UNDERWATER ORIENTATION IN THE SAND FIDDLER CRAB, UCA PUGILATOR

¹ Section of Neurobiology and Behavior, Cornell University, Ithaca, New York 14853

The semi-terrestrial sand fiddler crab, Uca pugilator, occasionally is forced by avian and mammalian predators to go into the water where it becomes vulnerable to aquatic predators. Therefore, it would be adaptive for Uca to possess some means of detecting the direction of the shore and its burrow while submerged. Using crabs from Florida and Long Island, New York, the identity of the cues used in underwater orientation and the possible hierarchial arrangement of these cues were ascertained.

Under various cue regimes, the crabs were individually observed in a long narrow tank that allowed the crabs to proceed either toward shore or away from it. The potential cues for shoreward orientation which were available to the submerged crab were celestial cues, landmarks, and gradient cues (hydrostatic pressure, light wavelength, light intensity and substrate slope). Each cue was isolated and then tested for its effectiveness in orientation. It was next presented to the crabs in a conflicting configuration with other cues to determine its relative effectiveness.

The crabs oriented toward shore when presented with either celestial cues, landmarks, or gradient cues. Celestial cues dominated both landmarks and gradient cues, while no clear dominance was observed between landmarks and gradient cues. Gradients of hydrostatic pressure, light wavelength, and/or light intensity were found to be ineffective cues for orientation. This suggests that the gradient cue used for shoreward orientation is the substrate slope. The behavioral differences between Florida and Long Island crabs were also examined.