The flora, fauna and sediments of the marine grass beds of Mahé, Seychelles

Summary

Extensive marine grass beds occur on both windward and sheltered reef flats off Mahé. Six species of angiosperms are found: their leaves being covered by epiphytic algae which help to trap fine sediment and to limit the growth of the grass plants. The grass rhizomes bind the sediments: well seen on the margins of the beds. The deposits are typically poorly sorted, coarse and medium skeletal sands, being richer in fines on the sheltered reef flats. On the margins of the beds the grain-size characteristics of the sediments are more typical of deposits in the adjacent environments. The dense cover of grass plants and stable sands provide an ideal habitat for a prolific infauna and epifauna. Gastropods and burrowing bivalves dominate the molluscan fauna: the density of the latter being greater on the sheltered reefs, apparently due to the higher proportion of fines in the sediments. Population densities are low when compared with those of burrowing bivalves in temperate regions. Echinoderms are abundant, especially holothurians, which are most common close inshore. Except locally corals are relatively unimportant. Crustaceans are abundant and are believed to be responsible for the numerous mounds on the surface of the beds. The sediments are subject to continual reworking by the burrowers and scavengers.