Comparative study on isopod parasites of fish in freshwater and marine environment

Abstract- The isopods are crustacean parasites which are found in all water bodies. The isopods can cause changes in the growth, reproduction and behavior of fishes. The isopods collected during the study period was Alitropus typus from the freshwater canals and freshwater farms of Thrissur and Ernakulam district, Nerocila indica and Nerocila depressa from the prawn filtration fields of KUFOS, which is a part of Vembanad lake. The marine isopod Norileca indica was collected from the local fish market. In the study period of six months,from January to June 2015 it was observed that 89% of the collected Channa striatus was infected with Alitropus typus and the parasite shows host and site. The parasite showed preference for the gills of Channa striatus, Wallagu attu, Catla catla and Koi carp(Cyprinus carpio koi). In the brackishwater environment 68% N.sunaica was found infecting Otolithes ruber.The parasite was also collected from the body surface of Ambassis commersonii, Alepes djedaba and Mugil cephalus.N.depressa was found infecting only on Thryssa mystax. The marine isopod Norileca indica was collected from the gills of mackerals. The observed histopatholgical changes on the gills of the parasite infested fish showed rupture of the lamellae, lifting of the lamellar epithelium, lamellar hyperplasia.vacoulisation and oedema. This is mainly because of the hook like dactylus of the parasite pereopod and the pressure that is created by the parasite on the gills of the host.The parasitic adaptation of the different parasites will vary depending on the site of attachment.The parasites collected from the skin had strong hook like pereopods for strong attachment on the host. The patterns of variation in occurrence of the isopods suggests that their distribution correlates with the environmental conditions. For the freshwater isopods, temperature is the limiting factor of distribution. When the temperature increased,the parasite numbers increased. The numbers reduced drastically after the rain during June. However in brackishwater the distribution of the isopods determined by salinity.The number of parasites appeared to be high during the high salinity period.