A Comparative Study of the Anatomy of Two West African Edible Bivalves, Aspatharia sinuata (Mutallidae: Unionacea) and Egeria radiata (Donacidae: Tellinacea).

Abstract

The need for a better understanding of the biology of Egaria radiata and 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 the 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 the Cross River area, Nigeria, and Aspatharia sinuata (an established holo-freshwater bivalve) obtained from inland waters. The results indicated that though the two species have certain anatomical features, 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 Bivalvia transition from salt to freshwater environments; suggesting different levels of adaptation to freshwater environment, probably, occasioned by variation in the period of arrival at such habitats by the two species. For example, the two species differed in structure and complexity of the stomach, presence/absence of caecum, association of the style sac with the midgut, length and coiling of the intestine and distribution of the gonads in the visceral mass. These variations probably indicate differences in feeding and breeding habits, related to differences in distribution and behaviour of the two species. The findings of this study revealed that E. radiata is on a definite transition from marine to freshwater habitat and it should be possible to hasten this migration by culturing the species entirely in freshwater captivity or, at least, transplanting the young clams from their present region of distribution at the lower reaches of inland waters, to new beds upstream to give them time to attain maturity before harvesting, in order to prevent extinction of the species, going by the reported limited area of its distribution and low production-to-mean-biomass (P/B ratio), resulting from over exploitation