Until now, 36 species belonging to the family Cymothoidae (Crustacea, Isopoda) were recorded from Indian fishes. In this study, ten additional cymothoids are reported in India, most of them for the first time. They parasitize nine fish species, several of them being new host species.

Cymothoid isopods are parasites that cause severe damage to the fish species. The present study was focused to carry out the parasitic infestation on the one of the important marine fish Strongylura leiura during the 2 years period between June 2009 and May 2011. 272 fish specimens collected and about 157 (57.72 %) fishes were infested by the cymothoid which were concealed under the gill arches of the host. Infested parasite was identified as Mothocya renardi. The prevalence rate of parasites during different seasons of the range of host specificity of isopods was studied. Variation of infestation rate also depends on the size, length and weight of the host. The sites of infestation by the parasitic crustaceans are highly specific. The infestation had affected the normal growth of the host fish and also probably leads the high levels of secondary infections.

Gut flora has a continuous and dynamic effect on the host's gut and systemic immune systems. The gut microbiota of fish has been studied extensively using various techniques; little information is available on the correlation between gut microbiota and host activity. In the present investigation three species of estuarine fishes were taken with regard to their microbial population in the gut region and enzymatic analysis of those gut isolates. The total heterotrophic bacterial load ranged from 0.8x10 6 to 4.5x10 6 CFU/g of gut sample and it was found to be the maximum of 4.5x10 6 CFU/g in Oreochromis mossambicus gut followed by 3.1x10 6 CFU/g in Oreochromis leucostictus gut and minimum of 0.8x10 6 CFU/g in Etroplus suratenis. The isolates were identified by various biochemical tests. Pseudomonas, Vibrio and Bacilli are the common bacteria found in all three fishes. The bacterial isolates were subjected to quantitative estimation of enzymes viz., amylase, lipase and protease. Varied kinds of results were obtained in different fish species. However it was observed that 20% of the isolates produced only amylase. A collection of 10 bacterial isolates from three fishes were screened for antagonistic activity against human and fish bacterial pathogens. Upon testing all these strains, it became apparent that strains isolated from the fish O.mossambicus, exhibited antimicrobial activity against Klebsiella pneumoniae (17mm) and Vibrio cholerae (12mm). Thus the present preliminary study showed the variation in the gut micro-flora of different fish species and the microbes from marine environment may be useful in development of probiotics, drugs and also in the industrial enzyme production.

The extracts from marine mollusks M. melo is the potential source of producing bioactive compounds against human pathogens and can be used for synthesis of new drugs.

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