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The Bivalvia — Proceedings of aMemorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. (Edited by Brian Morton). Hong Kong University Press, Hong Kong, 1990. HABITS AN D MORPHOLOGICA L ADAPTATION S O F MYTILIDS (MOLLUSCA : BIVALVIA ) FRO M COASTA L AND REEFAL ENVIRONMENT S I N SOUTHWES T MALAGASI A (INDIAN OCEAN ) Patrick M. Arnaud and Bernard A. Thomassin Centre d'Oceanologie de Marseille (CNRS/UA n° 41), Station Marine d'Endoume, rue de la Batterie des Lions, F-13007 Marseille, France ABSTRACT The Tulear reef complex was chosen as a model for this work because of its high biotope diversity, its mytilid richness, and more generally because of the large background knowledge about its faunistic assemblages and environments. Distribution through the reef complex , habits, shell shape and ornamentation of 13 representative species are analysed on the basis of data obtained during an ecological survey. Five types of substrate relationships, linked to characteristic shell features, are identified : A. Epifauna l (or epibyssate) on hard substrates, i.e., either 1, exposed or 2, cryptic species with large or medium modiolid o r mytilid shells , smooth in the former an d radially ribbed in the latter; B. Infauna l (o r endoby ssate) in hard substrates, i.e., either 3, borer s or 4, nestlers with elongate smooth shells in the former and modiolid shells with radial striae in the latter; C. Infauna l (o r endoby ssate) in soft substrates, i.e., 5, nestlers with thin glossy modiolid shells. The exposed epibyssate mussels, Modiolus auriculatus and M.elongatus, and borers of the genera Lithophaga and Botula are generally dominant, except in deeper waters where endobyssate mussel s predominate . Almos t al l habitat s ar e colonized , wit h fe w nich e overlaps. The development of Modiolusbeds in the lower intertidal appears linked to peculiar hydrological (laminar currents) and trophic conditions, and not predation. INTRODUCTION It was shown by Yonge (1962) that byssate, adult bivalves arose from burrowers by neoteny an d th e evolutio n o f th e byssa l apparatu s ca n explai n th e evolutio n 'fro m a n 334 PATRICK M . ARNAU D AN D BERNARD A . THOMASSI N isomyarian t o a heteromyarian an d then to a monomyarian condition' . This subject wa s reviewed b y Stanle y (1972 ) wh o pointed ou t the evolutionary tren d fro m semi-infauna l (endobyssate) to epifaunal (epibyssate) attachment, a hypothesis later discussed by Yonge (1976). Special interest in the Mytilidae in this context relates to the coexistence of semi-infaunal and epifaunal habits by boring, burrowing and nestling species. The variety of living habits makes the Mytilidae excellent material for the study of morphological adaptation to various benthic habitats. It was therefore interesting to 'test' morphological and byssal adaptation in an area of high ecological diversity and mytilid richness. The Tulear coast, south-west of Madagascar, was selected as a model for this study. It is characterized by a high diversity of shallow water environments and lined by a succession of coastal coral reef complexes (including barrier-, lagoonal- and fringing-reefs), variou s geomorphological features defined b y Battistini et al. (1975), wave-beaten sandy beaches and open bays with seagrasses and mangroves. Various kinds of biogenic (built and detrital) and terrigeneous substrates are available to the Mytilidae. Because of an ecological survey (material and sampling methods mainly in Plante, 1964; Peyrot-Clausade, 1977; Thomassin and Galenon , 197 7 an d Thomassin , 1978) , faunisti c assemblage s a t thi s sit e ar e wel l investigated, providing a good background for this work. A small number of species of mussels have been recorded from Tulear (Dautzenberg, 1929) but our work on available material enables us now to record 20 species. This richness is one of the highest known from a single, small-sized, area. In comparison, two other wellinvestigated Indo-Pacific areas revealed lower numbers of species: 12 around Inhaca Island, Mozambique (Boshoff, 1965) and only 14 around the far more extended Hawaii Archipelago (Kay, 1979). Only two, Hong Kong, with 28 marine species (Lee and Morton, 1985), and Sagami Bay, Japan, with 24 species (Kuroda et al, 1971) , are richer than the Tulear reef complex, presumabl y a s a resul t o f thei r locatio n i n th e ecoton e betwee n tropica l an d temperate realms (as confirmed by the occurrence of Mytilus edulis). Of th e 2 0...

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