Welsh Journals

OBSERVATIONS ON THE ADAPTIVE SIGNIFICANCE OF SHELL SHAPE AND BODY FORM IN DOGWHELKS (NUCELLA LAPILLUS (L.) ) FROM N. WALES. R. SEED INTRODUCTION Dogwhelks belonging to the gastropod family Thaisidae are important predators of barnacles and mussels on rocky shores throughout the world. Many species are exceedingly variable in their shell shape and body form e.g. Nucella lapillus (L.) from the Atlantic coasts (Moore 1936, Crothers 1975, Kitching 1977), various species of Thais from the N.W. Pacific (Spight 1973, Kitching 1976), Dicathais from Australia (Phillips, Campbell and Wilson 1973) and Lepsiella from New Zealand (Kitching and Lockwood 1974). Much of this recorded variation is considered to have survival value. Whereas many marine organisms possess an effective dispersal phase dogwhelks emerge directly as juvenile stages from horny egg-capsules laid in crevices in the mid-low shore. The absence of a pelagic larval stage in their life history together with a general lack of any marked migratory behaviour enables dogwhelk populations to evolve quite independently in response to local selective pressures. In this paper the variability in shell shape and body form of Nucella lapillus from several shores in Wales will be examined and the possible adaptive advantages of such variability considered with respect to two potentially important selective forces wave action and predation. ABUNDANCE AND POPULATION STRUCTURE Nucella populations on two quite dissimilar shores in N. Wales were studied in some detail during July 1977. Cable Bay on the W. 'coast of Anglesey is a very exposed wave-swept shore. It is relatively steep and supports dense populations of barnacles on which Nucella (density 101 ± 52/ m2 ) feeds. Small mussels are also present but these are restricted to cracks and crevices in the rock surface. Severe wave action and absence of refuge sites makes this type of shore extremely unsuitable for crabs and predation pressure from this source is probably minimal: no crabs were found at this site despite several prolonged searches. The shore at Menai Bridge, on the other hand, is sheltered from direct wave action and is dominated by a variety of macroalgae. The latter, together with the many loose stones and rocks, provide an ideal refuge for various species of crabs, notable Carcinus maenas (L.) and Cancer pagurus L. which are here present at high densities (25± 5/standard 10 min search). Barnacles occur on the steeper rocky outcrops and on the bridge supports but mussels are absent. Although Nucella is considerably less abundant at this site (29116/ m2) those which do occur are here noticeably larger than at Cable Bay. All observable dogwhelks in replicated 0.5m2 quadrats thrown at random in the mid-low shore were collected at these two sites. Shell height (Fig. 1) was measured to the nearest 0.1mm using sliding vernier calipers. Figure 2 shows the percentage size frequency distributions of Nucella from the two shores. In shelter small dogwhelks were uncommon and the population here consisted mainly of large individuals (>2.60cm in shell height) many of which were relatively thick-lipped with pronounced "teeth" immediately