Foraging patterns of two species of intertidal herbivorous fishes: Effect of food abundance and chemical composition

Herbivores are organisms that must meet their energy and nutrients requirem ents from a food source that typically has been characterized as having low nutritional value given their low concentration of nutrients and energy. T wo of the most important questions in relation to fish herbivory are: i) wh ich are the factors that determine the selection or rejection of a given al gal item? and ii) an herbivorous fishes capable of extracting the nutrients and energy of a macroalgal diet? In this work, we studied in two species o f herbivorous intertidal fishes, Scartichthys viridis and Girella laevifron s, the patterns of food selectivity in the field and in laboratory experime nts, the assimilation efficiency for different dietary algal items, and the relationship between the observed patterns and the chemical composition of the algae. The results showed that more than 90% of the diet of these orga nisms consisted of benthic macroalgae. In the field both species present a non-selective trophic behavior in summer and selective one in winter, chara cterized by the consumption of green algae in the later season. Furthermore , in the experiments of food selection both species showed a similar patter n characterized by the preference of green and red algae. The results of th e assimilation experiments, indicate that Girella laevifrons presents highe r values of this parameter than Scartichthys viridis, being in the former t he green algae Ulva and Enteromorpha, the items that present a higher effic iency of assimilation. Finally, the results obtained suggest in this herbiv orous species a strong relationship among the patterns of food selection an d the relationship between food composition and digestive characteristics.