Chimaera


Chimaeras are cartilaginous fish in the order Chimaeriformes, 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, their closest living relatives are sharks and rays, though their last common ancestor with them lived nearly 400 million years ago. Today, they are largely confined to deep water.

Description and habits

Chimaeras live in temperate ocean floors down to deep, with few occurring at depths shallower than. Exceptions include the members of the genus Callorhinchus, the rabbit fish and the spotted ratfish, which locally or periodically can be found at relatively shallow depths. Consequently, these are also among the few species from the Chimaera order kept in public aquaria. They have elongated, soft bodies, with a bulky head and a single gill-opening. They grow up to in length, although this includes the lengthy tail found in some species. In many species, the snout is modified into an elongated sensory organ.
Like other members of the class Chondrichthyes, chimaera skeletons are constructed of cartilage. Their skin is smooth and naked, lacking placoid scales, and their color can range from black to brownish gray. For defense, most chimaeras have a venomous spine 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. They also use electroreception to find their prey. However, unlike sharks, male chimaeras also have retractable sexual appendages on the forehead and in front of the pelvic fins. The females lay eggs in spindle-shaped, leathery egg cases.
They also differ from sharks in that their upper jaws are fused with their skulls and they have separate anal and urogenital openings. They lack sharks' many sharp and replaceable teeth, having instead just three pairs of large permanent grinding tooth plates. They also have gill covers or opercula like bony fishes.

Classification

In some classifications, the chimaeras are included in the class Chondrichthyes of cartilaginous fishes; in other systems, this distinction may be raised to the level of class. Chimaeras also have some characteristics of bony fishes.
A renewed effort to explore deep water and to undertake taxonomic analysis of specimens in museum collections led to a boom during the first decade of the 21st century in the number of new species identified. They are 50 extant species in six genera and four families are described; an additional three genera :




Tracing the evolution of these species has been problematic given the scarcity of good fossils. DNA sequences have become the preferred approach to understanding speciation.
The order appears to have originated about 420 million years ago during the Silurian. The 39 extant species fall into three families—the Callorhinchids, Rhinochimaerids and Chimaerids with the callorhinchids being the most basal clade. The families appear to have diverged during the late Jurassic to early Cretaceous.

Parasites

As other fish, chimaeras have a number of parasites. Chimaericola leptogaster is a monogenean parasite of the gills of Chimaera monstrosa; the species can attain in length.