The adaptive significance of aggregation behaviour in the dogwhelk Nucella lapillus (L.)

Summary Three kinds of aggregation behaviour were observed in an exposed shore population of Nucella lapillus. Aggregations on the open rock surface during the summer protected dogwhelks from water movement, and were not found on shores where the topography conferred protection. Feeding occurred mainly within these aggregations, probably because individuals experienced less disturbance there than when isolated. Physical contact was important in holding animals together.Winter and pre-breeding aggregations were usually found in clefts or pools. All age groups formed winter aggregations, but those of immature dogwhelks were not as permanent as those of adults, and the latter merged with the prebreeding aggregations. Winter aggregations protected dogwhelks from dislodgment when their ability to regain a foothold was reduced by low temperatures, while pre-breeding aggregations brought the sexes together for fertilization, but the permanence of adult winter aggregations suggested that reproductive activity may have been occurring within them.