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ABSTRACT Visually directed movements of juveniles of the mangrove crabs Aratus pisonii and Chlorodiella longimana and postlarvae (megalopae) of A. pisonii to black horizontal rectangles subtending visual angles from 5-350° were measured in a circular arena. The study tested the hypothesis that early stage crabs use visual cues for attraction to potential 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 attracted by relatively narrow rectangles subtending angles up to 30°. In contrast, juvenile C. longimana exhibited attraction to all dark sectors, except a 5° dark rectangle. Orientation away from large dark rectangles subtending angles of 90-350° was exhibited only 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 they were refuge sites, while A. pisonii differentiated between refuge and predators on the basis of horizontal extent.


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