Analysis of Y-axis orientation in an intertidal crab, Hemigrapsus nudus




Pasek, George J.

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Many animals that live near shorelines (X-axes) perfonn horizontal movements perpendicular (Y-axes) to the nearest shore, which is referred to as Y-axis orientation (Ferguson and Landreth, 1966). Shore-directed (Y-axis) orientation was studied in the intertidal purple shore crab, Hemigrapsus nudus. The crabs were collected fr001 several shorelines with different compass directions in southern British Columbia, Canada; i) in Barkley Sound, on the West Coast of Vancouver Island, ii) in the Nanaimo area on the east coast of Vancouver Island, and iii) in the Vancouver city area on the mainland coast. The directional preferences of the crabs were tested in orientation arenas. A seaward Y-axis orientation ability was established for crabs fr001 the Vancouver Island shorelines. Vancouver city crabs showed a bimodal orientation parallel to the beach (X-axis orientation). Potential factors influencing Y-axis orientation were investigated. The di recti anal preference and directivity of the orientation were not affected by the time of day, temperature, tidal cycles, level of activity, the moon, or the position of the observer. The crabs were presented with visual and non-visual orientation cues to determine the guiding mechanisms of the orientation. The results. indicate that visual cues present in clear day and night skies are important for Y-axis orientation since high cloud cover and experimental interference with the natural sky visibility severely disrupted the orientation ability. Y-axis orientation based on non-visual cues was tested by subjecting crabs to altered magnetic fields. Results of these and other experiments suggest that non-visual cues may also be used in Y-axis orientation. Transplant experiments were conducted in which crabs from one area were relocated to a new beach with a· different Y-axis. The directional preference in the seaward Y-axis direction of the original 11 home" beach persisted after 48 days on the new beach.



Intertidal, crabs, Vancouver Island