Adaptive Visually-Directed Orientation inUca pugilator

Abstract

Sand fiddler crabs are ill-equipped to inhabit either the terrestrial or permanently submerged littoral zones. They are obligate inhabitants of the intertidal region in which, at any point, extremes of conditions continually fluctuate. The crabs must constantly move about to specific areas in order to carry out life-supporting processes while avoiding detrimental physical conditons. For this reason, directional orientation is basic to their existence. The typical activities of adult sand fiddlers during diurnal low tides have charateristic directional components. Some directed movements have immediate survival value under conditions of stress, such as the approach of a predator. For example, crabs on the lower beach (well away from their burrow area) often run landward and enter burrows or vegetation, thereby obtaining refuge. Those which are chased offshore or inland are able to reorient to the beach. Experiments conducted both in the field and under controlled conditions indicate that these adaptive oriented movements are guided primarily by visual mechanisms. Adult crabs exhibit a time-compensated, menotactic orientation to the sun and polarized sky-light that enables them to maintain a heading coincident with the landward compass bearing of their particular shoreline. New directional preferences can be induced by holding crabs in simulated habitats with specific shore-water spatial configurations. The crabs also exhibit a telotaxis toward gross landmarks, such as mangroves or clumps of beach grass, that stand in optical contrast to the background. This orientation occurs in most individuals if celestial cues are obscured or if the animals become desiccated. Upon reaching the object, crabs enter suitable interstices affording refuge. Since they can orient by either celestial cues or landmarks, or both, there are probably few times in nature when they are disoriented from lack of guidance-stimuli.