EVIDENCE FOR PREDATION AS A FACTOR IN DETERMINING SHELL COLOR FREQUENCIES IN A MANGROVE SNAIL LITTORINA SP. (PROSOBRANCHIA: LITTORINIDAE).

It was hypothesized that in Littorina populations living on Avicennia marina in Moreton Bay, Queensland, yellow shelled individuals are at a selective advantage over other shell colors and that this advantage is due to differential selection by predators. Yellow shelled individuals were more likely to be recaptured than others, indicating a higher survival rate of yellows. When predation was restricted on ten mangrove trees, the apparent advantage of yellow shells was removed. After 18 months, the relative frequency of yellow shelled individuals was significantly lower on experimental trees than on control trees. A combination of selection for crypsis and for the less common morph is suggested as the mechanism maintaining the high levels of variation in this species.