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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.

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Differences in the vertical distribution of seaweed spores in a 20 m water column 30 km off the coast of North Carolina were documented. Spores were collected on plastic, and glass slides that were subsequently transferred to incubator cultures where germlings were grown and counted. Spores of green algae and bangiophycidean red algae were collected throughout the water column, whereas spores of brown algae and florideophycidean red algae occurred almost exclusively near the bottom. The occurrence of some kinds of spores in the upper water levels suggests relatively large dispersal shadows for those species. The taxa recovered in the upper levels were typically opportunistic organisms, many of them epiphytes. The collection of many Enteromorpha individuals in the upper and mid levels was of particular interest since locally this genus only occurs in abundance in shallow water habitats 35 km or more from the study site.

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