Diel variation of intertidal foraging by<i>Cancer productus</i>L. in British Columbia

Abstract

Scuba was used at high tide to observe the foraging activity of Cancer Productus L. on the rocky shores of British Columbia. Intertidal foraging occurred most frequently on shores protected from wave action. In the latter areas crabs displayed marked diel shifts in density, size, and sex ratio. Crabs did not forage intertidally at low water, but increased from 0·015 m-2 to 0·15 m-2 between day and night high tides. Day foraging crabs were predominantly males (86%) and significantly larger than night foraging crabs of either sex. Females were more common (64%) in the night. Juveniles and mating pairs occurred in the intertidal zone only on nocturnal high tides. Size/sex differences in movement patterns appear to be an important feature of the life histories of some cancrid crabs.