The clingfishes are denoted mainly by the presence of pelvic fins modified into a sucking disk. Unlike the gobies (Gobiidae, Perciformes), which possess pelvic fins modified into a suction cup, the clingfishes lack a spinous dorsal fin. With some exceptions, clingfishes are tadpole-like, with broad, depressed heads and flattened bodies. Some species, however, are elongate with pointed snouts. Clingfishes lack scales but shield themselves with a mucus that forms a protective coat around their bodies. The lateral line is well developed anteriorly but is either small or missing posteriorly.
There is a well-developed network of sensory pores on the head. There is one dorsal and one anal fin.
The sucking disc is of two types: a single disc, in which the anterior and posterior components form a single continuous structure, and a double disc, in which the larger anterior and smaller posterior components form two discs separated by a single wall or edge. The pectoral fins generally are small and are positioned posterior to the sucking disk. The dorsal nostril and, usually, the posterior nostril are both tubular. The swim bladder is absent. Color patterns are variable and range from black to orange, brown, green, or red. Additionally, there may also be contrasting stripes, bars, or spots of yellow, blue, green, brown, gray, or white. The larvae are elongate, torpedo-shaped, and laterally compressed, and they become more rounded with growth. The gas bladder, located above the gut, is present but is lost after settlement. Body sizes typically are small, less than 1.9-2.4 in (5-6 cm) in total length, but at least one temperate species from southern Africa, the rocksucker (Chorisochismus dentex), reaches 11.8 in (30 cm). Clingfishes are sexually dimorphic, with differences in the size of the urogenital papillae and also in body size—males generally are larger than females. Two species reportedly produce a skin toxin.
The singleslits are eel-like in shape, with diminutive pelvic fins and a vestigial sucking disc. Their dorsal and anal fins are modified to resemble those of true eels. Soft rays are present only in the caudal fin. The posterior nostril is absent in one species. Color patterns are also variable and range from translucent pink or green to light green, brown, gray, or black. Spots or bars of black, gray, brown, or yellow may also be present. Usually they are less than 1.9 in (5 cm) in length, but one species, the common shore-eel (Alabes dorsalis) of Australia, reaches 4.7 in (12 cm).
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