OBSERVATIONS ON DEFENSIVE INTERACTIONS BETWEEN PREDATORY DOGWHELKS, NUCELLA-LAPILLUS (L) AND MUSSELS, MYTILUS-EDULIS L

Dogwhelks, Nucella lapillus may be trapped by byssi of the mussel Myti lus edulis. Previous studies have suggested that such trapping contain s elements of selectivity, deliberation and cooperativity. The followi ng hypotheses were tested by field and laboratory studies: 1) that dog whelks avoid high mussel population densities; 2) that mussels trap do gwhelks but ignore other gastropod species; 3) that mussels selectivel y attach to particular parts of the dogwhelk shell so that dogwhelks a re 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, whil e dogwhelks took several hours to break free. In both species, escape was achieved simply by sustained pulling of the threads until they sna pped. Nucellas lapillus could escape from attachment by at least 18 by ssi; Littorina littorea from at least 14. Permanent entrapment occurs when byssi attach from different directions (and different mussels). D ogwhelks avoid dense mussel cover, but exploit the margins of mussel b eds, 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 ma trix of mussel beds are smaller still. Dogwhelks at the margins of mus sel beds had a mean of 5.15 +/- 6.71 (S.D.) byssi attached to the shel l; the value for N. lapillus within the mussel matrix was 24.00 +/- 19 .46. During held surveys a novel dogwhelk microhabitat, beneath mussel hummocks, was discovered. The area surrounding the dogwhelks was with out 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 bene ath each hummock, were encountered. Overall it is clear that entrapmen t of N. lapillus by Mytilus edulis is a more stochastic, less selectiv e process than claimed previously.