W. Lindsey Zemke-White ® J.H. Choat ® K.D. Clements A re-evaluation of the diel feeding hypothesis for marine herbivorous fishes Received: 24 August 2001 / Accepted: 11 March 2002 / Published online: 17 July 2002 Ó Springer-Verlag 2002 Abstract We re-evaluated the ‘‘diel feeding hypothesis’’ by measuring diel variation in starch, protein, and flor- idoside in three algal ‘‘types’’ collected from a fringing coral reef at Lizard Island, Great Barrier Reef, Austra- lia. Samples of two species of rhodophyte algae, Graci- laria arcuata and Acanthophora spicifera, and the turf assemblage from the territories of the herbivorous pomacentrid Stegastes nigricans were collected at four time periods through the day: 0630–0730, 1000–1100, 1330–1430, and 1630–1730 hours. We also measured the ability of several species of marine fish (the herbivores Acanthurus nigricans, A. lineatus, A. olivaceus, and Parma alboscapularis and the detritivore Ctenochaetus striatus) to hydrolyse floridoside by estimating a-ga- lactosidase activity in tissue from the anterior intestine. We detected no diel pattern in protein content of the algae but found a significant steady increase in starch content throughout the day. Floridoside content in- creased in the morning and decreased in the afternoon, a pattern that may be driven by midday photoinhibition of the algae. All the fishes tested could utilise floridoside. Our results support the diel feeding hypothesis. Al- though floridoside content decreased in the afternoon, our results suggest floridoside was used during the day by the algae to synthesise starch. Thus the algae in- creased in nutritional value until photoinhibition oc- curred at midday then subsequently maintained their nutritional value during the afternoon. This pattern of algal nutrients increasing to a midday peak and re- maining relatively constant throughout the afternoon correlates well with the diel feeding pattern in many species of marine herbivorous fish. Introduction Optimal foraging theory (MacArthur and Pianka 1966) predicts that natural selection will favour individuals that spend their foraging time most efficiently by ob- taining the best diet possible. Diel feeding periodicity, whereby feeding activity is greater in the afternoon than in the morning, has been observed in 20 species of ma- rine herbivorous fish (Table 1). Taborsky and Limberger (1980) suggested that the diel feeding pattern of her- bivorous fishes is an optimal foraging strategy, that the nutritional quality of algae is greater in the afternoon than in the morning, and that it is this temporal distri- bution of nutritional value that is driving the observed feeding patterns. A critical point that has not been ad- dressed to date is that, to support this ‘‘diel feeding hypothesis,’’ any observed diel variation in algal com- ponents must have nutritional significance for the fish; that is, it is not enough simply to measure algal com- ponents – it must also be determined that the fish can digest and assimilate those components. To date there is little evidence for a correlation be- tween feeding rate and algal nutritional value. Three studies have examined the diel feeding hypothesis. Polunin and Klumpp (1989) inferred an accumulation of photosynthetic carbon throughout the day by measuring the net oxygen flux of algal turf assemblages. They suggested that the fish might be targeting photosynth- ates. However, this must be contrasted with findings that many macroalgal species undergo photoinhibition (a reversible decrease in the effective photosynthetic quantum yield) at around noon, with recovery only coming in the late afternoon (Haeder et al. 1998). In addition, even if algae are shown to build up photo- synthetic carbon during the day, to support the diel Marine Biology (2002) 141: 571–579 DOI 10.1007/s00227-002-0849-y Communicated by G.F. Humphrey, Sydney W.L. Zemke-White (&) ® K.D. Clements School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand E-mail: l.zemke-white@auckland.ac.nz Tel.: +64-9-3733599 ext. 8483 Fax: +64-9-3737414 J.H. Choat School of Biological Sciences, James Cook University, Townsville, Queensland 4811, Australia