They inhabit tropical and temperate waters, and many are migratory. Pelagic sharks are constantly on the move, and rely on lift from their pectoral fins and buoyancy from the low density oils in their large livers to stop them from sinking.
Many pelagic sharks are the top predators in their range, and have few natural predators when fully grown. Pelagic sharks are carnivorous and mostly eat fish, including other sharks on occasion, and some will take turtles, seals and penguins. However, there are also three species of large plankton eating filter feeders – the whale shark, the basking shark and the megamouth shark.
The diversity of pelagic sharks is quite low at 39 species, far less than the hundreds inhabiting shallow coastal regions. Yet many are abundant and found across very wide expanses of the world’s oceans.
Overfishing is the overwhelming threat, with open ocean longlines using hundreds if not thousands of hooks each catching the greatest volume of sharks globally. While these fisheries may be primarily targeting tuna and billfishes such as marlin, the sharks caught are an important source of income, particularly their fins. Tuna purse-seiners also catch sharks, although these have a better chance of being released alive, while gill nets are an ecological disaster, catching almost anything in their path, including whales, dolphins, turtles and sharks.
While the oceans are vast, there are few refuges from industrial fishing, and some pelagic species, such as the oceanic whitetip have suffered massive population losses due to their inability to reproduce more quickly. Seventeen out of the 39 pelagic shark species are threatened with extinction.
The species featured here are found primarily in tropical waters. Some have home ranges of only a few square kilometres, while others wander more widely into other habitats found on continental shelves, into deep waters, and even across the open ocean to reach reefs elsewhere.
Large reef sharks are typically the top predators on tropical reefs. On pristine coral reefs, they can be may account for as much as 50 per cent of all the fish biomass (wet weight). All reef sharks are predators, typically hunting for fish, crustaceans, and other animals such as octopus. Some species are constantly on the move, while others spend much of their time lying on the bottom, and may feed mostly after the sun sets.
Overfishing is by far the biggest threat to most larger species, while damage to reef and other key habitats is also having an impact. The clearance of mangroves has a negative impact on species whose young use these as nursery grounds. The loss of living coral reefs due to sedimentation and fertilizer run-off from farmland, and climate change, will often reduce the amount of prey for sharks. Many kinds of food fishes that humans like to eat inhabit reefs, and so reefs are targeted by fishers, using types of fishing that also catch sharks.
Some 25% of all the 494 sharks and rays inhabiting coastal continental shelves, which includes all reef sharks, are threatened with extinction. There may be many more as the conservation status of 35% is not yet known.
Rays first appear in the fossil record about 200 million years later than the first sharks, and are thought to have evolved from flattened shark species. One of the most obvious differences with sharks, is that the pectoral fins of rays and skates form a large disc, which starts from the back of the skull.
There are around 540 species of rays and skates, inhabiting reefs, the open ocean, the deep sea and even freshwater. Rays form part of the diet of many species of predatory sharks.
Overfishing is the largest threat in the ray's marine realm, as it is for most sharks. Some of the most valuable fins in the shark fin trade are from shark-like rays, such as sawfishes and large guitarfishes, while the meat of many species of rays and skate are also eaten in coastal communities.
Five out of the seven families of elasmobranch most threatened with extinction are rays.