A Comparative Study of the Anatomy of Two West African Edible Bivalves, Aspatharia Sinuata (Mutellidae: Unionacea) and Egeria Radiata

Abstract

The need for a better understanding of the biology of Egaria radiatai to shed light on the sequence of events in the adaptation of this important west African fishery clam, which enabled it to first invade and gradually adapt itself to the freshwater environment, a pre-requisite to developing techniques for the expansion of sustainable production of species, through fisheries and aquaculture, informed this study. This paper therefore reports on a comparative study of the anatomy of E. radiata (a hemi-freshwater clam) collected immediately above the region of salt water penetration of Cross River State, Nigeria, and Apatharia sinuata (an established holo-freshwater bivalve) obtained from inland waters. The results indicated that though two species have certain anatomical features in common indicating strong evolutionary relationships and adaptation to freshwater environment, the species nevertheless differed considerably in certain features that get modified in the course of bivalve transition from salt to freshwater environment: suggesting different levels of adaptations to freshwater environment, probably occasioned by variation in the period of arrival at such habitats by the two specie. For example, the two species differed in structure and complexity of the stomach, presence/absence of caecum, occasioned by style of the sac with the mid gut, length of coiling of the intestine and distribution of the gonads in the visceral mass. These variations probably indicate differences in feeding habits., related to differences in distribution and behaviour of the two species. Findings od this study reveals that the E. radiata is on a definite transition from marine to freshwater habitat and it should be possible to hasten migration by culturing the species entirely in fresh water captivity or, at least, transplanting the young clams from their present region of distribution at the lower reaches of inland waters to new beds up stream to give them time to attain maturity before harvesting, in order to prevent extinction of the species, going by the report limited area of its distribution and low production-to-mean-biomass (P/B ration),resulting from over exploitation.