Monogenetic trematodes (Dactylogyridae: Ancyrocephalinae) on the gills of tilapia (a warm-water cultured fish) with special reference to Cichlidogyrus sclerosus Paperna and Thurston 1969

Abstract

Monogenetic trenatodes (Dactylogyridae : Ancyrocephalinae) on the gills of tilapia (a warm-water cultured fish) with special reference to Cichlidogyrus sclerosus Papema and Thurston 1969. The brief descriptions of Cichlidogyrus sclerosus. C. t. minutus and C. tilapiae by Paperna and Thurston (196 9) have prompted further taxonomic studies of these species of monogenetic trematodes on the gills of tilapia. There are few morphological variants between the species described with the archetypes of Paperna and Thurston (1969) except for differences in measurements. The pioneering efforts of Paperna and Thurston (196 9) have been complemented with further studies on some aspects of the biology of C. sclerosus. In this study the fecundity of C. sclerosus on excised gill arches is approximately 3; the period of incubation is 4 days at 25°C; host mucus and urea are good hatching inducers; pH 7 is found to be most favourable for development and hatching of the eggs. C. sclerosus, highly specific to the gills of tilapia, shows a definite preference for the anterior medial sections of hemibranchs of the first gill arches. C. t. minutus also shows preference for similar sites while juveniles of C. sclerosus are found randomly on the gills. The preference for the first gill arch could be attributed to the lack of microbranchiospines, which prevent passage of oncomiracidia over the gills. There is also a significant linear regression between lengths of fish and number of parasites. As the fish gets older the number of parasites will increase. The density experiments show the overdispersed distribution of C. sclerosus on S. mossambicus. The parasite numbers also appeared to build up after two weeks and decline after two weeks. This could be related to the life span of C. sclerosus and also a host defense mechanism. This is verified by the huge build-up of C. sclerosus after injection of host tilapia with hydrocortisone. Further elucidation of this host defense mechanism would be of great significance in the field of monogenetic trematode biology.