A comparison of parasites, pathological conditions and condition index of wild and farmed populations of Magallana bilineata (Roding, 1798) from Vembanad Lake, west coast of India

Abstract

This study forms the first attempt to assess the health status, studying parasites and pathologies in wild or natural and farmed or cultured populations of the Indian backwater oyster, Magallana bilineata (= Crassostrea madrasensis Preston, 1916), the mainstay of oyster farming in the country. The Condition index, prevalence of infection, intensity, and seasonal variations in parasites and pathological conditions were studied. Wild oyster population was infected/infested with protozoan (Perkinsus beihaiensis, Nematopsis sp., Sphenophrya sp., and Stegotricha sp.), metazoan (cestodes and crustaceans), and shell (Polydora spp. and Cliona spp.) parasites and pathological conditions (digestive tubule atrophy, ceroid bodies, haemocytic infiltration, tissue necrosis, neoplastic disorders, and prokaryotic inclusions). Farmed population exhibited only a single protozoan (P. beihaiensis), a metazoan (Polydora sp.) infection and two pathological conditions (digestive tubule atrophy and ceroid bodies) in the tissues. The occurrence, prevalence and intensity rates for parasites and pathological conditions were significantly lower in farmed population compared to the wild population at the study site. Prevalence for parasites and pathologies were observed to be higher during dry season. The condition index (CI) values, which indicate the general health status of the oysters, were also significantly higher for the farmed population. The exposure period and the location of oysters in the habitat had a significant influence on the condition index and parasitic load in the population. None of the parasites were OIE listed; however the presence of a potential pathogen (P. beihaiensis) and pathological conditions warrants regular monitoring of the stocks and water quality at the culture site.