Observations on defensive interactions between predatory dogwhelks, Nucella lapillus (L.) and mussels, Mytilus edulis L.

Abstract

Dogwhelks, Nucella lapillus may be trapped by byssi of the mussel Mytilus edulis. Previous studies have suggested that such trapping contains elements of selectivity, deliberation and cooperativity. The following hypotheses were tested by field and laboratory studies: 1) that dogwhelks avoid high mussel population densities; 2) that mussels trap dogwhelks but ignore other gastropod species; 3) that mussels selectively attach to particular parts of the dogwhelk shell so that dogwhelks are overturned; 4) that dogwhelks can break free from byssal entrapment. The data collected supported hypotheses 1 and 4, but not hypotheses 2 and 3. It was found that mussels could snare dogwhelks and winkles, Littorina littorea, but that winkles could escape within minutes, while dogwhelks took several hours to break free. In both species, escape was achieved simply by sustained pulling of the threads until they snapped. Nucellas lapillus could escape from attachment by at least 18 byssi; Littorina littorea from at least 14. Permanent entrapment occurs when byssi attach from different directions (and different mussels). Dogwhelks avoid dense mussel cover, but exploit the margins of mussel beds, where they may break free of byssal attachment. Dogwhelks at the margins of such beds are significantly smaller than Nucellas lapillus living on barnacle-encrusted rock, and dogwhelks trapped within the matrix of mussel beds are smaller still. Dogwhelks at the margins of mussel beds had a mean of 5.15 ± 6.71 (S.D.) byssi attached to the shell; the value for N. lapillus within the mussel matrix was 24.00 ± 19.46. During field surveys a novel dogwhelk microhabitat, beneath mussel hummocks, was discovered. The area surrounding the dogwhelks was without mussel attachment to the rock. The mussel hummock was underlain by barnacle and mussel shell debris which retained moisture efficiently and in which large dogwhelks (not snared by byssi), often several beneath each hummock, were encountered. Overall it is clear that entrapment of N. lapillus by Mytilus edulis is a more stochastic, less selective process than claimed previously.