Coral reef and associated invertebrate communities (mainly molluscan) around Mahé, Seychelles

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Abstract
The Seychelles Bank is an area of approximately 31000 km2of shoal water with an average depth between 44 and 65 m. The granitic island of Mahe is situated centrally on the Bank, it is 27 km long, 6 to 8 km wide and rises to a height of 900 m above sea level. The climate is oceanic with two distinct seasons; from April until October the strong South-East Trades blow with an average speed of 8 to 10 knots and from November until March the area is under the influence of the northwest monsoon, when calms are frequent and the average wind speed is 4.5 knots. The average temperature is 26.8 °G and the average rainfall at sea level is 2500 mm. The near-shore sea-water temperature varies between 27*5 and 32 °C but in tidal pools values up to 40.7 G were recorded, with a daily range of about 8 °G. Salinities in water with open circulation were close to that of normal sea-water but on reef flats after heavy rain values as low as 5.6 %o were recorded. Tides are mixed semidiurnal, with a maximum range of 1.8 m. Sites exposed to the dominant S. E. Trades experience the highest degree of wave action. Fringing coral reefs are developed around the island, on the eastern side they are generally wide and more or less continuous, but on the western side they are confined to narrow bay reefs. Submarine topography, wind direction and strength have influenced the development and resultant shape of the reefs. The most obvious feature of the flora and fauna occurring on and in the fringing reef is that there is a zonation parallel to the shore. This zonation may be related to that occurring upon rocky intertidal shores throughout the world: most of the reef lies in the sublittoral zone but with projections upwards into the eulittoral and littoral fringe zones. The upper limit of abundant coral corresponds with the appearance ofLaminariain temperate regions. A number of distinct communities of organisms can be recognized forming and inhabiting these zones. The area above highwater mark but with maritime influence is the supralittoral zone and around Mahé is colonized by salt-resistant plants such asIpomoea, Scaevola, CocosandCalophyllum. In creeks and sheltered areas mangrove patches occur, commonlyRhizophoraandAvicenniaand inhabited by many crabs such asUca, CardisomaandScylla, and the gastropodTerebralia. Where the shores are rocky, either granite or beachrock they are colonized by a typical rocky shore biota; in the littoral fringe blue green algae andLittorinaare common, below in the eulittoral zone green algae, barnacles, limpets, several species ofNerita, Isognomonand oysters occur. The area between the tide marks on sandy beaches is inhabited by the crabsOcypodeandCoenobitaand the burrowing bivalvesDonaxandAtactodea. On the reef flat proper, communities are almost completely sublittoral and only partly emersed at lowest spring tides. In sheltered areas around Port Victoria, where the once extensive mangrove swamps have now been cleared, the remaining sediment is blackened, has a high organic content and smells of hydrogen sulphide; the overlying water is turbid with generally reduced salinities. The area is inhabited by a large population of ‘fiddler’ crabs (Uca), green algae and the burrowing bivalvesGafrarium, QuidnipagusandPsammotea. On reef flats which are wide enough beds of marine angiosperms are developed which extend up to 300 m seawards from the base of the beach. They are mainly formed by the generaThalassia, CymodoceaandSyringodiumwhich by means of rhizome and root systems accumulate and fix sediment. These ‘grass’ beds support a large population of holothurians and burrowing bivalves, principallyCodakia, Quidnipagus, TellinellaandGafrarium. On windward reefs immediately seawards of the grass beds are open sandy areas which interdigitate with lower ridges of loosely bound calcareous algal cobbles and coral fragments arranged at right angles to the algal ridge with which they are in continuity. Brown algae such asSargassumandTurbinariaare abundant and attached to hard substrates;Halimedais common in the sandy areas. A great number of species of burrowing gastropods and bivalves inhabit the sandy areas, the fauna on the cobble ridges is very similar to that occurring on the algal ridge. The algal ridge is well developed only on windward reefs, it consists of a low ridge at the seaward edge of the reef flat made up of a cavernous growth of calcareous red algae such asLithophyllumandPorolithon. This substrate is clothed inSargassum, Turbinariaand many other algae. The associated fauna shows an extraordinary diversity with many species of gastropods, particularlyCypraea, Conus, Triphora, TurboandTrochus. Many crabs, echinoids, ascidians, sponges and cemented foraminifera are present. The extreme seaward edges of the reef flat, the reef front and the reef edge are the only environments where corals are the dominant organism in the community. The reef edge marks the seaward extent of the reef flat and slopes shallowly seawards from the algal ridge to the change in slope at the reef front and is the area where the waves break. The reef front is generally steep but around Mahé is only about 20 m high, descending to the shallow base platform outside the reefs. Two distinct reef edge and front communities can be recognized, theAcropora-Millepora-Stylophoracommunity characteristic of windward edges with good water circulation and dominated by branching corals, and thePorites-Favia-Leptoriacommunity is characteristic of reef edges in sheltered areas with massive, rounded, growth forms. An abundant molluscan fauna has been dredged from areas outside the reefs, but sufficient information is not yet available to warrant a division...

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