This study deals with comparison of morphology and in situ behaviour of three Alvinocarididae species from the Snake Pit hydrothermal area (mid-Atlantic ridge), explored during the Hydrosnake cruise (1988). Spatial distribution on the site is described and illustrated for the three species. Rimicaris exoculara highly predominates. Modifications, apparently more morphological than functional, allowed adaptation to an environment characterised by extreme physico-chemical conditions and richness 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. Rimicaris exoculata aggregates on active diffusers and affect the mineralization processes. Morphological comparison between the three carid species enables to recognize three different levels in their trophic adaptation. Rimicaris exoculata is considered as a primary consumer. Chorocaris chacei may be optionaly necrophagous and Alvinocaris markensis 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 assumptions about their ability in detecting trophic areas. The trophic specialization, represented by a morphological series of changes in the mouthparts, from Alvinocaris to Rimicaris through Chorocaris, goes with a gradual spread of a bilobed ocular organ, whose function is still unknown.