Sharks and Rays

The sharks are an ancient group of fishes that have been on earth for over 400 million years. The largest fish in the world is a shark - the plankton-eating whale shark. There is a great diversity of sharks and rays (about 350 species of sharks and about 425 species of rays, worldwide), but there are more species of bony fishes. Sharks are found in benthic and pelagic habitats and although they have a fearsome reputation, most of them are harmless to humans. Ironically, humans have a very serious impact on populations of sharks, due to their low rates of reproduction. Rays are essentially sharks with very large pectoral fins. Although some, such as the manta ray, live in pelagic environments, most live in close association with the bottom.

What do they look like?
Sharks: The sleekest of sharks are generally pelagic, and are clearly designed for speed. The fastest of sharks, such as the mako, have sprint speeds of over 20 body lengths per second. The shape of their tails varies considerably from the even lobes on the tail of makos to the extremely long upper lobe of thresher sharks (used to stun fish). Other sharks are just plain weird (eg. Hammerheads which have incredibly wide heads that may be as much as 1/3 the length of their bodies in width) or amazing, such as the worlds biggest fish, the whale shark, which can reach well in excess of 13m in length and is toothless, feeding only on plankton. Sharks that are closely associated with the bottom vary greatly in form from those that are pelagic. They tend to have blunt noses and a cumbersome swimming style. Banjo sharks have large pectoral fins and look like the cross between a ray and a shark. In contrast, rays appear to be all pectoral fins. A shark's "smile" is sure sign of the family they belong to. Teeth vary greatly and are partly used in the taxonomy of the group. The teeth of sharks originates from denticles of the skin (like fingernails) and many are shed during life. Some sharks have serrated, flesh cutting teeth (eg. great white sharks), others have spiky teeth for grabbing (eg. mako) and in the case of some benthic feeding sharks, flat plates that are used to crush prey. Rays: Rays differ from sharks because their pectoral fins (found behind the gills) are fused to the side of their heads, whereas sharks have fins that are separated from the sides of their head. Also, rays have grinding plates rather than true teeth so are unable to bite in the way that sharks are famous for. However, some rays can sting using a venomous barb in their tails (stingrays) or electrocute people, using electric organs in their wings (electric rays). Manta rays are particularly impressive with their huge wing span (often reaching more than 6m wide) and hooded mouth.

Where do they live?
Sharks and rays inhabit most marine environments on earth, but they are rare or absent at polar latitudes. Sharks are found in shallow water to deep oceanic trenches, kilometres below the surface (benthic habitats) and open ocean surface waters (pelagic habitats).

How and what do they eat?
Sharks: Many sharks eat other fishes, either bony fish or their own relatives, the sharks. Other species however are planktivorous, such as the whale shark and the rare "megamouth" (only 4 specimens have been found world wide). The type of teeth a shark has is a good indication of what and how it eats. For example, the cutting teeth of the cookie cutter sharks are well designed to remove chunks of flesh from their much larger prey (eg. whales and large sharks). Whalers, including tigersharks, slice their food using small serrated teeth and vigorous head shaking. Great Whites have large triangular shaped teeth which they use for biting chunks out of their food (see also section on "what do they look like"). Rays: Rays have no teeth as such, instead they have flat plates in their jaws which they use for crushing hard-shelled food, eg. shellfish and hermit crabs, which they often excavate from sand and mud. You can sometimes see "ray pits" on the bottom when diving. These are steep-sided holes with piles of broken shells alongside. Electric rays have modified this approach by using the electric organs in their wings to stun their prey before eating them. The exception to this general style of eating is the manta ray which is planktivorous and lives and feeds up in the water column.

What eats them?
Other sharks are probably the most common predators. For example, hammerhead sharks hunt for sting rays. Large bony fishes also eat sharks. For example, large grouper eat small reef sharks and small pelagic sharks are taken by large fishes such as jacks and bill fish. Although parasitism and disease can kill them, there is little information on the frequency of these or their effect on populations of sharks and rays. Humans are a major source of death for sharks in particular. Sharks have a fearsome and often unwarranted reputation for attacking humans that has resulted in massive shark hunts, and shark netting off beaches. While these approaches have given beachgoers a greater sense of security, in many cases this has had very serious consequences for shark populations because they have such low rates of reproduction.

How do they grow and reproduce?
It is easy to distinguish the different sexes in sharks and rays. Males have claspers on their anal fin and females do not. Sharks and rays mate, in that the female is inseminated with the clasper of the male. The young are either born alive as small sharks, or are released as eggs that may take up to 9 months to hatch. Some species, like Tiger sharks produce eggs which hatch inside the female and then emerge as live young. However, relationships inside the mother can be vicious with some early-hatching foetuses eating younger siblings. Some sharks, such as the Port Jackson shark, can migrate over 900km to aggregate in winter and mate. Growth rates have not been well documented for many species but some of the larger reef sharks have been reported to live for decades. Unlike humans who cannot replace their adult teeth should they fall out, shark teeth are regularly replaced by new ones forming from the skin behind the old ones and moving forward as needed.

Who do they live with?
Sharks live in pelagic and benthic environments (eg. sediment flats of continental shelves, coral and rocky reefs). Because there are more than 20x as many bony fish species as sharks, sharks and rays share most environments with these fish. Bony fish are, of course, an important source of prey. Many other organisms make up the environments and habitats in which sharks and rays live eg. the coral of reefs, the giant kelp of some temperate reefs. An interesting relationship has evolved between sharks and some species of bony fish - the remoras or suckerfish. These fish have a sucker pad on the top of their heads which they use to attach themselves to sharks. Remoras are basically opportunistic, on the lookout for food scraps whilst the shark is feeding, plus a free ride - the ultimate hitch hiker!

Their connection with people.
Sharks and rays are seriously overfished in most parts of the world. In fact one of the few records of an extinction in the marine environment was of the Irish skate. This happened because levels of fishing were set for cod with no regard for the skate which was also caught in cod nets. Sharks and rays are very vulnerable to over-fishing because they have low rates of reproduction. Most sharks and rays only have 10s to a couple of hundred young per year. In contrast one snapper may have 100,000s to millions in a year. Because of their poor public image, sharks in particularly have fairly negative interactions with humans, often being hunted or caught in shark nets around beaches. Some species of sharks, by being at the top of the food chain, also suffer from human activities because they accumulate pollutants and heavy metals (eg. mercury) in their tissues. A positive interaction with humans has been the increase in the numbers of sharks and rays now held in large public aquariums. This means that a great deal more people have the chance to learn about and see these amazing animals and appreciate their beauty rather than their perceived threat.