CHEMICALLY STIMULATED VISUAL ORIENTATION AND SHAPE-DISCRIMINATION BY THE HERMIT-CRAB CLIBANARIUS-VITTATUS (BOSC)

Orientation of the hermit crab Clibanarius vittatus (Bosc) to black ge ometric shapes of equal surface area was measured in a circular arena. This study tested the hypotheses that crabs can visually discriminate between different shapes and that attraction is related to the potent ial habitability of gastropod shells represented by the shapes. Test c rabs were untrained and exhibited chemically induced shell-seeking beh avior. This behavior indicated that they inhabit suboptimal gastropod shells (small relative to their body size). When presented with single silhouettes, crabs were significantly attracted to a horizontal recta ngle, horizontal diamond, square, semicircle, and triangle, but not to a vertical rectangle or a vertical diamond, When exposed to pairs, th e most and least attractive shapes were the horizontal rectangle and v ertical diamond, respectively. Since responses were a complex function of the dimensional characteristics of the shapes, it was concluded th at attraction was based on shape recognition. The hypotheses were furt her supported because crabs oriented poorly to the shape of the subopt imal shell which they inhabited (triangle), but oriented very well to shapes (horizontal rectangle, horizontal diamond, square, and semicirc le) that represented more optimal gastropod shells.