Rays and skates comprise more than species. They are closely related to sharks but can be distinguished from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface Figure.
Like sharks, rays and skates have a cartilaginous skeleton. Most species are marine and live on the sea floor, with nearly a worldwide distribution. Unlike the stereotypical sharks and rays, the Holocephali chimaeras or ratfish have a diphycercal tail equally sized fin lobes, with the tail vertebrae located between them , lack scales lost secondarily in evolution , and have teeth modified as grinding plates that are used to feed on mollusks and other invertebrates Figure b. Unlike sharks with elasmobranch or naked gills, chimaeras have four pairs of gills covered by an operculum.
Many species have a pearly iridescence and are extremely pretty. Members of the clade Osteichthyes , also called bony fishes, are characterized by a bony skeleton. The vast majority of present-day fishes belong to this group, which consists of approximately 30, species, making it the largest class of vertebrates in existence today. Nearly all bony fishes have an ossified skeleton with specialized bone cells osteocytes that produce and maintain a calcium phosphate matrix.
This characteristic has been reversed only in a few groups of Osteichthyes, such as sturgeons and paddlefish, which have primarily cartilaginous skeletons. The skin of bony fishes is often covered by overlapping scales, and glands in the skin secrete mucus that reduces drag when swimming and aids the fish in osmoregulation. Like sharks, bony fishes have a lateral line system that detects vibrations in water. All bony fishes use gills to breathe. Water is drawn over gills that are located in chambers covered and ventilated by a protective, muscular flap called the operculum.
Many bony fishes also have a swim bladder , a gas-filled organ derived as a pouch from the gut. The swim bladder helps to control the buoyancy of the fish. In most bony fish, the gases of the swim bladder are exchanged directly with the blood.
The swim bladder is believed to be homologous to the lungs of lungfish and the lungs of land vertebrates. Bony fishes are further divided into two extant clades: Class Actinopterygii ray-finned fishes and Class Sarcopterygii lobe-finned fishes.
Actinopterygii Figure a , the ray-finned fishes, include many familiar fishes—tuna, bass, trout, and salmon among others—and represent about half of all vertebrate species. Ray-finned fishes are named for the fan of slender bones that supports their fins. In contrast, the fins of Sarcopterygii Figure b are fleshy and lobed, supported by bones that are similar in type and arrangement to the bones in the limbs of early tetrapods.
The few extant members of this clade include several species of lungfishes and the less familiar coelacanths, which were thought to be extinct until living specimens were discovered between Africa and Madagascar. Currently, two species of coelocanths have been described.
The earliest vertebrates that diverged from the invertebrate chordates were the agnathan jawless fishes, whose extant members include the hagfishes and lampreys.
Hagfishes are eel-like scavengers that feed on dead invertebrates and other fishes. Lampreys are characterized by a toothed, funnel-like sucking mouth, and most species are parasitic or predaceous on other fishes. Fishes with jaws gnathostomes evolved later. Jaws allowed early gnathostomes to exploit new food sources.
Gnathostomes include the cartilaginous fishes and the bony fishes, as well as all other tetrapods amphibians, reptiles, mammals. Cartilaginous fishes include sharks, rays, skates, and ghost sharks. Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for part or all of their lives. The vast majority of present-day fishes belong to the clade Osteichthyes, which consists of approximately 30, species.
Bony fishes Osteichthyes can be divided into two clades: Actinopterygii ray-finned fishes, virtually all extant species and Sarcopterygii lobe-finned fishes, comprising fewer than 10 extant species, but form the sister group of the tetrapods. Daniel Cressey works for Nature magazine. Already a subscriber?
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Go Paperless with Digital. Get smart. Sign up for our email newsletter. Feed by suction with the help of a round muscular mouth and rows of teeth.
Have cylindrical and long bodies. Do not have paired fins and scales like most fish. There are two categories of jawless fish: hagfish and lampreys. Hagfish Hagfish usually feed on dead or dying fish.
This slippery mucus thus allows the hagfish to escape from the grip of predators. Hagfish can also twist their bodies into a knot, which provides additional leverage to feed. Sometimes hagfish enter the bodies of dead animals and eat carcasses from the inside out! Interestingly, they do not have a stomach!
Hagfishes have a cartilaginous skull, as well as a fibrous and cartilaginous skeleton, but the major supportive structure is the notochord that runs the length of the body. In hagfishes, the notochord is not replaced by the vertebral column, as it is in true vertebrates, and thus they may morphologically represent a sister group to the true vertebrates, making them the most basal clade among the skull-bearing chordates.
Figure 2. The class Petromyzontida includes approximately 40 species of lampreys, which are superficially similar to hagfishes in size and shape. However, lampreys possess extrinsic eye muscles, at least two semicircular canals, and a true cerebellum, as well as simple vertebral elements, called arcualia —cartilaginous structures arranged above the notochord. These features are also shared with the gnathostomes —vertebrates with jawed mouths and paired appendages see below.
Lampreys also have a dorsal tubular nerve cord with a well-differentiated brain, a small cerebellum, and 10 pairs of nerves.
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