Feeding rates of Littorina striata and Osilinus atratus in relation to nutritional quality and chemical defenses of seaweeds
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Algae frequently suffer attack by herbivores and in some cases complete destruction has been observed. Despite the role of herbivores in structuring marine algal communities, herbivore food choice and the algal traits that herbivores use to choose foods are poorly documented. There has been much discussion and disagreement regarding the best method of assessing food preference. Experiments designed to detect preferential feeding involve offering a selection of food types to one or more individual consumers. Ideally, the potential foods are presented simultaneously to the consumer within a single experimental arena so that the consumer has the opportunity to express a dietary choice. However, some experimental designs allow individual consumers to feed for a constant time on individually presented alternative foods, which permits comparisons of feeding rates on these different foods. Such an experiment is not a true feeding preference experiment because no food choice is provided, but the results can reflect differences in attractiveness or palatability of various potential foods that may also be expressed when choices are offered. In this work, "discrimination" in selection of algae was tested in two invertebrate species common on intertidal rocks in Gran Canaria (Littorina striata and Osilinus atratus) by measuring their feeding rates on 20 common algal species in no-choice laboratory experiments. The algae represented the major floral elements of the local intertidal community. L. striata and O. atratus exhibited high feeding rates on Enteromorpha muscoides and Ulva rigida, whereas Alsidium corallinum and Laurencia corrallopsis were refused. The majority of algal species were readily consumed by these two generalist herbivores. No clearly defined trends in the concentrations of ash, nitrogen, carbon, soluble carbohydrates, soluble proteins, and caloric content in the seaweeds were found to account for these differences in feeding rates. In contrast, feeding rates varied according to the presence of secondary metabolites. The low susceptibility to grazing by snails in the least-consumed algae seems to be associated with the presence of secondary metabolites. We suggest that feeding rates of L. striata and O. atratus on algae are primarily due to avoidance of algal chemical defenses, whereas positive aspects of food quality probably play a lesser role in determining consumption.
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