Ratfish

The other major subgroup of cartilaginous fishes, the subclass Holocephali, comprises about twenty-five or thirty living marine species, most or all of which are placed in a single family, the Chimaeridae. They have a single gill opening on each side, like the bony fishes, but unlike them have a soft (rather than bony) gill cover. These fishes, commonly called chimaeras or ratfish (because of their long, slender tails), live and feed on the ocean bottom, usually in deep water. They have pavementlike teeth for crushing their mollusk and crustacean food, and they have a venomous spine on the leading edge of the first dorsal fin (the forwardmost of the unpaired fins on the upper surface of the body) for defensive purposes. Male ratfishes have a fingerlike barbed clasper, of unknown function, on the top of the head, and two pairs of claspers on the ventral (belly) side of the body.At least one pair of these ventral claspers is involved in mating. The female lays eggs in leathery capsules somewhat like skate egg cases.

Ichthyology
The study of sharks, rays, and related species is part of the larger discipline known as fish biology, or ichthyology. This science has its origins in the writings of Aristotle more than twenty-three centuries ago. He was the first to report, for example, that the sex of sharks can be determined by the structure of the pelvic fins, that is, by the presence of claspers in the male. Aristotle also contrived some rather fanciful interpretations of shark anatomy and behavior, as in his explanation for the fact that the shark mouth is on the under side of the head, far back from the tip of the snout, unlike the mouths of most other fish. In his view, this made it difficult for the shark to feed on its prey, requiring it to turn on its back, and thus nature allowed some chance for the poor animals to escape the jaws of this ravenous predator. Modern study of cartilaginous fishes, like fish biology in general, involves several disciplines. Ichthyology, or systematic ichthyology, is particularly concerned with the naming and classifying of species and higher taxa (taxonomic categories) and determining their interrelationships. Living cartilaginous fishes are probably better known (that is, more of the extant species have been discovered) than living bony fishes, simply because they tend to be larger, more conspicuous, and less secretive.Yet it was not until 1976 that one of the largest shark species, a deep-water filterfeeding species called the megamouth shark (Megachasma pelagios), was discovered near Hawaii. There may exist many additional Chondrichthyes species in deep ocean waters and remote coral reef areas. Chondrichthyes systematics (that is, the classification of the fishes) has undergone many changes and revisions as more has become known about fossil representatives and about the characteristics of the anatomy, biochemistry, and the like, of the living species. Studies of fossil cartilaginous fishes are limited almost entirely to samples of teeth, since these are virtually the only body parts durable enough to be preserved in the fossil record. Nevertheless, there is enough information in the characteristics of the teeth so that knowledge of the interrelationships of fossil species, both among one another and with living species, is quite advanced. It is known, for example, that the enormous hand-sized fossil teeth of Carcharodon megalodon, which lived about twenty million years ago, are so similar to the smaller teeth of the white shark (Carcharodon carcharias) that they both belong in the genus Carcharodon. Systematic study of modern species requires collection of specimens, generally by means of nets, traps, hooks, and lines, or spearing. Specimens are then preserved in some way and maintained in a museum collection. Entire specimens, if they are relatively small, can be maintained in diluted alcohol after fixation in formalin (formaldehyde solution). Other specimens, especially large ones, are dissected, and only certain parts are preserved, particularly the head skeleton with jaws and teeth. Certain new techniques provide taxonomic information from samples of living tissue. Karyotyping (analysis of the chromosomes), protein analysis (determination of the amino acid sequence), and DNA hybridization (estimation of genetic similarities) are all techniques that can elucidate interrelationships among the Chondrichthyes. Other disciplines concerned with the study of cartilaginous fishes include fisheries biology (the science of management and exploitation of commercially important fish species) and comparative physiology.