Differential susceptibility to grazers in two phases of an intertidal alga: Advantages of heteromorphic generations

The intertidal red alga Gigartina papillata (C. Ag.) J. Ag. alternates between two major forms in its life cycle, an upright blade and a fleshy crust. The evolution and functional significance of such alternation of heteromorphic generations is little understood. This study, done on the outer coast of Washington state (U.S.A.), considers the rôle of physical factors, measured by intertidal position, and a biological factor, interactions with herbivores, as selective pressures potentially responsible for this pattern. Observations along transects and a herbivore exclusion experiment showed that the two phases of this alga simultaneously occupy similar intertidal positions but differ significantly and oppositely in their response to grazing. The number of crust plants declined in the absence of grazers, probably due to being overgrown and shaded by a thick algal film. It is, therefore, suggested that the crust form is grazer-dependent. In contrast, the number of blade phase plants increased in the absence of grazers. “Bet-hedging” is suggested in explanation of the maintenance of two phases in one cycle; the crust is more persistent but susceptible to overgrowth, while the more productive blade is more susceptible to grazing.