The chimaeras (order Chimaerae, class Bradyodonti) are a most peculiar looking group of fish that live near the sea bed off continental shelves and in deep offshore waters at a depth of 985-1,640 mi (300-500 m). Collectively these species form a small, cohesive group of about 25 species. They are all exclusively marine species. Closely related to sharks, rays, and dogfish, chimaeras are characterized by their cartilaginous skeletons-in contrast to the bony skeletons of most fish. One feature that distinguishes them from rays and dogfish is the fact that the upper jaw is firmly attached to the cranium. They also have flattened teeth (two on the upper jaw and one on the lower) that are modified for crushing and grinding their food. There is one gill opening on either side of the head, each of which is covered with a fleshy flaplike cover.
Also known as rabbit fish, rat fish, or elephant fish, these species have large heads, a tapering body and a long, rat-like tail. Relatively large fish, they range from 2 to 6 ft (0.61 to 2 m) in length. Unlike their larger relatives—the sharks, for example—they are all weak swimmers. The fins are highly modified: one striking feature is the presence of a strong, sharp spine at the front of the first dorsal fin. In some species this may be venomous. When the fin is relaxed, for example when the fish is resting, this spine folds into a special groove in the animal's back, but it may be quickly erected if disturbed. Chimaeras have a very reduced, elongated tail fin. The large pectoral fins, unlike those of sharks, play a major role in swimming, while additional propulsion is gained through lateral body movements.
Chimaeras have a smooth skin and one gill opening on either side—a morphological change that lies between that of a shark and the bony fish. A series of mucus-secreting canals occur on the head. The males of some species such as Callorhinchus have a moveable club-shaped growth on the head, the function of which is not known.
Little is known about the ecology of chimaeras. Most are thought to feed on a wide range of items, including seaweed, worms, crabs, shrimp, brittle stars, molluscs and small fish. Most rat fish are thought to be nocturnal—one explanation for their peculiarly large eyes. Male chimaeras have small appendages known as claspers, used to retain their hold on females during copulation; this act is also observed in sharks and rays. Unlike the fertilized eggs of bony fishes, those of chimaeras are enclosed in toughened capsules. In some species these capsules may measure 6 in (15 cm) in length and are pointed to stick into the soft substrate, possibly to prevent the eggs from drifting too far.
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