Heteromorphic Life Histories of Certain Marine Algae as Adaptations to Variations in Herbivory

Abstract

Many of the annual or ephemeral algae of the mid to high intertidal zones have heteromorphic life histories, existing as upright morphs during seasonal algal blooms and as crustose or boring morphs during other portions of the year. Experimental removal of herbivores on the coasts of New England and Oregon resulted in the occurrence of the upright morphs in the times of year when they were normally absent (summer in our areas), demonstrating that such uprights can survive the summertime physical regime (contrary to earlier speculation). We suggest that the upright and crustose or boring stages of these algae represent mutually exclusive adaptations to fluctuations in grazing pressure: the upright stages are adapted for high rates of growth and reproduction when grazing pressure is low, and the crustose and boring stages are adapted for surviving through times of high grazing pressure. We predict isomorphic species of algae would predominate in these sorts of habitats if grazing pressure were more constant.