TIDAL RHYTHM AND THE ROLE OF VISION IN SHELTER-SEEKING BEHAVIOR OF THE HALF-CRAB PETROLISTHES ELONGATUS (CRUSTACEA, ANOMURA, PORCELLANIDAE)

Based on observations and tests in the field, we conclude that during the day Petrolisthes elongatus has a considerably greater tendency to conceal itself at low rather than high tide. We provide evidence that vision is involved in detecting potential rock shelters. To small crab s (carapace lengths no more than 5 mm) a small rock close by, is appar ently more attractive than a bigger one at greater distance subtending the same sector of are. In large crabs (carapace length at least 15 m m) a tendency for the reverse was seen with bigger as well as smaller rocks close by being less attractive than similar shelters 1 m away. W e believe that the greater desirability of more distant targets in lar ger crabs is related to their greater physical stamina and visual sens itivity to extended sources. This allows them to run away from a distu rbance further than a small crab.