RETINAL ANATOMY OF CHOROCARIS-CHACEI, A DEEP-SEA HYDROTHERMAL VENT SHRIMP FROM THE MID-ATLANTIC RIDGE

Exploration of deep-sea hydrothermal vents over the past quarter centu ry has revealed that they support unique and diverse biota. Despite th e harsh nature of the environment, vents along the Mid-Atlantic Ridge are dominated by large masses of highly motile Bresiliid shrimp. Until 1989, when it was discovered that the vent shrimp Rimicaris exoculata possesses a hypertrophied dorsal eye, many believed that animals popu lating hydrothermal vents were blind. Chorocaris chacei (originally de signated Rimicaris chacei) is a Bresiliid shrimp found at hydrothermal vent fields along the Mid-Atlantic Ridge. Like R. exoculata, C. chace i has a hypertrophied retina that appears to be specialized to detect the very small amount of light emitted from the orifices of black smok er hydrothermal vent chimneys. C. chacei lacks the sophisticated compo und eyes common to other decapod crustaceans. instead, it has a smooth cornea, with no dioptric apparatus, apposed by a tightly packed, mass ive array of photosensitive membrane. Photoreceptors in the C. chacei retina are segmented into a hypertrophied region that contains the pho tosensitive membrane and an atrophied cell body that is roughly ten ti mes smaller in volume than the photosensitive segment. The microvillar photosensitive membrane is consistent in structure and ultrastructure with the rhabdoms of decapod and other invertebrate retinas. However, the volume density of photosensitive membrane (greater than or equal to 60%) exceeds that typically observed in invertebrate retinas. The r eflecting pigment cells commonly found in decapod retinas are represen ted in the form of a matrix of white diffusing cells that exhibit Tynd all scattering and form an axial sheath around the photoreceptors. All photoreceptor screening pigment granules and screening pigment cells are restricted to the region below the photoreceptor nuclei and are th ereby removed from the path of incident light. No ultrastructural evid ence of rhythmic cycling of photosensitive membrane was observed. The morphological adaptations observed in the C. chacei retina suggest tha t it is a high-sensitivity photodetector that is of functional signifi cance to the animal. (C) 1997 Wiley-Liss, Inc.