Host Sexual Dimorphism and Size Polymorphism in Susceptibility of the Nest-Breeding, Nile Green Tilapia (Tilapia zilli) Gervais, 1848 to the Monogenoidean Gill Parasites

Abstract

A detailed account has been made of the sexual dimorphism and size polymorphism in susceptibility of the nest-breeding, Nile green tilapia (Tilapia zilli) Gervais, 1848 to cichlidogyrid monogeneans at Mansouria Canal, Nile Delta, Egypt. The spawning period extended from early April to late August in 2016. Only four gill monogenean species, namely Cichlidogyrus arthracanthus, C. aegypticus, C. tilapiae and C. halli typicus were encountered on the gills of the green tilapia. The two genders of the breeding tilapia acquired significantly higher numbers of monogenean worms than their conspecifics of the non-breeding tilapia. However, the infection variables (prevalence, mean intensity and abundance) showed no significant variation between the two genders in the breeding or non-breeding forms of tilapia. Extensive field observations revealed that the breeding forms of the green tilapia were resident, mostly quiescent during the spawning time, indicating increased opportunities for monogenean populations to proliferate on the likely captive host individuals. The prevalence of the most dominant monogenean, Cichlidogyrus arthracanthus varied significantly among different classes of the host condition factor in the breeding males, non-breeding males and non-breeding females. However, the infection variables of other monogeneans attained no significant variation among different classes of the host condition factor. The condition (class I) acquired greater number of the monogenean Cichlidogyrus arthracanthus, followed by class II and class III. Factors contributing to the sexual dimorphism and size polymorphism in monogenean infestation on the breeding forms of the green tilapia are discussed.