Analysis of Y-axis orientation in an intertidal crab, Hemigrapsus nudus
Date
2018-08-01
Authors
Pasek, George J.
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Abstract
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.
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Keywords
Intertidal, crabs, Vancouver Island