Ostariophysans possess segmented, branched, flexible, soft rays in the fins, unlike the stiff spines of perchlike fishes. In some species, however, soft ray elements may fuse during development and give rise to a spinous ray (usually called a spine), commonly found in the dorsal and pectoral fins of most catfishes and in the dorsal and anal fins of some Old World cyprinids. The presence or absence of these spines may be diagnostic for genera and families.

An adipose fin consists of a small to elongated fleshy or fatty structure without fin ray supports, located dorsally between the rayed dorsal fin and caudal (tail) fin. It is present in most ostariophysan fishes.

Barbels

Diverse morphological differences in the mouth region are related to the type of diet and to the modes of locating, capturing, and ingesting food. Barbels are short to filamentous, fleshy, fingerlike projections located at the corners of the mouth or on the snout and chin of many suctorial and bottom-feeding fishes (some minnows, loaches, and catfishes). Barbels are highly sensitive to touch, and they bear numerous taste buds. Taste and touch probably function together in the selection of food before ingestion.

Teeth

Teeth may be present along the jaws, in the roof of the mouth, on the tongue, or in the pharynx, or they may be entirely absent. In the minnows (Cyprinidae) and suckers (Catostomidae), the mouth is toothless, but an array of teeth is borne on a pair of branchial bones, the lower pharyngeals, located in the throat. In the minnows the pharyngeal teeth, arranged in one, two, or three rows, press or bite against a horny pad in the roof of the mouth. They have undergone specialization paralleling the diversity found in jaw teeth of other fishes. Vegetarians such as the carp have grinding, molarlike teeth; carnivores have pointed or hooked teeth. Suckers have numerous pharyngeal teeth aligned in a single row. Oral and pharyngeal teeth are of great value in classifying many families of ostariophysans.

Secondary sexual characteristics

With the onset of the breeding season, many secondary sexual characteristics develop: size differences, nuptial coloration, enlarged and modified fins, breeding tubercles, and contact organs. These features are related chiefly to courtship and mating, but differences in size obviously play a role in guarding nests and care of the young; the sex that exercises parental care is usually the larger. Brilliant red, orange, yellow, green, and blue coloration may develop on various parts of the head, body, and fins, especially in the males. Some characins and cyprinids are among the most beautiful of all fishes. The male usually has larger and more brightly coloured fins than the female. In some characins, the median and pelvic fins of the males may possess small hooks or contact organs, which aid in maintaining contact with the female during spawning. In the cypriniforms, breeding tubercles, or pearl organs (epidermal excrescences), develop on the head, body, and fins of males under the influence of sex hormones. The tubercles function in maintenance of body contact during spawning, in defense of nests and territories, and possibly in the stimulation of females during breeding.

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Why Do Animals Have Tails?

Sexual differences among the siluriforms are more marked in the highly specialized families. Pelvic fins of female ariid catfishes and, to a lesser extent, of ictalurid catfishes show specialized developments whose functions are not yet fully known. Some male loricariid catfishes develop elaborate dermal, branching growths and spines around the head; in others, the lower lip is enlarged to accommodate the transport of eggs.

Swimming

The body of most ostariophysan fishes is more or less streamlined, taking the most efficient form for movement through water. In this highly diversified group, however, a large array of adaptations occurs. Lateral compression (flattened from side to side) is common, especially among characins and cyprinids that inhabit quiet, weedy lakes, ponds, and backwaters. Extreme examples are the flying hatchetfishes (Gasteropelecidae) and the knifefishes (Rhamphichthyidae and Apteronotidae). Depressed body form (flattened from top to bottom), especially in the head region, is widespread among fishes spending much time on or near the bottom or under rocks and similar objects (most catfishes) or among those inhabiting torrential mountain streams (Balitoridae, some Loricariidae). An elongated eel-like form has evolved in certain loaches (Cobitidae) and electric eels (Gymnotidae), fishes that live on soft, muddy, and sandy bottoms or in rock crevices.

The common form of locomotion among ostariophysans is swimming by lateral undulations of the body, resulting from the contractions of muscles along the sides of the body and base of the tail. These undulating flexures culminate in a powerful back and forth sweeping of the caudal fin, which produces as much as 85 percent of the total thrust. Some fishes have departed from the normal horizontal swimming posture. The headstanders (Anostomidae) move with the head pointing downward at a slant; some of the pencil fishes (Hemiodontidae) assume a tail-standing position. Most bizarre of all are the upside-down catfishes (Mochokidae) of Africa, which can swim either in the normal position or inverted, with the belly uppermost; in one species, Synodontis batensoda, the coloration of the belly is darker than the back, a reversal of the usual pigmentation pattern. Displacement of the swim bladder toward the underside is a further adaptation to this unusual swimming behaviour.

In fishes with specialized modifications of body form and habits, the fins are frequently modified and used for propulsion. The electric eels and knifefishes (Gymnotiformes) have lost the dorsal fin and, in some cases, the caudal fin. Slow forward and backward movements are made possible by undulations of an extremely long anal fin.

Associated with locomotion is the need for maintaining position in the water, particularly in the rapid torrents of mountain streams. A variety of modifications have evolved that function as holdfasts, anchoring the fish to rocks or similar objects. The hill stream loaches (Balitoridae) of southeastern Asia possess a large ventral suction disk formed by the expanded pectoral and pelvic fins. Some of the mountain stream catfishes (Sisoridae) of Asia have an adhesive organ on the thorax (chest). Mountain-inhabiting catfishes of South America may use a suckerlike mouth (Loricariidae) or employ a combination of a disklike mouth and disklike paired fins (Astroblepidae) for adhesion to the surface.