Abstract
Most herbivorous coral-reef fishes feed slower in the morning than in the afternoon. Given the typical scarcity of algae in coral reefs, this behavior seems maladaptive. Here we suggest that the fishes' slow feeding during the morning is an outcome of highly selective feeding on scarcely found green algae. The rarity of the food requires longer search time and extended swimming tracks, resulting in lower bite rates. According to our findings by noon the fish seem to stop their search and switch to indiscriminative consumption of benthic algae, resulting in apparent higher feeding rates. The abundance of the rare preferable algae gradually declines from morning to noon and seems to reach its lowest levels around the switch time. Using in situ experiments we found that the feeding pattern is flexible, with the fish exhibiting fast feeding rates when presented with ample supply of preferable algae, regardless of the time of day. Analyses of the fish's esophagus content corroborated our conclusion that their feeding was highly selective in the morning and non-selective in the afternoon. Modeling of the fishes' behavior predicted that the fish should perform a diel diet shift when the preferred food is relatively rare, a situation common in most coral reefs found in a warm, oligotrophic ocean.
Highlights
Most of the world’s coral reefs are found in oligotrophic seas, where low nutrient concentrations restrict primary production [1], rendering food availability a potential limiting factor for benthic herbivores
Our work examines the reasons for the slower feeding rates exhibited by herbivorous coral-reef fishes in the early morning, Figure 3
The results of our in situ experiments imply that the fish switch their feeding behavior from selective consumption of rare but highly preferable food items in the morning to a generic, non-selective diet that includes less preferable, albeit more abundant, items in the afternoon
Summary
Most of the world’s coral reefs are found in oligotrophic seas, where low nutrient concentrations restrict primary production [1], rendering food availability a potential limiting factor for benthic herbivores. Many herbivorous fishes in both coral reefs and temperate habitats exhibit a diurnal pattern consisting of slow feeding during the morning, gradually increasing later in the day [5,6,7,8,9,10]. Taborsky and Limberger [11] were the first to suggest that this diurnal pattern optimizes the fish’s feeding efforts, as the increase in feeding rate coincides with a diurnal increase of the energy content in the algae. Zemke-White et al [10] corroborated the hypothesis, showing that the content of starch and floridoside, main sources of edible energy for herbivorous fishes, gradually increases after the initiation of photosynthesis in the morning, and reaches high values in the afternoon
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