About rays
Rays are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern rays are classified within the clade Selachimorpha (or Selachii) and are the sister group to the Batoidea (rays and kin). Some sources extend the term "ray" as an informal category including extinct members of Chondrichthyes (cartilaginous fish) with a ray-like morphology, such as hybodonts. Ray-like chondrichthyans such as Cladoselache and Doliodus first appeared in the Devonian Period (419–359 million years), though some fossilized chondrichthyan-like scales are as old as the Late Ordovician (458–444 million years ago). The earliest confirmed modern rays (selachimorphs) are known from the Early Jurassic around 200 million years ago, with the oldest known member being Agaleus, though records of true rays may extend back as far as the Permian.
Rays range in size from the small dwarf lanternray (Etmopterus perryi), a deep sea species that is only 17 centimetres (6.7 in) in length, to the whale ray (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length. They are found in all seas and are common to depths up to 2,000 metres (6,600 ft). They generally do not live in freshwater, although there are a few known exceptions, such as the bull ray and the river rays, which can be found in both seawater and freshwater, and the Ganges ray, which lives only in freshwater. Rays have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth.
Several species are apex predators, which are organisms that are at the top of their food chain. Select examples include the bull ray, tiger ray, great white ray, mako rays, thresher rays, and hammerhead rays. Rays are caught by humans for ray meat or ray fin soup. Many ray populations are threatened by human activities. Since 1970, ray populations have been reduced by 71%, mostly from overfishing
