Fish anatomy

Fish anatomy is primarily governed by the physical characteristics of water, which is much denser than air, holds a relatively small amount of dissolved oxygen, and absorbs light more than does air.

Nearly all fish have a streamlined body, which is divided into head, trunk, and tail, although the dividing points are not always externally visible.

The head includes the snout, from the eye to the forwardmost point of the upper jaw, the operculum or gill cover, and the cheek, which extends from eye to preopercle. The lower jaw defines a chin. The head may have several fleshy structures known as barbels, which may be very long and resemble whiskers. Many fish species also have a variety of protrusions or spines on the head. The nostrils or nares of almost all fishes do not connect to the oral cavity, but are pits of varying shape and depth. The outer body of the fish is covered with scales.

The caudal peduncle is the narrow part of the fish's body to which the caudal or tail fin is attached. The hypural joint is the joint between the caudal fin and the last of the vertebrae.

The fins are the most distinctive features of a fish.

• The dorsal fins are located on the back. A fish can have up to three of them. There are two types of dorsal fin rays, spiny and soft. A fin can contain only spiny rays, only soft rays, or a combination - if the latter the spiny rays are always anterior. These comments about fin rays also apply to the anal, pectoral, and pelvic fins.
• The caudal fin is the tail fin, located at the end of the caudal peduncle.
• The anal fin is located on the ventral surface behind the anus.
• The paired pectoral fins are located on each side, usually just behind the operculum, and are homologous to the forelimbs of tetrapods.
• The paired pelvic or ventral fins are located ventrally below the pectoral fins. They are homologous to the hindlimbs of tetrapods.
• The adipose fin is a soft, fleshy fin found on the back behind the dorsal fin and just forward of the caudal fin. It is absent in many fish families, but is found in characins and catfishes.
• The "horns" of manta rays and their relatives are called cephalic fins.
• Some types of fast-swimming fish have a horizontal caudal keel just forward of the tail fin. This is a lateral ridge on the caudal peduncle that provides stability and support to the caudal fin. There may be a single keel or a pair above and below.
• Finlets are small rayless non-retractable fins between the last dorsal or anal fin and the caudal fin. They are found on fast swimming fish such as tuna.

For every fin, there are a number of fish species in which this particular fin has been lost during evolution.

There are four types of fish scales.

1. Placoid scales, also called dermal denticles, are similar to teeth in that they are made of dentin covered by enamel. They are typical of sharks and rays.
2. Ganoid scales are flat, basal-looking scales that cover a fish body with little overlapping. They are typical of gar.
3. Cycloid scales are small oval-shaped scales with growth rings. Bowfin and remora have cycloid scales.
4. Ctenoid scales are similar to the Cycloid scales, with growth rings. They are distinguished by the spines that cover one edge. Halibut have this type of scale.