Sharks and rays float without a swim bladder by carrying an oily liver
Most bony fish stay buoyant with a gas-filled swim bladder. Sharks, rays, skates and sawfish have none. Instead they carry enormous livers packed with oil, which keeps them from sinking and doubles as a food reserve when meals are scarce. It is one of several quirks defining the elasmobranchs.
Elasmobranchii is the branch of cartilaginous fish holding modern sharks and the flattened batomorphs. The name, coined by Charles Lucien Bonaparte in 1838, joins Greek words for plate and gill, after their broad, flat gills. Members have five to seven pairs of gill slits, each opening separately to the outside, stiff dorsal fins and skin armoured with tiny tooth-like placoid scales. Teeth grow in multiple rows, and the upper jaw is not fused to the skull. Their eyes have a reflective tapetum lucidum, and in males the pelvic fins are grooved into claspers that transfer sperm.
How the jaws hang from the skull differs between groups and helps specialists tell lineages apart. Four basic arrangements are recognised, from amphistyly, where ligaments and a joint behind the eye carry most of the load, to euhyostyly, where the jaw cartilages have no ligament link to the cranium at all and a single pair of hyomandibular cartilages supports everything.
The lineage is ancient. Phoebodus, the oldest undisputed member of the wider group, dates to around 383 million years ago in the Middle Devonian. In the Carboniferous some ctenacanths grew to about 7 metres, rivalling a great white. Xenacanths thrived in rivers and seas through the Carboniferous and Permian, and hybodonts dominated during the Triassic and Early Jurassic before fading at sea, hanging on in fresh water until the very end of the Cretaceous. Modern sharks and rays existed by the Triassic but only diversified widely from the Early Jurassic, when today's orders appeared.
Defining the group has caused headaches. Bonaparte's original version covered all living cartilaginous fish, including chimaeras; during the 20th century chimaeras were dropped and many fossil forms added. In 2012 John Maisey argued the name should cover only modern sharks, rays and their common ancestor, a set Leonard Compagno had called Neoselachii in 1977, while other researchers use it for everything closer to sharks than to chimaeras.
Source: Elasmobranchii
