VISUAL ORIENTATION OF POSTLARVAL AND JUVENILE MANGROVE CRABS

Visually directed movements of juveniles of the mangrove crabs Aratus pisonii and Chlorodiella longimana and postlarvae (megalopae) of A. pi sonii to black horizontal rectangles subtending visual angles from 5-3 50 degrees were measured in a circular arena. The study rested the hyp othesis that early stage crabs use visual cues for attraction to poten tial refuge sites represented by silhouettes of mangrove roots and for avoidance of large dark areas that represent predators. The habitats of the 2 species differ in that A. pisonii lives in the supralittoral/ littoral area among the roots and branches of mangrove forests, while C. longimana lives sublittorally in this area and is rarely exposed to air. Megalopae and juveniles of A. pisonii were significantly attract ed by relatively narrow rectangles subtending angles up to 30 degrees. Tn contrast, juvenile C. longimana exhibited attraction to all dark s ectors, except a 5 degrees dark rectangle. Orientation away from large dark rectangles subtending angles of 90-350 degrees was exhibited onl y by A. pisonii. These different responses are interpreted to reflect the demands for shelter and predator avoidance in the habitat of each species. Chlorodiella longimana responded to all dark objects as if th ey were refuge sites, while A. pisonii differentiated between refuge a nd predators on the basis of horizontal extent.