ENIGMA OF THE TROPHIC ADAPTATION OF THE S HRIMP ALVINOCARIDIDAE IN HYDROTHERMAL AREAS ALONG THE MID-ATLANTIC RIDGE

This study deals with comparison of morphology and in situ behaviour o f three Alvinocarididae species from the Snake Pit hydrothermal area ( mid-Atlantic ridge), explored during the Hydrosnake cruise (1988). Spa tial distribution on the site is described and illustrated for the thr ee species. Rimicaris exoculara highly predominates. Modifications, ap parently more morphological than functional, allowed adaptation to an environment characterised by extreme physico-chemical conditions and r ichness in bacteria. This species has developed a culture of bacteria on its mouthparts and under the carapace to complement nutrient input drawn from bacterial production from the surface of active chimneys. R imicaris exoculata aggregates on active diffusers and affect the miner alization processes. Morphological comparison between the three carid species enables to recognize three different levels in their trophic a daptation. Rimicaris exoculata is considered as a primary consumer. Ch orocaris chacei may be optionaly necrophagous and Alvinocaris markensi s is necrophagous. The latter two species do not have the same spatial distribution as Rimicaris. We notice a possible relation between the different pigmentation of carids and their diet and we make assumption s about their ability in detecting trophic areas. The trophic speciali zation, represented by a morphological series of changes in the mouthp arts, from Alvinocaris to Rimicaris through Chorocaris, goes with a gr adual spread of a bilobed ocular organ, whose function is still unknow n.