Nutritional ecology of nominally herbivorous fishes on coral reefs

Abstract

Nominally herbivorous acanthurids (surgeonfishes) and scarids (parrotfishes) have often been considered a 'homogeneous' functional group that consumes and digests algae. Recent work demonstrates that many of these fishes consume detritus. The objective of this study was to investigate the composition of dietary nutrients targeted by these and other fishes in terms of feeding behaviour, diet and short chain fatty acids (SCFA). We undertook a nutritional analysis of a range of species including detritivores, algivores, omnivores and planktivores from north eastern Australia. We calculated assimilation efficiencies for total protein amino acids (TAA), carbohydrate and lipid, and measured TAA in gut fluid along the intestine. Nutrients were assimilated similarly to their dietary proportions, with planktivores assimilating a high proportion of TAA, a moderate proportion of lipid and little carbohydrate. Omnivores assimilated moderate proportions of TAA and carbohydrate, and a low proportion of lipid. Algivores assimilated a low proportion of TAA and lipid, but a high proportion of carbohydrate. Detritivorous scarids and acanthurids differed significantly from algivores, assimilating a high proportion of TAA, a low proportion of carbohydrate and a moderate proportion of lipid. TAA levels in gut fluid of all species were highest in the anterior and lowest in the posterior intestine. Gut segments with highest TAA values were compared between dietary groups and followed a similar trend to TAA assimilation. Planktivores had high concentrations of TAA, while omnivores had intermediate, and algivores the lowest, concentrations. The highest gut fluid TAA concentrations were found in detritivorous scarids and acanthurids, and were significantly higher than in algivores. A significant negative correlation was found between anterior intestinal fluid TAA and posterior intestinal SCFA values. Detritivores had the highest levels of TAA but the lowest levels of SCFA. Planktivorous species had high levels of TAA and low to intermediate levels of SCFA. Omnivores had moderate levels of both TAA and SCFA. Algivores had low levels of TAA but high levels of SCFA. This indicates that there are major differences in the food resources targeted by detritivorous and algivorous fish species on coral reefs.

Food habits of the farmer damselfishStegastes nigricansinferred by stomach content, stable isotope, and fatty acid composition analyses

“…On coral reefs, the lipid contents of Polysiphonia algae, detritus, and algal mats that consist mainly of filamentous rhodophytes are reported to be very low: 2.1-3.6% in dry weight for Polysiphonia (Montgomery and Gerking 1980), 0.8-1.0% for detritus (Wilson 2002, Crossman et al 2005, and 0.6-2.1% for algal mat (Montgomery and Gerking 1980, Wilson et al 2001, Crossman et al 2005. On the other hand, benthic animals such as copepods contain more lipids, i.e., 1.6-6.6% (Shansudin et al 1997) or 3.3-13.3% (Toledo et al 1999), which is 2-10 times as much as those in algal mats.…”

Ultraviolet sunscreens in reef fish mucus

“…Although speculative, the presence of high asterina-330 levels in both C. sordidus and L. dimidiatus mucus found here may explain this preference and suggest a potential source of MAAs in cleaner fish. Some parrotfish are known to eat algae and hard coral (Bruggemann 1994), which can contain MAAs (Sinha et al 2001), while others mainly eat detritus from the sand surface (Crossman et al 2005). The diet of Thalassoma lunare is mainly composed of benthic invertebrates and small fishes (Grant 2004), a food source with unknown MAA levels.…”

Ecosystem structure and processes at Kaloko Honokohau, focusing on the role of herbivores, including the green sea turtle Chelonia mydas, in reef resilience

“…Recent evidence from the South Pacific demonstrates that few of the fish species previously classified as 'herbivores' are indeed exclusively herbivorous (Choat et al 2002, Cvitanovic et al 2007). Many appear to predominantly feed on detritus, only incidentally removing turf from the reef in the process (Crossman et al 2001(Crossman et al , 2005. Data on the extent of algae removed but not consumed are needed to correct our current assumption that all algal material removed by herbivorous fish was ingested.…”