The effect of predation on the life histories of Littorina obtusata and Littorina mariae

Abstract

Littorina obtusata is an epiphytic gastropod which lives for 3-4 yr on its host Ascophyllum nodosum, a long-lived brown alga. Most of the population consists of adults, present throughout the year. Newly hatched individuals appear between April and October with a peak in July; the immature winkles grow to merge with the adult cohort in May. Littorina mariae is an annual; newly hatched individuals grow to maturity by the winter of their first year. Those reaching maturity overwinter as adults but very few live beyond spring of their second year. The population dynamics of L. mariae are closely linked with changes in the biomass of the alga Fucus serratus on which it lives. Both species suffered heavy losses to crabs on the low shore, where predation pressure was highest. Crabs on the low shore indiscriminantly ate both species of winkles, including juvenile and adult L. obtusata. Predation may limit the downshore extension of L. obtusata. Littorina mariae survives this intense pressure by maturing whilst predation pressure is reduced (due to the crabs overwintering offshore) and their shells being thickened prior to reproduction. -Author