Algal life-history strategies and resistance to digestion

Abstract

The capacity to resist digestion is emerging as a widespread adaptation of benthic algae, espec~allyamong some opportunistic forms. This study examines differences in digestion resistance among macroalgae having bfferent degrees of tissue differentiation and different life strategies. Opportunistic Chlorophyta and Bangiophycidae are able either to release protoplasts or to regenerate new tissues from undigested algal remains, suggesting 100 % grazing mortality to be a rare event among these species. In species with differentiated reproductive structures, swarmer and protoplast release does not occur in algal fragments: survival depends on the capacity of menstematic cells to pass through the digestive tract undamaged. Sunrival after digestion is higher among densely branched, filamentous fornls with apical cell or diffuse growth, and lower in late successionists where meristematic tissues are restricted to particular regions of the thallus. Reproductive cells in all species with differentiated reproductive organs show greater sensitivity to digestion than vegetative, meristematic cells. Since resistance to digestion is closely related to morphological organizahon, life-history phases in heteromorphic species (e.g. leafy and filamentous phases of Porphyra) may have completely different sensitivity to digestion. Overall, these results suggest that algae presently recognized as opportunistic forms include species with very different morphological and reproductive responses to grazing. bkewise algal mortality induced by grazing vanes significantly among species of grazers. Some grazers induce high algal mortality while others stimulate swarmer and protoplast release, allowlng survival of apical and intercalary meristems. Some of these grazers may have important ecological roles in dispersing, redistribubng and even increasing the number of macroalgal propagules being dispersed.