Orientation of the hermit crab Clibanarius vittatus (Bosc) to visual cues: effects of mollusc chemical cues

Abstract

Orientation of the striped legged hermit crab Clibanarius vittatus (Bosc) to black horizontal areas of different sizes was studied in a circular arena. Crabs were tested either in the presence or absence of chemicals that signal the potential availability of a gastropod shell. The chemical cue was generated by freezing and thawing muscle tissue from the gastropod Busycon carica Gmelin. Upon pre-experimental exposure to the chemical cue, crabs could be differentiated into two categories: those showing investigative interest in gastropod shells (Investigating), and those with no evident interest (Non-Investigating). The chemicals stimulated increased activity in the Investigating groups but not in the Non-Investigating groups. Clibanarius vittatus exhibited a uniform circular distribution when exposed to an all white horizon, independently of the presence of the chemical cue. Both crab categories showed little interest in black sectors subtending angles of 5, 10, 20 and 30° when the chemical cue was not present in the experimental arena. However, Investigating crabs were attracted to these sectors in the presence of the chemical cue. This response ceased once the dark sector exceeded ≈40° in size and both groups showed an avoidance response, with increasing number of crabs moving towards the white section of the arena as the dark sector increased in size. The attraction towards the small black sectors was interpreted as a shell investigating response, for which the visual cue seems to be activated by the chemical cue. The avoidance of increasingly larger black sectors was interpreted as predator avoidance reaction, which was not affected by the chemical cue. These interpretations were verified as Investigating crabs were attracted to a gastropod shell and both groups generally avoided a large blue crab.