Chimaera
Chimaeras[1] are cartilaginous fish in the order Chimaeriformes /kɪˈmɛrɪfɔːrmiːz/, known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.
At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks and rays, though their last common ancestor with them lived nearly 400 million years ago.[2] Today, they are largely confined to deep water.[3]
Chimaeras live in temperate ocean floors down to 2,600 m (8,500 ft) deep, with few occurring at depths shallower than 200 m (660 ft). Exceptions include the members of the genus Callorhinchus, the rabbit fish and the spotted ratfish, which locally or periodically can be found at shallower depths. Consequently, these are also among the few species from the chimaera order kept in public aquaria.[4] They live in all the oceans except for the Arctic and Antarctic oceans.
They have elongated, soft bodies, with a bulky head and a single gill-opening. They grow up to 150 cm (4.9 ft) in length, although this includes the lengthy tail found in some species. In many species, the snout is modified into an elongated sensory organ.[5]
Like other members of the class Chondrichthyes, chimaera skeletons are composed of cartilage. Their skin is smooth and naked, lacking placoid scales (except in the claspers), and their color can range from black to brownish gray. For defense, most chimaeras have a venomous spine in front of the dorsal fin. They use these fins to "fly" through water.
Chimaeras resemble sharks in some ways: they employ claspers for internal fertilization of females and they lay eggs with leathery cases. They also use electroreception to find their prey.[6] However, unlike sharks, male chimaeras also have retractable sexual appendages on the forehead (a type of tentaculum)[7] and in front of the pelvic fins.[5] The females lay eggs in spindle-shaped, leathery egg cases.[1]
