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Blennies are small, primarily tropical and subtropical marine fish. They are elongated and often eel-like in shape, with a dorsal fin running from the back of the head almost to the tail fin, and small abdominal fins; the pelvic fin is often completely absent. Many species also lack scales. The blenny's anatomy is well suited for hiding in cracks and crevices along shallow, rocky shorelines, their preferred habitat. Some species, however, dwell in deeper waters. Living and foraging close to the ocean floor, blennies are either carnivorous (meat eaters) or omnivorous (eating both meat and vegetation). A wide variety of body shapes, colors, patterns, and behaviors are displayed by the more than 732 species in the six families belonging to the suborder Blennioidei.

Many species of scaleless blennies (family Blenniidae) are distributed throughout the world. Scaleless blennies are often found in tide pools and complete their entire life cycle in one general location. These species are divided into two groups—the subfamily Blenniinae, which have immovable teeth firmly rooted in the jaw, and the subfamily Salariinae, which have moveable teeth rooted in the gums.

Scaled blennies belong to the very large family Clinidae, or clinids. They inhabit temperate oceans primarily south of the Equator. Dazzling and varied colors and markings differentiate the species. The largest clinid, one of the many pointy-headed blennies, is the 24-in (61 cm) kelpfish (Heterostichus rostratus), which inhabits the Pacific shoreline from British Columbia to Southern California, while the 8-in (20-cm), blunt-headed, hairy blenny (Labrisomus nuchipinnis) lives in the tropical waters off both Atlantic coastlines. The pike blenny (Chaenopsis ocellata) is a tube-dwelling species found in Florida. Male pike blennies jealously defend their territories from other intruding males by aggressively displaying a stiffly raised dorsal fin and a widely gaping mouth. Two males may literally face off, gaping mouths touching, until one snaps its mouth shut on the other. Some of the smallest blennies are also found in the Clinidae family—the female of the species Tripterygion nanus found in the Marshall Islands, is fully grown at less than 0.75 in (1.9 cm) in length.

The largest blennies, often reaching 9 ft (2.7 m) in length, are found among the nine species of wolf fish and wolf eels belonging to the family Anarhichadidae. These cold water fish are found in the northern hemisphere. They have prominent canine teeth in the front of their jaws and massive grinding teeth in the back of their mouths. Two species, the Atlantic wolf fish (Anarhichas lupus) and the spotted wolf fish (A. minor), are fished commercially along the European coasts.

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