Molluscan Biodiversity of the North-eastern Region of Bangladesh

Abstract

Mollusca, the second largest phylum in the animal kingdom are characterized by soft, slimy body and glandular body fold of skin or mantle. The mantle secrets a shell which often gives the mollusc’s partial or complete shelter for its body. Molluscs are the members of very ancient group whose origin goes back to the earliest fossil bearing rocks of the Paleozoic Era (the Cambrian period, some 500 million years ago). They reached the peak of their development in the Cenozoic Era, but even today molluscas are still, numerically, a very large group. The number of species in the phylum probably about 120,000, but this figure includes extinct species. A common estimate is 80,000 species, the majority of which live in the sea and about 75% of this total are gastropod (Pfleger and Chatfield 1988). The number of living molluscan species today varies from 80,000 (Boss, 1973) to 135,000 (van Brugen, 1955), of which 31,000 to 100,000 are marine, 1,4000 to 35,000 terrestrial and about 5,000 are freshwater (Abbot, 1989; Seddon, 2000). Moluscs are markedly diverse with regard to their external morphology and size they range from microscopic clams and snails (smaller than 1 mm) to giant oceanic squid and the massive tridacnid clams oflndopacific reefs (Hochachka, 1983). Habit and Habitats of Molluscs Molluscs range from limpets clinging to the rock, to snails, which crawl or dig or swim, to bivalves which anchor or burrow or bore, to cephalopods which torpedo through the water or lurk watchfully on the bottom. They penetrate all habitats: the abyssal of the sea, coral reefs, mudflats, deserts and forest, rivers, lakes and underground. They may become hidden as parasites in the interior of other animals (Morton; 1968). The molluscs are found in a wide range of habitats, from tropical to the polar seas, at altitudes exceeding 7000 m like Himalayas, in flood plains, ponds, lakes, beels, baors, streams, paddy fields, forest beds, mud flats, mangroves, rivers, and the seas from pelagic to benthic depths. The land molluscs are adapted to all kinds of weather and dwell on ground habitats ranging f om bushes, parks' gardens, vegetations and forests, particularly inhabits that offer sufficient shady and moist places. Their primary habitats are damp shady corners among vegetation, under fallen leaves, litters, and logs and in crevices. Some species crawl on damp walls, trees trunks, petioles and leaves and algal growths. A few prefers salty environments, while some are adopted to leaves and branches of trees or inside the hollow trees bark .Slugs having no protective shell often take shelter inside the petioles or folds of leaves. The young one is usually found attached to under side of the herbs. Freshwater molluscs are common in ponds, paddy fields, irrigation canals, lakes, hill streams (chhara) and rivers. Marine molluscs live in a wide range of habitats, from supra tidal mark of a coast to the extremely deep water. Most of the Bangladeshi marine molluscs live in the Bay of Bengal coast and represent diversified life style, including free -living, sedentary, bottom-dwellers, burrowers, borers and pelagic form. A few are also arboreal on mangrove trees and grasses. The majority of the species, however, inhabit shallower waters (Encyclopaedia of Flora and Fauna of Bangladesh, 2007). Molluscs as human food and animal food In Guadeloupe, Mauritius, China, Japan and even in France, certain species of bivalve are used for human consumption (Preston, 1915).The use of fresh water molluscs, protein-rich food very much in practice in a number of countries viz. Mexico (Garia­Cubas, 1989, Valvas-Gonzalez 1989), Taiwan and Formosa (Wu 1989), the Philippine (Talavera and Faustino, 1933, Pagulayan and Cruz, 1989), and Kewajam, 1986). Pila globosa, Bellamya beagalensis, Brotia costula, Paludomus conica, Lamellidens marginalis, Solenia soleniformes, Trapezoides exolescens are consumed in West Bengal, India (Subba Rao, 1989). Land snails are also eaten by people of France. France also consumes 5 million pounds of escargot (a large tree snail Helix aspersa) every year (Bourquin et. al, 1999). Along the Worlds miles of coastlines, man has always had a readily available food source - high in protein and trace of minerals, because of the many kinds of molluscs to be found there. Mussel and oyster beds, clam­flats and other abundant shell fishes have always provided and easy source of food……………………………………..