Regulation of Morphological Plasticity in Tropical Reef Algae by Herbivory

Many tropical algal species exhibit considerable morphological variation associated with different coral reef habitats, but the factors contributing to such variation have not been identified. Two strikingly distinct morphologies are described here for the common Caribbean alga Padina jamaicensis. These distinct morphologies are characteristically found in different reef habitats, and are shown to represent phenotypic responses to different levels of herbivorous fish grazing. Experimental reduction of grazing intensity in a high—herbivory reef habitat resulted in a rapid (96 h) morphological shift from a prostrate, highly branched turf morphology to an erect foliose morphology. Transplant experiments indicated that foliose Padina plants were preferentially consumed by herbivorous parrotfishes. These results suggest that morphological plasticity may represent an important adaptive strategy in some tropical algal species, enabling plants to persist in intensely grazed reef habitats while maintaining the ability to respond rapidly to spatial or temporal fluctuations in herbivory.