Adaptive Visually-Directed Orientation in Uca pugilator

Sand fiddler crabs are ill-equipped to inhabit either the terrestrialor permanently submerged littoral zones. They are obligate inhabitantsof the intertidal region in which, at any point, extremes ofconditions continually fluctuate. The crabs must constantlymove about to specific areas in order to carry out life-supportingprocesses while avoiding detrimental physical conditons. Forthis reason, directional orientation is basic to their existence. The typical activities of adult sand fiddlers during diurnallow tides have charateristic directional components. Some directedmovements have immediate survival value under conditions ofstress, such as the approach of a predator. For example, crabson the lower beach (well away from their burrow area) oftenrun landward and enter burrows or vegetation, thereby obtainingrefuge. Those which are chased offshore or inland are able toreorient to the beach. Experiments conducted both in the field and under controlledconditions indicate that these adaptive oriented movements areguided primarily by visual mechanisms. Adult crabs exhibit atime-compensated, menotactic orientation to the sun and polarizedsky-light that enables them to maintain a heading coincidentwith the landward compass bearing of their particular shoreline.New directional preferences can be induced by holding crabsin simulated habitats with specific shore-water spatial configurations. The crabs also exhibit a telotaxis toward gross landmarks, suchas mangroves or clumps of beach grass, that stand in opticalcontrast to the background. This orientation occurs in mostindividuals if celestial cues are obscured or if the animalsbecome desiccated. Upon reaching the object, crabs enter suitableinterstices affording refuge. Since they can orient by eithercelestial cues or landmarks, or both, there are probably fewtimes in nature when they are disoriented from lack of guidance-stimuli.