the intact host weight. The mean number of plerocercoids per fish was approximately 0.75, or 1.5 per infected fish, and the maximum number per fish was five. The mean incidence of infection was approximately 50%. The effects of these cestodes on the host fish included a reduction in general body condition, an inhibition of gonadal maturation, and an inhibition, during the reproductive season, of movements to the spawning areas. The possibility that this cestode may have been intro- duced by man together with exotic fishes from the northern hemisphere is discussed. The nematode, usually found encysted in the visceral peritoneum, was identified as the larval form of Eustrongylides sp. (probably gadopsis), family Dioctophymidae. The mean number of nematodes per fish was 4.4, or 5.4 for infected fish, and the maximum number per fish was 44. The mean incidence of infection was approximately 80%. One of the effects of this nematode on its host was the inhibition of egg laying by female fish when encysted in the ovary or vent. This species was also found in an eleotrine goby and in rainbow trout, and a similar form (probably an intermediate-adult stage of the same species) in the cormorant Phalacrocorax varius, from the same lake.