Nearly all fish have a streamlined body plan, 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, (absent in sharks) 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 many fish is covered with scales. Some species are covered instead by scutes. Others have no outer covering on the skin; these are called naked fish. Most fish are covered in a protective layer of slime (mucus).
The lateral line is a sense organ used to detect movement and vibration in the surrounding water. It consists of a line of receptors running along each side of the fish.
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.
Photophores are light-emitting organs which appears as luminous spots on some fishes. The light can be produced from compounds during the digestion of prey, from specialized mitochondrial cells in the organism called photocytes, or associated with symbiotic bacteria, and are used for attracting food or confusing predators
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 dorsal fin serves to protect the fish against rolling and assist in sudden turns and stops.
- 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. This fin is used to stabilize the fish while swimming.
- 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 Salmonidae, characins and catfishes.
- 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, usually composed of scutes (see below), that provides stability and support to the caudal fin. There may be a single paired keel, one on each side, or two pairs above and below.
- The "horns" of manta rays and their relatives are called cephalic fins.
- Finlets are small, rayless, non-retractable, fins between the last dorsal and/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.
- 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.
- Ganoid scales are flat, basal-looking scales that cover a fish body with little overlapping. They are typical of gar.
- Cycloid scales are small oval-shaped scales with growth rings. Bowfin and remora have cycloid scales.
- 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.
Another, less common, type of scale is the scute, which is:
- an external shield-like bony plate, or
- a modified, thickened scale that often is keeled or spiny, or
- a projecting, modified (rough and strongly ridged) scale, usually associated with the lateral line, or on the caudal peduncle forming caudal keels, or along the ventral profile.
Some fish (eg pineconefish) are completely or partially covered in scutes.
- The gas bladder, or swim bladder, is an internal organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth, ascend, or descend without having to waste energy in swimming. It is often absent in fast swimming fishes such as the Tuna and Mackerel families.
- The gills, located under the operculum, are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. They are not usually visible, but can be seen in some species eg the frilled shark.
- Gill rakers are bony, finger-like projections of the gill arch filaments which function in retaining food organisms.