Size-related decrease in spatial refuge use by Iceland scallops Chlamys islandica: ontogenetic behavioural changes or decreasing refuge availability?
David J. Arsenault, John H. Himmelman*
We examined whether the size-related shift from refuges to exposed surfaces by Iceland scallops Chlamysislandica in the Mingan Islands, northern Gulf of St. Lawrence, eastern Canada, is caused by behavioural changes in their tendency to use
refuges or by decreasing refuge availability as they increase in size. A laboratory experiment indicated that the frequency of refuge use did not vary with scallop size, when the entrance to available refuges was sufficiently large, which suggests that
the tendency of scallops to use refuges does not change during ontogeny. The size of crevices used increased with scallop size, indicating that the size structure of refuges potentially could determine size-specific refuge use. Our field observations
using SCUBA indicated that the availability of suitably sized refuges (shells of the bivalve Spisula
polynyma) decreased markedlywith increasing scallop size. Size-related changes in the frequency of refuge use were
positively correlated with refuge availability, suggesting that the shift from refuges to exposed surfaces is caused by decreasing availability of suitably sized refuges as scallops increase in size. However, the density of refuges was generally greater
than the density of scallops, which suggested that refuges were utilized below their carrying capacity. The proportion of adequately sized refuges which were occupied by scallops was low (<15%) and decreased with increasing scallop size, possibly because
scallops had increasing difficulty in locating refuges of suitable size as they increased in size. That the shells used as refuges by scallops covered only 6.4% of the bottom should decrease the probability of their being found. Further, because the
frequency of suitable refuges decreased with increasing scallop size, scallops should progressively have increasing difficulty in locating refuges. This would likely increase the time scallops are exposed to predators while searching for refuges, which is
likely to be critical for small scallops given their high vulnerability to predators. Hence, encounter rates with refuges could potentially produce a demographic bottleneck in respect to survival of recruits.
Chlamysislandica · Scallop · Habitat · Refuges · Crevices · Size · Scaling