Ontogenetic changes in foraging behaviour of the dogwhelk Nucella lapillus (L.)

The distribution of dogwhelks among habitats on an intertidal reef was determined by quadrat sampling. Dogwhelks were categorized into early juveniles of 5.0-9.9 mm shell height, smaller juveniles 10.0-14.9 mm, larger juveniles 15.0-19.9 mm, subadults 20.0-24.9 mm and adults > 25.0 mm. Juveniles and subadults were much more abundant on the side of the reef exposed to wave action, whereas adults were about equally distributed on the exposed and sheltered sides. More dogwhelks were present in shaded than in open habitats, but there was a tendency of early juveniles to be found more commonly in the open. Early juveniles, subadults and adults were more abundant below mean tide level, whereas smaller and larger juveniles were slightly more abundant above. This may correspond with sequential upshore and downshore shifts in distribution during ontogeny. Foraging behaviour of smaller juveniles, larger juveniles, subadults and adults was monitored on thirty successive, daytime low tides. All sizes of dogwhelks were more active during neaps, although the difference between activity on spring and neap tides was greater in juveniles and subadults than in adults. Changing weather conditions during the study period had little effect on foraging activity. Dogwhelks foraged in bouts of 1-2 days, between which they took refuge in crevices. Bout length increased with size of the dogwhelk. During ontogeny, mussels became more predominant in the diet, accounting for 75% of consumed flesh in smaller juveniles and 90% in adults. Prey size generally was correlated with predator size. Adult dogwhelks, however, took smaller barnacles, perhaps owing to local scarcity of larger individuals of these prey. Ontogenetic changes in foraging behaviour were interpreted in terms of increasing energy demand as the body grows. A foraging bout continues until a threshold amount of flesh has been ingested. Thus, bout length was found to be volumetrically related to size of the dogwhelk. The time needed to fill the gut will be minimized by selecting larger prey, hence the observed dietary shift towards larger mussels. The vertical shift in distribution on the shore during ontogeny, may represent a compromise between the benefits of high consumption rate, maximizing productivity, and of avoiding predators, maximizing survivorship during critical phases of the life history.