Marine Biology68, 299-319 (1982) MARINE BIOLOGY Springer-Verlag 1982 Feeding Capabilities and Limitation of Herbivorous Molluscs: A Functional Group Approach R. S. Steneck 1,2,, and L. Watling 3 1 Marine Systems Laboratory, Smithsonian Institution; Washington, D.C. 20560, USA 2 Department of Earth and Planetary Sciences, The Johns Hopkins University; Baltimore, Maryland 21218, USA 3 Department of Zoology and Oceanography Program, Darling Center, University of Maine; Walpole, Maine 04573, USA Abstract The susceptibility of an alga to an herbivorous mollusc depends, in part, upon the size and toughness of the plant relative to the feeding ability of the mollusc. In this study, algae are subdivided into seven functional groups based on these and other physiological characteristics. Herbivo- rous prosobranchs and chitons are subdivided into four functional groups based on the structure of their feeding apparatus. Distinct patterns in the diets of these molluscs are evident when feeding data, based on these functional groups, are examined. Most herbivorous mollusc species eat algal forms that are either minute (i.e., micro- and fila- mentous algae) or very large and expansive (kelp-like or crustose algae). Algae of intermediate size (erect forms 1- to 10-cm tall) are eaten to a lesser extent, possibly because they are too large to be rasped from the substratum and too small for most herbivores to occupy. Herbivorous archaeogastropods (excluding limpets) and mesogastro- pods tend to eat filamentous and microscopic algal forms predominantly, whereas limpets and chitons feed on large, leathery and crustose algae. These dietary differences reflect functional differences in the feeding apparatus of these herbivore groups. Radulae of herbivorous mesogas- tropods function like rakes and can ingest larger, tougher algae than can radulae of nonlimpet archaeogastropods. The latter function more like brooms by sweeping the substratum broadly, but exerting little force. Limpets and chitons have superior excavating abilities because their radulae have: robust buccal muscles surrounding them, a reduced number of points of contact on the substratum, and minerally hardened teeth. The feeding apparatus of chitons is most versatile since it possesses features found in all herbivorous gastropod functional groups, and thus, it can sweep and excavate simultaneously. This functional * Present address: Department of Zoology and Oceanography Program, Darling Center, University of Maine; Walpole, Maine 04573, USA group approach suggests various hypotheses concerning algal community structure, plant/herbivore and herbi- vore/herbivore interactions, the relative importance of structural defenses in algae, and the evolution of special- ized grazers. These hypotheses are examined using data from published accounts. Introduction The complexity and variability of natural communities often make descriptions, interpretations, and predictions of their structure difficult if not impossible. Although each species is thought to occupy a unique ecological niche (Hutchinson, 1957), commonly there are groups of species that utilize certain aspects of the environment in similar ways. That is, there are groups of functionally similar species (functional groups) that occupy similar adaptive zones (sensu, Stanley, 1979) although they may be geo- graphically and evolutionarily distinct. Recognizing such groupings may release ecologists from the necessity of studying individual species in a community in order to understand and predict the outcome of interspecific inter- actions and to interpret patterns in community structure. Furthermore, functional groups can be recognized in the fossil record, thereby facilitating interpretations of past community structure. Predators (both carnivores and herbivores) are known to affect the distribution, abundance, and fitness of their prey. The diets of predators vary, in part, because of dif- ferences in their feeding abilities relative to the differential susceptibilities of their potential prey. In marine com- munities, distribution patterns of benthic algae are heavily influenced by herbivory (Paine and Vadas, 1969; Paine, 1977; Lubchenco and Menge, 1978; Vance, 1979). In this paper, we examine functional group interactions between benthic marine algal prey and their molluscan herbivores. Specifically, we group herbivorous proso- 0025-3162/82/0068/0299/$ 04.20