RETINAL ANATOMY OF A NEW SPECIES OF BRESILIID SHRIMP FROM A HYDROTHERMAL VENT FIELD ON THE MID-ATLANTIC RIDGE

A new species of shrimp (Rimicaris sp.) was recently collected from th e Snake Pit hydrothermal vent held on the Mid-Atlantic Ridge. Until th e discovery in 1989 that the deep-sea, hydrothermal vent species, Rimi caris exoculata, possessed a hypertrophied dorsal eye, everyone believ ed that animals recovered from vent environments were blind, Like R. e xoculata, Rimicaris sp., a small orange bresiliid shrimp, has an enlar ged dorsal eye specialized for detecting light in a very dim environme nt instead of the expected compound eye. The individual lenses charact eristic of a compound eye adapted for imaging have been replaced in Ri micaris sp, by a smooth cornea underlain by a massive array of photose nsitive membrane. The number of ommatidia in this species is about the same as in shrimp species that live at the surface: however, the phot oreceptors are larger in the deep-sea species and the shape of the pho toreceptors is markedly different. The light-sensitive region of the p hotoreceptor is much larger than those of other shrimp and the rest of the receptor is much smaller than normal. All screening pigment has m oved out of the path of incident light to a position below the retina, and the reflecting pigment cells have adapted to form a bright white diffusing screen between and behind the photoreceptors, The ultrastruc ture of the microvillar array comprising the rhabdom is typical for de capod crustaceans; however, there is a much greater volume density of rhabdom (80% to 85%) than normal, There is no ultrastructural evidence for cyclic rhabdom shedding or renewal. Rimicaris sp. has apparently adapted its visual system to detect the very dim light emitted from th e throats of the black smoker chimneys around which it lives.