Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish (not to be confused with the rattails), spookfish (not to be confused with the true spookfish of the family Opisthoproctidae), or rabbitfishes (not to be confused with the true rabbitfishes of the family Siganidae).
They may be the "oldest and most enigmatic groups of fishes alive today". At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks, though in evolutionary terms they branched off from sharks nearly 400 million years ago and have remained isolated ever since. Today they are largely confined to deep water.
Chimaeras live in temperate ocean floors down to 2,600 metres (8,500 ft) depth, with few occurring at depths shallower than 200 metres (660 ft). Exceptions include the members of the genus Callorhinchus, the rabbit fish and the spotted ratfish, which locally/periodically can be found at relatively shallow depths. Consequently, these are also among the few species from the Chimaera order that are kept in public aquaria.
Like other members of the class Chondrichthyes, chimaeras have a skeleton constructed of cartilage. Their skin is smooth and largely covered by placoid scales, and their color can range from black to brownish gray. For defense, most chimaeras have a venomous spine located in front of the dorsal fin.
Chimaeras resemble sharks in some ways: they employ claspers for internal fertilization of females and they lay eggs with leathery cases. However, unlike sharks, male chimaeras also have retractable sexual appendages on the forehead (a type of tentaculum)
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