Species List - Carpet Sharks (Order Orectolobiformes)
This small group of sharks consists mostly of well-camouflaged, bottom-dwelling, frequently nocturnal and often rather small animals; the body is often patterned, accounting for the common name. Two dorsal fins occur towards the rear of the body, and distinguishing features of the group include the placement of the gill slits, the presence of a small anal fin, and the shape of the tail fin. Most species have fleshy barbels around the mouth. Typical carpet sharks include the Indo-Australian wobbebongs (Orectolobidae).
The best-known of the 39 species is also the most unusual for this group of sharks. As well as being the world's largest shark, in an order of mostly small species, the whale shark is an open-water, migratory species, a filter feeder rather than an ambush predator, and although patterned, it lacks the camouflage of more typical carpet sharks.
NURSE SHARKS (GINGLYIMOSTOMATIDAE)
Only three species occur in this family, which ranges along both the Atlantic and Pacific coasts of the Americas and across the Atlantic to West Africa. Nurse sharks adopt a typical carpet shark lifestyle, foraging at night for bottom-dwelling invertebrates. The group's most distinctive feature is a mouth located far ahead of the eyes, well-placed for foraging on benthic invertebrates.
Species List - Ground Sharks (Order Carchariniformes)
Over a fifth of elasmobranchs are ground (or whaler) sharks, a total of around 270 species belonging to eight families. The group's best-known representatives are the hammerhead sharks (Syrphinidae), and many of the typical, large warm-water sharks belong to this order, among them tiger, bull, reef, and whitetip sharks. Ground sharks have a distinctly elongated snout, giving them a sleek appearance, two (rather than one) dorsal fins, and a membranous transparent 'eyelid' (actually a nictitating membrane) that protects the round eye from damage when seizing prey. In common with most other sharks, five rather than six gill slits are present.
Whaler sharks are typically live-bearing, and exhibit a variety of reproductive modes from ovoviviparity (in which young develop within eggs that hatch shortly before birth) to true viviparity in which young develop to term within the mother and connected to her bloodstream by placental tissue.
From the preliminary data available on shark declines, whaler sharks are especially badly-affected by fishing pressures. Atlantic hammerhead populations have declined by over 90% from historical levels, while oceanic whitetip, once the most abundant warm-water sharks in the world,. occur at only 1% of their historical densities in the Gulf of Mexico.
REQUIEM SHARKS (CARCHARINIDAE)
Around 50 species belong to this group, mostly fully viviparous, migratory sharks of warm marine environments, and the family has a worldwide distribution. One member of this family, the bull shark, has been blamed for more attacks on humans than any other species; another, the tiger shark, is also listed in the top three. Unusually, the bull shark has a high tolerance for freshwater and will travel further upriver than any other species of marine shark.
Whitetip Reef Shark
Species List - Rays and Skates (Order Rajiformes)
Rays and skates are unmistakeable animals, characterised by a disc- or diamond-shaped body formed by pectoral fins enlarged into 'wings' that extend from a point ahead of the snout to the pelvic region. There is no pronounced dorsal fin, and generally none at all, and as a result the entire animal appears flattened. Gill slits occur on the animal's lower surface rather than along the sides of the head as in sharks. The head is mounted on top of the disc, and distinctively a spiracle is present beneath each eye. This feature allows the animal to take up water to oxygenate its gills while resting on the seabed.
The largest rays, the mantas, are pelagic filter-feeders, but most species are strictly carnivorous and many adopt an ambush strategy, lying motionless and sometimes concealed by sand on the seabed ready to seize passing fish.
Others actively hunt sand-dwelling or burrowing animals, including molluscs and worms, using beats of their wings to clear sand and other debris from their feeding area to reveal prey items.
