Clutch Sizes of Two Marine Snails with a Changing Food Supply

When food becomes more abundant, most species can produce more offspring, and individuals that do so should be more fit than ones that do not. Fecundities of two marine snails, thais lamellosa and Thais emarginata, were monitored to determine how these snails responded in increases in their common food supply. For both species, clutch size is directly proportional to body size. Each T. lamellosa spawns once each year, and therefore annual fecundity is proportional to body size. Adults responded to a steady increase in food supplies, 1968—1973, by growing, and consequently, the average clutch increased from 930 to 1,428 eggs/♀. In contrast, T. emarginata adults remained about the same size. However,individuals spawned many times during the year, and spawning frequency was closely correlated with food supply. At Shady Cove, each T. emarginata ♀ produced about as many eggs as each T. lamellosa ♀ during 1969 and 1970. The species that responds to increasing food supplies by growing also has a longer juvenile period, a larger adult size, and a longer life—span than the species that responds by spawning more frequently. The high—stress upper shore environment of T. emarginata precludes a long juvenile period, and, therefore dictates clutch size and spawning frequency. Each clutch requires 23—37% of the prespawning body weight, and requires a higher proportion of the body weight of larger snails.