Introduction  

  Acanthurids are a circumtropical class of teleosts commonly known as surgeonfish or tangs. They are very popular in the aquarium trade due to their bright coloration and hardy disposition. These fish are laterally compressed with continuous dorsal and anal fins that end at the caudal peduncle. The class and genus names are derived from the Latin akanthos, meaning thorn. This is appropriate, as these fish possess very sharp spines on their caudal peduncle that can be everted and used defensively. Anyone who handles these fish should be wary of these spines because they are capable of inflicting deep wounds.

Acanthurids are an important group of reef fishes because of their impact as herbivores on reef ecology (Carpenter, 1986), and their popularity as display animals. Three species are found in the waters of the Florida Keys: Acanthurus coeruleus (blue tang), A. bahianus (ocean surgeon), and A. chirurgus (doctorfish). Along with parrotfish (Scaridae) and damselfish (Pomicentridae), these animals are primarily herbivores, and together they form the largest portion of fish biomass of reef systems (Thresher, 1980). Tangs are browsers, with lips and dentition for snipping off the tips and branches of algae. However, anatomical differences between species, particularly with the digestive tract is evident, leading one to suspect varying dietary preferences between these species. The blue tang has a long thin-walled digestive tract, while the doctorfish and ocean surgeonfish have sand-filled, muscular gizzard-like stomachs (Tilghman et al., 2001). Herbivorous fish are not known to produce cellulase or other enzymes to digest cell wall components (Lobel, 1981). However, they are capable of digesting the materials inside plant cells if they have developed mechanisms to break the cell walls. This can be done in two ways. One method is trituration, which occurs through chewing, or the material is masticated in a muscular, gizzard-like stomach. An alternative strategy is the use of acidic stomach secretions, typically secreted by thin-walled stomachs (Lobel, 1981). Acanthurids found in the Florida Keys possess both stomach types (Tilghman et al., 2001), so it is likely that they utilize both mechanisms to break algal cell walls.

This histological atlas focuses on A. coeruleus and A. bahianus and includes major organs and tissues. Particularly note the stomach tissues of both species, which illustrate the difference in digestive strategies of the Caribbean Acanthurids. Acanthurus chirurgus was intentionally left out of this atlas, as its tissues are very similar to those of A. bahianus.

The surgeonfish used for this atlas were obtained from the Florida Keys between 1999 - 2000. They ranged in size from 68 - 318 mm total length. Tissues were collected immediately after euthanasia with 1 g/L buffered methane tricaine sulfonate (MS-222) and fixed with commercially available 10% buffered formalin (Fisher Scientific, Atlanta, GA). Calcified structures found in skin and gill tissues were decalcified with Cal-Ex® (Fisher Scientific, Atlanta, GA) prior to histological processing. The tissues were then embedded in paraffin, cut into five micron sections and stained with 7211 hematoxylin and eosin with phloxine (Richard Allen Scientific Co., Kalamazoo, MI). Photographs of tissue were taken with an Olympus DP-11 digital camera (Olympus Optics); stomach photos were taken with a Nikon S2 digital camera (55mm Nikkor Optics).