Effects of Tidal Exposure on Bivalve Molluscs: First Stab at the Case of West African Mangrove Oyster (Crassostrea tulipa) in Benya Lagoon, Elmina, Ghana

Abstract

It is widely recognized that growth of sessile intertidal organisms is largely driven by tidal exposure. However, data supporting exact manifestation of this effect on Crassostrea tulipa is lacking, limiting management of the species to enhance nutrition and livelihood opportunities for coastal communities in West Africa. We investigated the growth of the species at 3 tidal heights [13 – 22 cm, 23 – 32 cm and 33 – 41 cm] above neap tide mean low water (MLW)] focusing on 3 parameters, i.e. shell height, condition index (CI) and gonadal development. Three hundred and thirty-three individual oysters from 10 colonies randomly selected from 2 stations, 500 and 600 m from the sea, around an open lagoon were investigated. CI was determined as the ratio between dry meat weight and inner shell volume; gonadal development was determined via histological analysis of gonad tissues. Shell height and CI of the oyster increased with tidal elevation irrespective of whether they were closer to or further from the sea. These findings agree with reports on other bivalves, suggesting that short tidal inundation promotes growth of C. tulipa. In terms of gonads, five stages of development were identified; all occurred among oysters located at lowest tidal levels (13 – 22 cm above MLW), possibly due to continuous breeding as a result of increased food supply by long inundation. The proportion of ripe gonads increased with tidal height; ≈67% of gonads found at the highest tidal levels were ripe suggesting discontinuous breeding in the higher-level oysters.