N. lapillus may be trapped by byssi of the mussel M. 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.