Recruitment of mud clam Polymesoda erosa (Solander, 1876) in a mangrove habitat of Chorao Island, Goa

Abstract

Habitat-specific settlement success results either from active habitat selection or differential mortality after non-selective settlement. The mud clam Polymesoda erosa (Solander, 1876) is an ecologically and economically important benthic invertebrate with high abundance in the mangrove forests of Chorao Island, Goa, India. A one-year study (Jul 2004-Jul 2005) was conducted to characterize the patterns of post-larval settlement and survival of P. erosa in the mangrove habitat. The mean density of settling post-larvae was 28 no.m sup(-2) with a majority of settlers during Sept 04. The density of adult clams in the landward zone ranged from 7-12 no.m sup(-2) (mean: 9 + or - 6 sd; n=122). In the seaward region, adults were completely absent at the low tide level. Higher densities of juveniles observed at the low and mid-tide levels were assumed to be due to frequent inundation which allowed the young individuals to feed adequately and attain the critical sizes at which vulnerability to physical and biological constraints is substantially reduced, whereas, at high-tide level, increased desiccation may account for the mortality of settlers. It is speculated that initially the settlers settle according to the hydrodynamic conditions for sedimentation; however, adult survival is affected by their recruitment in a suitable habitat (e.g. substrates near Avicennia sp.). Thus, habitat dependent viability during the post-settlement phase could be the best explanation for the non-random, patchy distribution of P. erosa observed in the Chorao mangrove swamp.