Food Selectivity and Diet Switch Can Explain the Slow Feeding of Herbivorous Coral-Reef Fishes during the Morning Ruth Khait 1,2 *, Uri Obolski 3 , Lilach Hadany 3 , Amatzia Genin 1,2 1 The Interuniversity Institute for Marine Sciences of Eilat, Eilat, Israel, 2 Department of Ecology, Evolution & Behavior, the Hebrew University of Jerusalem, Jerusalem, Israel, 3 Department of Molecular Biology and Ecology of Plants, Tel-Aviv University, Tel Aviv, Israel 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. Citation: Khait R, Obolski U, Hadany L, Genin A (2013) Food Selectivity and Diet Switch Can Explain the Slow Feeding of Herbivorous Coral-Reef Fishes during the Morning. PLoS ONE 8(12): e82391. doi:10.1371/journal.pone.0082391 Editor: Gabriele Sorci, CNRS, Universite ´ de Bourgogne, France Received May 24, 2013; Accepted November 1, 2013; Published December 17, 2013 Copyright: ß 2013 Khait et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This research was supported by a grant from the Israel Nature and Park Authority and by a grant 840/08 from the Israel Science Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: ruth.khait@mail.huji.ac.il Introduction 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. Accordingly, animals tend to maximize their foraging effort. For example, many diurnal fishes, including planktivores [2], coralivores [3], and benthivores [4] start feeding at high rates as soon as light levels allow prey detection, short time after sunrise. On the other hand, 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 [5–10]. 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. Here we examine the hypothesis that the fishes’ diurnal feeding pattern is an outcome of a shift in their diet from selective feeding on rare, highly preferable algae in the morning to a general, bulk feeding on abundant but less preferable species in the afternoon. The raise in algal energy content due to photosynthesis can further promote the switch since it narrows the gap in nutritional value between the algae. Our hypothesis is based on reports from temperate seas showing that for many herbivorous fish, the quality, rather than quantity of the food is the key determinant of its value [12–16]. Methods Ethics statement This study was carried out under a permit from the Israeli Nature & Parks Authority (permit #2009/32928) and strictly complied with the regulations of the Hebrew University Commit- tee for experiments with animals. Location The study was carried out across the 1–15 m depth range of the fore-reef in front of the Steinitz Marine Biology Laboratory in Eilat, Israel (29u369N, 34u569E), northern Gulf of Aqaba, Red Sea [4,17]. Our work focused on the two most abundant herbivorous fishes in the region: the surgeonfishes (acanthurids) Acanthurus nigrofuscus (brown surgeonfish) and Zebrasoma xanthurum (yellowtail surgeonfish). Observations The fishes’ diurnal feeding behavior was recorded during four diurnal intervals: 0.5 h after sunrise and every 3 hours afterwards, after the fish arrived at the study site and were foraging; that is, past their migration from their nocturnal shelters to the feeding PLOS ONE | www.plosone.org 1 December 2013 | Volume 8 | Issue 12 | e82391