Visual detection of predators by the sand fiddler crab, Uca pugilator

The purpose of this study was to determine the aspects of visual stimuli that the fiddler crab, Uca pugilator, uses to detect its predators. The study was conducted under natural conditions on a beach near Naples, Florida. During low tide predator models made of styrofoam were presented to crabs closely associated with a burrow. Shape, position, movement, and proximity to the crab were aspects of the models that were manipulated. Crabs were also exposed to conspecifics retreating from a predator model and their reactions observed. Escape reactions were quantified by measuring the time spent in the burrow after a stimulus was presented.

Plover and ellipsoid shaped models elicited the same response while rectangular models elicited a greater response. It is suggested that model contrast with the background or model novelty is important in this response. Models elevated abve the ground on a dowel leg were more effective than models situated directly on the ground. Moving models always elicited the same strong response regardless of model shape. This suggests that movement is an extremely important cue for predator detection. Crab response to a still model declines exponentially with increasing distance to the model. This suggests that the angular size of the model may be an important cue for detection of predators.

Crabs respond to retreating conspecifics by returning to the entrance of their burrows. The response disappears if the retreeating crabs are over 50 cm away. This suggests that crabs may be using the behavior of other members of their group to aid in the detection of predators.