Distribution, population structure and feeding of subtidal seastars in the northern Gulf of St. Lawrence

Abstract

The distribution and size-structure of seastar populations in the Mingan Islands, northern Gulf of St. Lawrence, were examined using quantitative sampling by SCUBA Asterias vulgaris and Leptasterias polaris are the most abundant seastars. In both species, individuals measuring 1 to 5 cm in diameter are concentrated in the rocky subtidal zone, a t a slightly greater depth for A. vulgans (4 to 7 m) than for L. polaris (0 to 3 m). With increasing size and probably the onset of sexual maturity, both species aggregate near the low tide level and join concentrations of several earlier year classes. This shift in distribution is most marked for A vulgaris. Finally, adults measuring > 15 to 20 cm in diameter are associated with mixed and sediment substrata at greater depths. The aggregation in shallow water upon attaining 5 cm in diameter probably occurs because the seastars are attracted to the mussel beds in this zone, whereas the shift to habitats at greater depths at size 15 to 20 cm may b e caused by a change in food preference towards larger prey which are concentrated in the sediment zone (e.g. large bivalves). Crossaster papposus and Solaster endeca are rarer species and also show different patterns of size partitioning related to depth and substratum. Seastars appear to be the dominant predators In the northern Gulf and probably play an important role in determining community structure. A variety of interactions (competition, predation, facilitation) exist between A. vulgaris and L. polaris, and between these seastars and the most abundant benthic carnivore, the whelk Buccinum undatum.