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).