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

Orientation of the hermit crab Clibanarius vittatus (Bosc) to black geometric 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 potential habitability of gastropod shells represented by the shapes. Test crabs were untrained and exhibited chemically induced shell-seeking behavior. 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 rectangle, horizontal diamond, square, semicircle, and triangle, but not to a vertical rectangle or a vertical diamond, When exposed to pairs, the most and least attractive shapes were the horizontal rectangle and vertical diamond, respectively. Since responses were a complex function of the dimensional characteristics of the shapes, it was concluded that attraction was based on shape recognition. The hypotheses were further supported because crabs oriented poorly to the shape of the suboptimal shell which they inhabited (triangle), but oriented very well to shapes (horizontal rectangle, horizontal diamond, square, and semicircle) that represented more optimal gastropod shells.