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The diversity of the crabs in the Pichavaram mangroves for the first time has shown that there are 46 species of crabs from the five different stations. The neritic zone (Chinnavaikal) was rich in species composition (35 species). The freshwater zone was poor in species composition (14 species) and population density. Among the crab species Sesarma and Uca species were dominant in almost all the stations. Grapsid crabs were the most dominant species. Neritic and Avicennia zones showed more number of crab species but less number of species was noted in slightly saline zone. The substrate suitability, tidal inundations mangroves distribution and salinity were the possible factors for distribution of crabs in Pichavaram mangroves.
The incidence of parasitic isopods in the fish Sphyraena obtusata was studied. The reduction in gill raker count and gill surface area was observed as a function of infestation was found to be statistically significant. Maximum reduction in respiratory surface area was observed in Ist gill arch (10.7%) and minimum (9.2%) in the IIIrd gill arch. The percentage reduction of surface area in the IInd and IVth gill arches was 10.2 and 9.7, respectively. The infestations such as, hemorrhagic lesions, anemia; encapsulation; inflammation and penetration of dactylus usually pressure atrophy often accompanied by the presence of larger parasites. This may lead to huge economic losses in commercial species of fish.