J. Exp. Mar. Biol. Ecol., 149 (1991) 109-122 © 1991 Elsevier Science Publishers B.V. 0022-0981/91/$03.50 109 JEMBE 01598 Trapping of three drilling whelks by two species of mussel R.W. Day, A. Barkai and P.A. Wickens Zoology Department, University of Cape Town, Rondebosch, South Africa (Received 27 July 1990; revision received 30 January 1991; accepted 18 February 1991) Abstract: The mussels Choromytilus meridionalis (Krauss) and Mytilus galloprovincialis (Lamarck) coexist with the predatory drilling whelks NuceUa cingulata (Linnaeus), N. dubia (Krauss) and N. squamosa (Lamarck) on the west coast of southern Africa. Samples revealed that these whelks are trapped by byssus threads in intertidal mussel beds. Nucella of each species were enclosed with high and low densities ~f each mussel. Both mussels attached byssal threads to large numbers of all the Nucella species, and trapped some of those attacked. When M. galloprovincialis were enclosed with the non-drilling whelks Burnapena catarrhacta (Gmolin) and B. limbosa (Lamarck), the mussels attached few threads to the Burnapena, and did not trap any. C. meridionalis, which has weaker threads than M. galloprovincialis but produces them more rapidly, trapped Nucella more effectively when in dense clumps, and was most effective in trapping N. squamosa, which is the most common species of Nucella in beds of C. meridionalis. Small numbers of M. galloprovincialis trapped Nucella as effectively as dense clumps. M. gailoprovincialis has strong byssus threads, and appears to aggregate rapidly into clumps. These features may have been important in its recent invasion of rocky shores in the region. Key words: Choromytilus; Invading species; Mytilus gailoprovincialis; Nucella; Prey defense; Rocky shore INTRODUCTION Drilling whelks are found in association with mussels worldwide, and are regarded as major mussel predators, in some cases controlling the population size and zonation pattern of their prey (Seed, 1976; Griffiths, 1981; Wickens & Griffiths, 1985). Laboratory observations by Wayne (1980, 1987) and Carriker (1981) have revealed that Mytilus edulis Linnaeus respond to the presence of Nucella spp. by attaching byssus threads to these drilling whelks, and can immobilise them. This appears to be a defensive behaviour directed specifically at drilling whelks (Wayne, 1987). Whether other mussel species can trap their predators is not known. Both Wayne (1987) and Petraitis (1987) noted that Nucella spp. are often found with broken byssus threads attached to the shell, suggesting that they have escaped entrapment by mussels. Thus to trap their predators the mussels must both attach byssus threads to them and prevent them from breaking free, presumably by attaching enough threads within a short time to restrain the snail. Conversely the Nucella may survive by preventing the attachment of threads, or by breaking free before many threads are attached. Another strategy may be to avoid the mussels. In Petraitis" (1987) study of trapping ofNucella lapillus (Linnaeus) by M. edulis Correspondence address: R.W. Day, Zoology Department, University of Melbourne, Parkville, 3052, Australia.