Feeding Electivity Indices in Surgeon Fish (Acanthuridae) of the Florida Keys
IAAAM Archive
G. Christopher Tilghman; RuthEllen Klinger; Ruth Francis-Floyd
University of Florida Department of Fisheries and Aquatic Sciences, Gainesville, FL


Three species of surgeonfish A. canthurus coeruleus, A. bahianus, and A. chirurgus were captured in the waters of the Florida Keys (24°03'N, 81 °40'W). Items found in the stomachs of these fish were identified and analyzed for percent occurrence. These values were compared to percent occurrence values of the same items from random bottom transects taken at the point of capture to quantify any forage selectivity or avoidance behavior of these fish. A. bahianus selected for sand and chlorophytes and avoided phaeophytes. A. coeruleus also showed selection for sand and chlorophytes, while some A. chirurgus sampled selected for phaeophytes and others avoided them. A. coeruleus avoided sand and selected for rhodophytes, phaeophytes, and chlorophytes.


Acanthurids are an important group of reef fishes, both because of the ecological impact of herbivores on reef ecology,2 and also because of their popularity as aquarium fish. The acanthurids are extremely abundant tropical and subtropical marine fish. Along with parrotfish (Scaridae) and damselfish (Pomacentridae), these animals are mostly herbivores, and together they form the largest part of the fish biomass of most reefs.5

Acanthurids are browsers, with lips and dentition for snipping off the tips and branches of algae. They also have long thin-walled digestive tracts and some species have a sand-filled, muscular gizzard-like stomach. These are all adaptations for foraging on relatively soft algal filaments and blades. Tangs, particularly when young, have a stringent requirement to feed almost continuously, undoubtedly because of a relatively poor utilization of their algal food.5 While the long, thin-walled intestine is probably well adapted to absorbing the contents of crushed algal cells; it may be poorly suited to handling cellulose. This has considerable bearing on their survival in model ecosystems, since they must have a food source that allows almost continuous foraging.1

The purpose of the reported study was to identify the natural foods of Atlantic acanthurids that have economic value to Florida's ornamental fish trade. This information is part of a larger study designed to define normal parameters for free-ranging acanthurids from Florida waters. The long-term goal of this work is to improve captive husbandry of acanthurids and other herbivorous reef species. This paper will describe surgeonfish feeding behavior in their natural habitat, and quantify their selectivity and avoidance of various forage items.

Materials and Methods

Fish and transect data were collected at several sites near Marathon Key, Florida (24°03'N, 81°40'W), on reefs using scuba gear. Once a suitable site was chosen, a diver would lay a 25 m transect line in a random direction and record all substrata and benthos (to the phylum level) along the line. The distance along the line that each phylum was present was also recorded. These data were later analyzed for percent occurrence.

A barrier net was deployed in a "C" shape for capturing the surgeonfish. A diver would locate a surgeonfish and attempt to corral it into the net, at which time the fish was collected with a hand net and transferred to a ventilated holding box on the bottom. Once ashore, captured fish were euthanized with buffered MS-222 (tricane methanesulfonate).

Stomach contents were analyzed for percent occurrence for each phylum present for each fish. Ivlev's electivity index4 for food selection was employed to quantify any selectivity or avoidance in the surgeon fishes' feeding behavior. The relationship is defined as E = (ri - pi) / (ri - pi), where E is the measure of electivity, r the relative abundance of prey item i in the gut (percent occurrence), and pi he percent occurrence of the same item along the transect measured on the bottom. The index has a possible range of 1 to - 1 , with 1 indicating active selection, zero indicating random selection or no consumption of the item, and-1 indicating avoidance of the item.4


A. bahianus had the most (seven) phyla present in their stomachs. This species has a very muscular stomach with a relatively small lumen, which was always found full of sand. A. chirurgas also showed selection for sand. Its muscular stomach is similar to that of A. bahianus, and was similarly filled with sand, among which were a few bits of algae. Strong selection for chlorophytes was observed. A. coeruleus differ from the previously discussed species in that they have a thin-walled, relatively high volume stomach. These fish clearly avoided sand and strongly selected rhodophytes, phaeophytes and chlorophytes.


Results of this study demonstrate that of three species of Atlantic surgeonfish studied, there were clear differences in natural diet and preference for different food items. The gross anatomy of the stomach and feeding habits of the A. chirurgas and the A. bahianus were similar, however the A. coeruleus was markedly different from the other species.

The stomachs of the ocean surgeonfish and doctor fish were grossly similar in that both species had a very muscular stomach with a relatively small lumen. These species both actively ingested calcareous sand, and the sand made up most of the material in the lumen of all samples taken. In addition to the ingestion of sand, the diet of these species was more varied than the blue tang, with foodstuffs from seven phyla being identified. One may speculate that they gather essential nutrients from these organisms that are absent in a strictly vegetarian diet. Alternately, it may be that these items have been incidentally eaten while browsing on algae, and not selected for at all. These items, which may be missing from commercially available diets, may be important for optimal health of captive acanthurids.

In contrast, the stomach volume of the blue tang was comparatively large, and the structure was characterized as thin-walled and tubular. The blue tangs actively avoided ingestion of sand. These animals had a demonstrated preference for red algae, which was surprising as it is considered to have a lower nutritional value than other algal groups. 3 It is possible that by seeking out rhodophytes they are trying to meet some unknown nutritional requirement.

These findings may eventually for captive acanthurids be applied to the improvement of husbandry programs and diets


The authors wish to thank: Florida Sea Grant, Roy Hemdon, John Than, Charles "Chuck" Cichra, Chris and Jane Bowen, and Brian Bowen.


1.  Adey, H. and L. Loveland. 1991. Dynamic Aquaria. Building Living Ecosystems. Academic Press, San Diego, Pp. 643.

2.  Carpenter, R.C. 1986. Partitioning herbivory and its effects on coral reef algal communities. Ecological Monographs 56"343-363.

3.  Montgomery, W.L. and S.D. Gerking. 1980. Marine macroalgae as foods for fishes: an evaluation of potential food quality. Environmental Biology of Fishes 5:143-153.

4.  Strauss, R.E. 1979. Reliability estimates for Ivlev's electivity index, the forage ratio, and a proposed linear index of food selection. Transactions of the American Fisheries Society 108:344-352.

5.  Thresher, R. 1980. Behavior and ecology on the reef and in the aquariums. Palmetto Publishing, St. Petersburg, FL, Pp. 171.

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G. Christopher Tilghman

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