Associations d'ostracodes d'un environnement récifal envasé: Le lagon de l'île de Mayotte (Archipel des Comores, Océan Indien occidental)

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Ostracode assemblages have been collected from sediments dredged in the lagoon of Mayotte Island in order to study effects of mudflows from land to lagoonal zones due to anthropisation. Two main facies occur: bioclastic sandy bottoms in the external part and muddy deposits which progressively invade the major part of coastal bays and proximal lagoonal zones. Previous geological analyses are available dealing with some micropalaeontological (foraminifera) and sedimentological aspects. A systematic approach of the ostracode content is proposed within the same context; records obtained constitute a preliminary account concerning this area; nevertheless, a few data already given in the selected bibliography may be used. In such sedimentary environments characterized by a high evolutionary rate, the knowledge of present-day assemblages and their qualitative/quantitative composition will be of great interest because they may be considered as reference for the future. One of the prior applications will be to put in a prominent position the effect of occasional variations of physico-chemical parameters related with the development of human activities. The ostracode fauna is composed of thirty-nine recorded species belonging to thirty-five genera (four indetermined) and ranging mainly into the Cytheracea and Bairdiacea; Cypridacea are less common. Poorly local documentation and occurrence of rare and/or badly preserved specimens does not permit an accurate ostracode check-list. Accordingly, many species are left in open nomenclature. Within the two sedimentary type-deposits, ostracode assemblages show many qualitative differences; diversity is maximum in medium mud-content areas (transitional facies between sands and pure muds). From a quantitative point of view, it is noteworthy the more important scarcity of microfaunas on bioclastic (shelly) sands and several well-muddy substrates. An assessment is made of the biogeographic point of view; mahoran microfaunas show strong affinities with those of North Madagascar and Reunion Island. Common species have been described or located in the Western Pacific Ocean, especially Australia. Relationships with South African coasts are not clearly demonstrated.