"Fishes" are the most numerous and diverse of major vertebrate groups. "Fish" are the dominant vertebrates in water. The total number of species is most likely in region of 28,000 in approximately 30 orders and 400 families.
Size is measured in terms of lengths and mass. A variety of lengths are measured: total length, standard length, volume1/3.
Longitudinal form is quantified as the fineness ration, FR. usiform, elongate and ovate, truncated, or gibbose.
Cross-sectional form is described in terms of the relative sizes of depth and width. Verbal descriptors are: compressed, depressed, and round.
There is no special terminology to describe the caudal peduncle, and its relative size is probably most easily expressed as a percentage of the maximum depth of the body. In general, a narrow caudal peduncle minimizes recoil during cruising and sprinting, and hence tends to be smallest in cruisers such as tuna and other pelagic species. The caudal peduncle may also be streamlined to minimize drag as it moves from side-to-side. The caudal peduncle tends to large in fish specialized for lunging at prey, for example cottids. Sometimes the caudal peduncle shows special modification. For example it is very narrow in sticklebacks, probably to facilitate nest ventilation during reproduction.
Most fish have scales. Modern agnathans lack scales. Elasmobranchs have placoid scales. Less derived actinopterygians, have ganoid scales. More recent fishes have bony-ridged scales, cycloid, or ctenoid.
The fins of fish vary in structure, distribution, and shape. Modern fishes have fin-patterns based on two median dorsal fins, one median ventral (anal) fin, a caudal (tail) fin, and two sets of paired fins, the anterior pectoral fins and usually more posterior pelvic fins. Elamsobranch fins are supported by ceratotrichia. Ray-finned fish fins are supported by rays. Sometimes several anterior soft-rays in less derived malacopterygian fishes are fused to create a spine; e.g. catfishes. More derived acanthopterygian fishes have thickened, usually sharp-pointed spiny rays, given them the name of spiny rayed fishes.
Fins may be described as: truncated (square), indented (e.g. trout), forked (e.g. many minnows and shiners), rounded (e.g. sculpin, bowfin), or pointed (e.g. lamprey, eel). The slenderness of the fin is quantified as aspect ratio, AR.
The orientation of the fin base also affects how the fins are used. Fins used rowing tend to have more vertical fin-bases. Wing-like fins tend to beat vertically, and the fin-base tends to be more horizontal.
The location of paired fins varies among bony fishes. In less derived fishes, including the soft-rayed malacopteryigian teleosts, the pectoral are ventral and anterior and the pelvic fins are ventral and abdominal. In the more derived spiny-rayed acanthopterygian fishes, the pectoral fins are lateral. The pelvic fins have moved forward. The pelvic fin location is described as thoracic when they lie beneath the pectoral fins, jugular when below the gill cavity, and mental when under the chin or eye.
The most prominent sense systems are usually the eyes.
Fish have a pair of external nostrils which open into a nasal pit containing an olfactory rosette. Fish have taste buds in the mouth. Barbels are common.
Some fish can sense electric fields.
Currents and pressure variations are sensed by the acoustico-lateralis system. The ear is the major pressure sensor, used for hearing.
The
mouth itself may terminal (at the anterior tip of the head; e.g. many minnows),
inferior (opening ventrally; e.g. suckers) or superior (opening dorsally; e.g.
topminnows). The mouth may have lips and barbels of various sizes. The size of
the mouth is measured as the gape.
Fish have many teeth on dorsal and ventral bones of the mouth, skull, and gills. Tooth shape roughly relates to diet. Teeth may be: cardiform, villiform, incisorform, molariform, or canine.
The anterior faces of the gill arches have gill rakers.
The gills are the primary organs for aquatic gas exchange in juvenile and adult fish. The skin is sufficient for gas exchange in many larvae. Some fish have structures for exchanging gases with the air.