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Some aspects of the ecology of the mollusc Nerita senegalensis in Lagos harbour, Nigeria

Nerita senegalensis Gmelin is a marine epibenthic prosobranch mollusc (Archaegastropoda: Neritidae) occurring abundantly on the artificial rocky shores of Tarkwa Bay in Lagos harbour along with other common snails such as the keyhole limpet Fissurella nubecula L. and Patella safiana Lamarck. N. senegalensis is one of the commonest species of gastropods on West African rocky shores, occurring from the low tide level to the upper level of the shore. Although like most species of the family Neritidae it usually hides in crevices when the tide is out, N. senegalensis is often found in shady areas on exposed rocky shores (Oyenekan 1975). Edmunds (1978) found the species in pools and under stones in Ghana and rocky estuaries in Sierra Leone. Although we have now a good understanding of the occurrence and distribution of the benthic fauna of Lagos harbour and lagoon, our knowledge of the ecology of the constituent species of the various benthic communities, particularly those of the rocky shores, remains sparse. Apart from the description of shell characteristics, classification and geographical distribution (Edmunds 1978, Nickles 1950), little is known about the biology of N. senegalensis and yet this mollusc is of great importance locally as several tonnes are collected each year to supplement the diet of the people. The main objective of this study was to investigate the survival of N. senegalensis in water of differing salinities and the tolerance of the mollusc to constant and to fluctuating temperatures. The investigation also sought to explain whether the exposure of the animal out of water contributed to its distribution in Lagos harbour. Description of the study area. Tarkwa Bay lies at the southwest end of Lagos harbour (Figure 1). The rocky shore there is artificial, having being built to prevent the erosion of the sandy beach by wave action of the Atlantic ocean.