Aspects of the ecology of an exposed shore population of dogwhelks Nucella lapillus (L.)

Summary

Up to the third year of life Nucella lapillus (L.) could be aged in the laboratory and in the field using shell characters. Monthly counts along transects on an exposed rocky shore have shown that the age groups behave differently, dogwhelks in their first year migrating upshore, and returning to lower levels in the second year. Adults (three years or older) lived and laid egg capsules in the low shore. The most favourable situation for subsequent hatching of these capsules was where they were permanently submerged in sea water.

The first requirement of recently hatched young was probably protection from water movement, but they soon gegan feeding on small mussels. The shell height attained by the end of the first winter was related to winter temperatures, but the main growing season was June to November. Adult size was attained in two years, after which the individuals did not usually grow, but those which did grow during the third or succeeding years may have been nonbreeders or have been castrated by the trematode Parorchis acanthus.

The age structure of the population suggested that mortality rate decreased with age, and this was confirmed by calculating annual mortality rates for the first three years of life. Heavy mortality in the low shore during the first winter was due mainly to predation by purple sandpipers. In succeeding winters mortality was low, and predation, the main mortality factor, was restricted to the summer months. At this time Cancer pagurus was the most voracious predator, but it was active mainly in the low shore, and the upshore distribution of second year dogwhelks, which were preferred to other age groups, age groups also led to a more economical use of environmental resources, namely food, winter aggregation sites and breeding sites.