Jeffrey Walker for their love and support, and for their love of snorkeling which led to some of the best fish-times of our lives. Grantly R. Galland would like to thank Gale and Bud Galland for showing him his first fishes and teaching him their names. Finally, we would all like to thank our numerous mentors who over the years have schooled us in our unwavering appreciation of fishes.
ANATOMY OF FISHES
While their anatomy varies greatly, all fishes have several features in common. In this section, we briefly review and illustrate the major features of fish anatomy, focusing on those that are most important for distinguishing among lineages and groups.
External Anatomy
Several external regions of fishes have specific names.
SNOUT The area of the head between the tip of the upper jaw and the anterior margin of the orbit.
CHEEK The area of the head below and posterior to the eye, anterior to the posterior margin of the preopercle.
NAPE Dorsal area just posterior to the head.
OPERCULUM Plate-like structure covering the branchial chamber and consisting of four bones: the opercle, preopercle, subopercle, and interopercle.
BRANCHIOSTEGALS Slender, bony elements in the gill membrane, slightly ventral and posterior to the operculum.
ISTHMUS Area of the throat ventral to the gill openings.
LATERAL LINE Sensory system consisting of pores and canals along the head and body for the detection of vibrations and water movement, often associated with perforated scales along the body.
CAUDAL PEDUNCLE Area of the body between the insertions of the dorsal and anal fins and the base of the caudal fin.
ANUS (VENT) Terminal opening of the alimentary canal.
Body Shapes
Many fishes are somewhat elongate, laterally compressed, and oval in cross section. Several specialized shapes are recognized, including the following primary examples:
COMPRESSED Flattened laterally, sometimes strongly so, and often deep-bodied.
DEPRESSED Flattened dorsoventrally.
GLOBIFORM Rounded, often spherical.
ANGUILLIFORM Greatly elongate and usually tubular.
FUSIFORM Roughly bullet-shaped, often tapering both anteriorly and posteriorly.
Fins
The fins of fishes are either unpaired or paired. The unpaired fins, also called median fins, include the dorsal, anal, and caudal fins, as well as the adipose fin in some fishes. The paired fins include the pectoral and pelvic fins.
Fin-ray Elements and Dorsal-fin Configurations
The fins of actinopterygian fishes are composed of two types of rays: soft rays, which have evident segments, are bilaterally divided, are often branched, are typically flexible, and are usually connected by a fleshy membrane; and spines, which lack segments, are not bilaterally divided, are never branched, and are usually stiff and sometimes pungent. These fin-ray elements are derived from dermal tissues and are collectively called lepidotrichia. The dorsal fin of actinopterygians may be composed of soft rays only or of both spines and soft rays. In the latter case, the two parts of the fin may be continuous, separated by a notch, or completely separate. The fin rays of chondrichthyan fishes are flexible, unsegmented, and derived from epidermal tissues; they are called ceratotrichia.
Pelvic-fin Positions
The pelvic fins of fishes vary considerably in their position on the body, a feature useful in distinguishing many groups.
ABDOMINAL Inserted well posterior to the pectoral fins.
THORACIC Inserted slightly posterior to or directly under the pectoral fins.
JUGULAR Inserted slightly anterior to the pectoral fins.
MENTAL Inserted far forward, often near the symphysis of the lower jaw.
Caudal-fin Shapes
The caudal fins of fishes come in a variety of shapes that are roughly related to a species’ swimming behavior. Slow moving fishes often have rounded caudal fins, while fast swimming fishes have deeply forked fins with stiff upper and lower lobes. Most sharks and the early lineages of ray-finned fishes have a heterocercal caudal fin in which the vertebral column is deflected dorsally and extends along the upper, larger, caudal-fin lobe. Most ray-finned fishes have a homocercal caudal fin, which is externally symmetrical and supported by a series of laterally flattened bones. A few specialized groups such as the flyingfishes have a hypocercal caudal fin in which the lower lobe is larger than the upper lobe. Shapes of caudal fins include the following examples:
ROUNDED No sharp or straight edges, convex posteriorly.
TRUNCATE Posterior profile vertical.
EMARGINATE Upper and lower rays slightly longer than central rays.
FORKED Separate upper and lower lobes that join at a sharp angle.
LUNATE Crescent-shaped posteriorly, with extremely large upper and lower lobes.
HETEROCERCAL Vertebral column is deflected dorsally and extends along the upper, larger caudal-fin lobe.
Mouth Positions
In addition to the size of the gape and the size and type of teeth, the position of a fish’s mouth provides clues to its feeding habits. These include the following:
TERMINAL Mouth located at the tip of the snout.
SUBTERMINAL Mouth located below the tip of the snout.
INFERIOR Mouth opens ventrally, well posterior to the snout.
SUPERIOR Mouth opens dorsally.
Oral
