Effects of whelk (Nucella lapillus (L.)) predation on mussel (Mytilus trossulus (Gould), M. edulis (L.)) assemblages in tidepools and on emergent rock on a wave-exposed rocky shore in Nova Scotia, Canada

The whelk Nucella lapillus is the most abundant predator of intertidal mussels (Mytilus trossulus and M. edulis) on rocky shores along the Atlantic coast of Nova Scotia, Canada. Environmental differences among intertidal habitats, such as tidepools and emergent rock, may influence the intensity of predation and its effect on community structure. We manipulated densities of both recruits (< 5 mm shell length, SL) and post-recruits (≤ 5 mm SL) of N. lapillus in tidepools and on emergent rock to examine the effects of whelk predation on mussel assemblages on a rocky shore near Halifax, Nova Scotia. Mussels > 10mm SL were more abundant in plots where the density of whelk post-recruits was reduced than in control plots where their density was not manipulated. Percentage cover of mussels remained stable where the density of post-recruits was reduced but declined in control plots, more so on emergent rock than in tidepools. This between-habitat difference probably reflects differences in the density of whelk post-recruits since feeding rates of whelks enclosed in cages did not differ significantly between tidepools and emergent rock. Predation by whelk post-recruits could not fully account for the reduction in mussel cover and abundance on emergent rock or in tidepools. This discrepancy is probably due to dislodgment by wave action of mussels killed by whelks, as well as the live mussels surrounding the empty shells. We could not detect an effect of recently recruited whelks on mussel cover or size distribution. Laboratory experiments indicated that the size of Nucella lapillus could be predicted from the diameter of the drill hole they create when feeding on a mussel. In the laboratory, feeding rate was linearly related to body size for recruits but not for post-recruits. Mean size of mussels consumed increased with increasing whelk size for both recruits and post-recruits. In the field, the size distribution of shells drilled by post- recruits differed from that of live mussels, but the distribution of shells drilled by recruits was generally similar to that of live mussels. Analysis of field-collected drilled shells indicated that whelk post-recruits will prey on mussels as small as 2 mm SL, while recent recruits can prey on mussels as large as 22 mm SL. Given the relatively high feeding rate of recent recruits and broad size range of mussels they consume, we conclude that these small whelks are potentially important in limiting mussel recruitment on these shores.