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 al...

Mixed species diets enhance the growth of two rocky intertidal herbivores

“…Because we held the total amount of algae offered to each herbivore constant, the amount of each algal species consumed was less in mixture than in the corresponding single species diet treatments (the most preferred species composed, at most, 50% of diet in mixtures). As a result, the greater performance of herbivores on mixed diets was not solely the result of including a single high-quality food, but was instead likely due to complementarity among algal species in nutritional benefit or dilution of algal chemical or morphological features that negatively impact herbivore fitness (Kitting 1980, Pennings et al 1993, CruzRivera & Hay 2003. Crabs preferred and had the highest growth on Cladophora columbiana and Ulva sp.. Because 72% of the total consumption in mixed diet treatments was of these 2 species, the mixing of these 2 species may be important for crabs.…”

“…Additionally, herbivore-deterrent chemistry or morphology that can reduce performance in some herbivore species, such as phenolics in Pelvetiopsis limitata and dimethylsulfoniopropionate in Ulva spp. (Kitting 1980, Steinberg 1985, Van Alstyne et al 1999, 2009), could be less harmful when diluted in mixed diets. Our experiments did not explicitly test for the mechanisms underlying this complementarity, but we will discuss possible characteristics of the algae that might underlie this effect.…”

Interplay of encrusting coralline algae and sea urchins in maintaining alternative habitats

“…Some authors have reported that encrusting algae rely on the presence of herbivores to prevent overgrowth by erect forms (Kitting 1980, Paine 1980, Steneck 1982. Others (Bertness et al 1983, Sebens 1983, Masaki et al 1984, Johnson & Mann 1986, Dethier 1994 and references therein) have shown that crusts were able to reduce settlement of potential competitors, suggesting that their persistence does not strictly depend on the control of erect species by grazers.…”

Biological interactions and their role in community structure in the rocky intertidal of Helgoland (German Bight, North Sea)

“…Considering the dominant species, their feeding habits and mechanisms of adaptions to environmental factors, four types of biological interactions may influence the community patterns: (1) P r e d a t i o n (see Connell, 1970, I975;Dayton, 1971Dayton, , 1975Paine, 1974Paine, , 1976Menge, 1976Menge, , 1978aMenge, , 1978bMenge, , 1983Menge & Sutherland, 1976Connell & Slatyer, 1977;Lubchenco & Menge, 1978;Hughes, 1985;Marsh, 1986;Petraitis, 1987). (2} H e r b i v o r y (sometimes also called "grazing", though this means something different to a lotof scientists: see Paine & Vadas, 1969;Dayton, 1971Dayton, , 1975Connell & Slatyer, 1977;Lubchenco, 1978Lubchenco, , 1980Lubchenco, , 1982Lubchenco, , 1983Lubchenco & Menge, 1978;Kitting, 1980;Lein, 1980;Lubchenco & Cubit, 1980;Underwood, 1980Underwood, , 1984aUnderwood, , 1984bLubchenco & Gaines, 1981;Robles & Cubit, 1981;Underwood & Jernakoff, 1981;Gaines & Lubchenco, 1982;Steneck, 1982;Bertness et al, 1983;Hawkins & Hartnoll, 1983;Cubit, 1984;Hartnoll & Hawkins, 1985;Jernakoff, 1985a…”