Herbivore‐Plant Interactions of Individual Limpets Maintaining a Mixed Diet of Intertidal Marine Algae

Abstract

Plate limpets, Acmaea (Notoacmea) scutum, are common in central California on mid—intertidal vertical faces, where they ignore erect algae and consume primarily two encrusting algae, Petrocelis middendorffii (=P. franciscana) and Hildenbrandia occidentalis. This mid—intertidal system offers a manageable diversity of species, slow—moving consumers, and a two—dimensional, stationary food resource. Such an assemblage of organisms enables detailed studies on food selection within localized sets of available foods, and provides an example of how an animal utilizes a particular, mixed assortment of foods as it forages. Previous theories of diet selection predict that an animal will maintain a fixed hierarchy of preferences; less preferred foods would be consistently passed over if a preferred food is encountered frequently. However, individual Acmaea scutum preferentially consume mixtures of foods, even when each of these foods is readily available. Adjacent individuals tend to select the same mixture of foods while sharing the same foraging area, without intraspecific partitioning of food species. A family of simple graphic models illustrates hypothetical tendencies to select a particular (proportioned) mixture of foods, and predicts how food preferences would differ where relative abundances of the foods differ. Avoiding either too much or too little of a given type of food, relative to other foods, yields a mixed diet. Unlike foraging patterns leading to increased diversity of coexisting foods, maintaining a mixed diet involves seeking out separate components of the diet, particularly when a component is locally rare. Hypothetically, this would drive rare foods toward local extinction, decreasing diversity of available foods within each foraging area. The models of food selection were tested in the natural environment using adult limpets on a series of isolated rocks where percentage cover (probability of random encounter) of each food differed. Feeding of labeled individuals on these rocks was observed repeatedly during successive high tides. Acmaea scutum is seen to fit the mixed diet model with about 60% of its algal diet consisting of Petrocelis and 40% Hildenbrandia; this is true over a wide range of availabilities of these two foods. Maintaining this mixed diet appears to avoid excessive tooth wear associated with eating tough Hildenbrandia, though this food is palatable in limited quantities. Hypothetical consequences on the food resources due to diet mixing were tested by manipulating population densities of limpets on a series of boulders, and determining overall effects of foraging on various sets of algal abundances. However, even unusually high experimental densities of limpets failed to decrease algal abundances detectably. Algal declines on both experimental and control boulders are attributed to physical factors. Simple underwater listing techniques reveal feeding activities of individual adult molluscs. Contrary to visual observations, local molluscs almost always bite part way into the macroscopic algae rather than simply brushing off and consuming microscopic epiphytes. Time lapse photographs of individual limpets indicate that their feeding usually causes negligible damage to these encrusting algae, but the limpets sometimes revisit specific foraging sites and produce visible wounds on the plants. Monthly photographs of damaged plants indicate that slow vegetative reproduction, not new settlement, usually recolonizes patches of rock exposed by feeding. Such maintenance of mixed diets may evolve generally among animals that rarely decreases the abundance of available foods.