Abstract

In the present study, the seasonal variation in physic-chemical parameters, population density and distribution pattern of Nerita crepidularia in the mangroves of Vellar estuary, Tamil Nadu was studied for a period of one year (January to December). The physico-chemical parameters such as temperature (atmospheric and water), pH, salinity, dissolved oxygen and rainfall were estimated. The atmospheric and water temperature ranged from 24 to 38 and 22 to 36°C, respectively. The pH ranged between 6.7 and 7.4. The dissolved oxygen was ranged from 4 to 5.1 ml/L. Salinity fluctuated between 4 and 38 ppt. The value of rainfall ranged between 240 and 6070 mm. Further the physic- chemical parameters in various seasons was subjected to analysis of covariance and was calculated as F = 3.055484 (P<0.05). Quadrate sampling was carried out monthly. The population density was found to be lesser (average 2 numbers / quadrate / 0.5 m²) in monsoon (November, 2008) and higher (average 7 numbers / quadrate of 0.5 m²) during post monsoon (February, 2008). Further the density of Nerita crepidularia in different seasons was subjected to analysis of covariance and was calculated as F = 0.0215 (P<0.05). In the present study, the correlation coefficients obtained between the population density and the hydrobiological parameters were non significant. This gastropod species is found enjoying a variety of habitats and interesting movement along the tides within the same mangroves. It is found attached to the mangrove plants on the stems and moving up and down along the tidal water, that is, during the high tide when the water level goes up to the height of 2.5 m above the ground level, the animals are also moving up and when the water level is coming down during the low tide, the animals are moving down along the tidal water. Further the animals are also found crawling in the intertidal muddy substratum during low tide.

Key words: Mangroves, physico-chemical parameters, Nerita (Dostia) crepidularia, population density, distribution pattern, Vellar estuary.