Grain-size distribution, texture (mud content) and quantitative bioclastic component analyses from outer barrier reef and lagoonal environments of Mayotte Island show that reefal and lagoonal sediments are clearly separated. Analysis of major bioclastic components allows the recognition of five sedimentary groups of "facies", each of them being significant of a particular zone. They are (1) coral-mollusc on the outer barrier reef slope, (2) coral-calcareous red algae on reef flats, (3) mollusc-coral and/or (4) mollusc-Halimeda at the lagoonal bottoms with coral build-ups or lagoonal plains with free living corals, and (5) mollusc-foraminifera on lagoonal plains; mollusc fragments pervade muddy substrates. In the lagoon the dominance of molluscs reflects the trophic regime related to the island terrigeneous influx. The bioclastic sediment composition generally reflects the nature of the benthic community living in this lagoon. This observation as well as textural and granulometric evidence show that the bulk of the sandy bioclastic material is in situ and has suffered little transport. Some relict grains are present in littoral muddy sands while probably older particles (black/green result of diagenesis) occur in the deeper parts of the lagoon. Textural parameters are more environmentally significant than granulometric ones.

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Distribution of the Phoronida in the littoral of coral ree! complexes and terrigeneous soft bottoms of the Mozambique Channel (SW Indian Ocean). The distribution of the Phoronida in three wellstudied regions of the Eastern Mozambique Channel (in Malagasy: southward the Tulear region and northward the Nasy-Be region; and around the Islands of the NE Mozambique ChanneL Mayotte 1., Clorious Is., Geyser and Zelee banks) allows definition of the ecological requirements of the endopsammic species, Phoronis muelleri, P. psammophila, P. pallida, Phoronopsis albomaculata and Ph. californica, and of the commensal one, Phoronis austraUs, living in cerianthid hlbes. Among the endopsammic phoronids there is a succession of species related to sedimentological parameters (grainsize, sand and silt ratios, organic matter) reflecting water movements: Phoronopsis californica in organegeneous gravels and coarse sands, then Phoronopsis albomaculata and Phoronis psammophila in biogenic and terrigeneous medium ta fine sands (the fonner prefering the coarsest sediments), then P. paUida in biogenic well-sorted medium and fine sands, and finally P. muel1eri in terrigeneous muddy sediments, although this species can also occur in biogenic clean medium sands washed by waters with high levels of suspended material, as at Mayotte I.

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