A little under half of the described elasmobranchs are rays and their relatives, mostly within the 11 families of the Rajiformes. Many species possess a spine at the end of the tail, usually serrated, which injects venom into potential attackers, usually when stepped on. Mostly the venom itself is non-lethal to humans, and rays are rarely aggressive, but species with long spines have been known to inflict significant mechanical damage; most infamously, in 2006 the naturalist Steve Irwin was killed by an unprovoked attack from a stingray whose spine pierced his heart. This was an exceptional case, and rays are far more likely to swim away when they perceive a threat than to attack.
A cosmopolitan family of rays with venomous spines and more-or-less rounded discs. Most are species of coastal waters but a number of species, including some of the largest, occur in the river systems of tropical Asia. Size in this family is highly variable, from the width of a human hand to over two metres in diameter; some may have a total length of 5 metres. Some species are brightly-coloured or patterned, but typical stingrays are shades of grey, brown or green. A number of species appear to be threatened by pollution and habitat loss, but several are also harvested as food.
Blue-spotted Ribbontail Ray
The elasmobranchs (or elasmobranches), strictly defined, consist of the sharks, skates and rays, a group of more than 1,000 described species of cartilaginous fish. For convenience, the term is sometimes extended to include the 35 species of chimaera, a related but distant lineage of jawed fish with cartilaginous skeletons. Defined principally on the basis of a skeleton made of dense connective tissue (cartilage) rather than bone, sharks and rays are often regarded as more primitive than other classes of vertebrates (backboned animals), and sometimes as unchanging 'living fossils'. In fact, the earliest sharks evolved at much the same time as the first bony fishes, and are not ancestral to the lineage that produced the better-known vertebrate groups (bony fish, amphibians, reptiles, mammals and birds). Instead they represent a separate evolutionary radiation from a common ancestor. While possible ancestral sharks occur as far as 455 million years ago, following periods of diversification and evolutionary experimentation, recognisably modern sharks evolved relatively recently, first appearing in the fossil record around 180 million years ago.
Most modern shark lineages can be traced to the mid-Cretaceous, around 100 million years ago. Today, the group includes the world's largest fishes, the whale and basking sharks, as well as the great white shark, one of the most iconic animals in the sea, and numerous other species of shark, wobbegong, dogfish, sawfish, skates and rays. The majority of species live in the sea, often in open water, but a number live in estuarine habitats and may travel inland along rivers. Sawfish and some species of ray have become fully adapted to freshwater habitats.
Elasmobranchs possess several features popularly associated with "more advanced" vertebrates. Many sharks and rays give birth to live young, and in some species, hammerhead sharks being the best-known, a placenta develops connecting growing offspring with the mother's bloodstream, a reproductive system best-known from eutherian mammals. One lineage of sharks is 'warm-blooded' (endothermic), generating internal heat to raise their body temperature above that of the surrounding water. Elasmobranchs are also known for their ability to detect prey using a sense that picks up the electrical activity of living organism, and for keen senses of sight and smell. Most elasmobranchs are carnivores, but some of the largest members of the group, including whale sharks, basking sharks and manta rays, feed on plankton that they filter through their gills.
It has been heavily publicised that sharks are in the midst of a global decline crisis; while it is often pointed out in defence of these often unpopular animals that shark attacks are rare, very few sharks are killed on the grounds of human safety. Many are caught as bycatch (incidental captures that are discarded, almost invariably dead) in fishing operations, but above all rising demand for shark fin soup, mainly in China, is responsible for active persecution of these animals on an industrial scale. Shark meat itself has little value, while the highly-prized fins are simply a mass of cartilage (the same tissue as the human nose) that thickens soup but is reportedly without flavour. The tragic consequence is that sharks are caught and finned, with the rest of the carcass often discarded and populations decimated for a (literally) tasteless luxury.
There are as yet no global figures available for the extent to which sharks are threatened by this trade, but as an estimate over 90% of large predatory fish, many of which are sharks, have been wiped out by overfishing. The overall picture emerging from research into elasmobranch declines is that large, open-water shark species, many charismatic animals that are both important apex predators in marine ecosystems and a valuable source of revenue from diving tourism, are declining throughout the world, a situation exacerbated by the low reproductive rates and long development period in most species.