Observations on the Ecology and Feeding Behavior of the Anchialine Shrimp Procaris ascensionis

The shrimp Procaris ascensionis occurs in landlocked pools that are under tidal influence. Individuals occur in shaded areas in deep crevices and almost never occur in sunlit or open areas. Although this species has been considered to be rare, our field observations during March 1983 revealed densities of 18.2 ± 4.9 individuals m$^-3$ in lava caves leading off surface pools. Size segregation is evident in pools and caves, with smaller individuals spending most of their time in crevices and among the coenocytic alga Valonia and branching green algae. Large individuals primarily spend a large portion of their time in the water column swimming upside down using pereiopodal exopods and pleopods. While on the cave bottom large individuals move through algae, extending the achelate pereiopods laterally, apparently searching for prey. When a prey item is encountered, Procaris extends all five pereiopods and pulls the prey ventrally, trapping it against the gnathothorax with the aid of the third maxillipeds. Procaris then swims upside down holding the prey within a cage formed by the spinous pereiopods. Detection of prey is probably accomplished by mechano- or chemoreceptors borne on the pereiopods and third maxillipeds. Sections through the reduced eye revealed no ommatidia, suggesting that this shrimp is blind. Procaris is a voracious predator on the gammarid amphipod Melita and the atyid shrimp Typhlatya rogersi, although detailed gut analyses produced equal amounts of animal parts and plants (filamentous algae and benthic diatoms). The reproductive biology of Procaris remains unknown.