Chimaera

Chimaeras are cartilaginous fish in the order Chimaeriformes kᵻ'mErᵻfɔrmiːz, 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. Living species are largely confined to deep water. Chimaeras are soft-bodied, shark-like fish with bulky heads and long, tapered tails; measured from the tail, they can grow up to in length. Like other members of the class Chondrichthyes, chimaera skeletons are entirely cartilaginous, or composed of cartilage. Their skin is smooth and relatively free of scales or unique features, lacking the keeled, tooth-like placoid scales (denticles) present on sharks and rays. However, there are still exceptions, as hatchlings retain a few rows of denticles on the back, and adult males bear denticulated sexual organs. The gill arches are condensed into a pouch-like bundle covered by a sheet of skin (an operculum), with a single gill-opening in front of the pectoral fins. The pectoral fins are large enough to generate lift and a relaxed forward momentum, giving the chimaera the appearance of "flying" through the water. Further back on the body are also a pair of smaller pelvic fins, and some genera bear an anal fin in front of the tail. In chimaerids and rhinochimaerids, the tail is leptocercal, meaning that it is thin and whip-like, edged from above and below by fins of similar size. In callorhinchids, the tail is instead heterocercal, with a larger upper lobe inclined upwards, similar to many sharks. There are two dorsal fins: a large triangular first dorsal fin and a low rectangular or depressed second dorsal fin. For defense, some chimaeras have a venomous spine on the front edge of the dorsal fin. In many species, the bulbous snout is modified into an elongated sensory organ, capable of electroreception to find prey.