The distribution and abundance of the intertidal prosobranchs Littorina scutulata (Gould 1849) and L. sitkana (Philippi 1845)

An attempt was made to explain the distribution and abundance of the intertidal prosobranch snails Littorina scutulata (Gould, 1649), and L. sitkana (Philippi, 1845) on beaches near the city of Vancouver, in the Gulf Islands and on the west coast of Vancouver Island, British Columbia. L. scutulata has a planktonic dispersal stage and is widely distributed. L. sitkana develops directly from benthic egg masses and tends to be more restricted in its distribution. The egg masses of L. sitkana are susceptible to desiccation at low tide, and consequently this species thrives best in damp places such as mud flats, tide pools and crevices. L. sitkana appears to be selected against in wave-swept places, because it offers more resistance to wave action than does the comparatively streamlined L. scutulata. Experimental manipulation of densities and species composition indicated that food limitation may take place in the summer (decreased growth rates at higher densities of snails and low food abundance) but not in the winter. Density dependent survivorship and natality were found for L. sitkana, indicating that a regulatory mechanism may be operating. L. scutulata showed no such density dependent response at the densities examined. The presence of L.scutulata reduced the survival of L. sitkana and vice versa in comparison to control populations consisting of each species alone. Possible evolutionary survival strategies and competitive relationships of these two species are discussed.Science, Faculty ofZoology, Department ofGraduat