麻豆传媒

THE beach at Muizenberg outside Cape Town is a Mecca for wannabe surf bums. But when the beach siren sounds, surfers and swimmers alike tend to lose their cool. That distinctive rolling wail is a warning that sharks may be nearby. Everyone knows the drill 鈥� get out of the water as quickly as you can.

The mere suggestion of a shark is enough to conjure fear in many of us. But sharks also inspire awe. It isn鈥檛 just their elegance or physicality; equally impressive is their tenacity. As a group, sharks have been around for at least 420 million years, meaning they have survived four of the 鈥渂ig five鈥� mass extinctions. That makes them older than humanity, older than Mount Everest, older than dinosaurs, older even than trees.

It is possible that sharks just got lucky in the lottery of life. But over the past few years, scientists have discovered that the fish possess some unusual qualities that allow them to be super-adaptable in the face of change, including a fondness for hanging out around underwater volcanoes. The big question now is whether these qualities will help sharks survive the current sixth mass extinction, triggered by human activities. Today, sharks face a new challenge, far deadlier than any they have ever encountered.

Sharks, along with rays, skates and chimaeras, make up a group of fish known as chondrichthyes, characterised by a cartilaginous skeleton. Fossil scales found in Siberia indicate that sharks originated in the Silurian period, which began about 440 million years ago. It was a time when the world was warm, sea levels were high and corals reefs were starting to appear. Since then, thousands of shark species have existed, culminating in a golden age about 360 million years ago, when they dominated the oceans, taking many weird and wonderful forms. Today, there are more than 450 shark species, ranging from well-known ones such as great whites and hammerheads to the exotic and bizarre, including goblin sharks, cookiecutter sharks and Japanese wobbegong.

鈥淎s well as letting very few nutrients go to waste, sharks are surprisingly unfussy eaters鈥�

Of course, many shark species are now extinct, but that is to be expected. It is estimated that extinction has been the fate of more than 99 per cent of all plants and animals that have ever lived. The puzzle instead is how sharks as a group have survived for so long. Palaeoecologist Sora Kim at the University of California, Merced, who studies ancient and modern sharks, sees one clue in their faeces. 鈥淲hen a shark poops, there鈥檚 hardly any solids,鈥� she says. 鈥淚t鈥檚 more of a clearish goo.鈥� This indicates that they possess a highly efficient digestive system able to process almost all of what they eat. That can be helpful if food gets scarce, says Kim, which is likely to happen during a mass extinction event. Earth鈥檚 third and biggest mass extinction, for example, which happened about 252 million years ago, saw upwards of 96 per cent of all marine life disappear.

As well as letting very few nutrients go to waste, sharks are also surprisingly unfussy eaters. A few years ago, Kim and her colleagues by analysing chemical signatures in their backbones. 鈥淲hen I started the project, I thought, well, white sharks eat seals and sea lions,鈥� she says. That was the received wisdom. 鈥淚 was really surprised that that鈥檚 not what I saw.鈥� They don鈥檛 turn their nose up at these animals, but they seem just as happy consuming other prey such as squid. 鈥淓ven though we think of them as being apex predators at the top of the food chain, they definitely aren鈥檛 that all of the time,鈥� says Kim. In fact, research published last year reveals one shark species, the bonnethead, is omnivorous, consuming copious amounts of seagrass along with shellfish. Such dietary flexibility would have worked in sharks鈥� favour when the going got tough.

More evidence of shark adaptability comes from their teeth. Unlike their cartilaginous skeleton, the teeth are extremely hard, which gives them a good chance of being preserved in the fossil record. In Canada鈥檚 Northwest Territories, teeth belonging to sand tiger sharks litter ancient sediments near the Muskox and Eames rivers. They are between 53 and 38 million years old, dating from the Eocene, an epoch when . Ice caps melt in warmer worlds, and their freshwater drains into the oceans. , which can be a problem for fish species that require specific salt levels to survive. But it doesn鈥檛 seem to have bothered the Eocene sand tiger shark: the chemistry of its teeth suggests that it was than its counterparts inhabit today.

鈥淎 lot of people think of the Eocene as where we could be going with [modern] climate change,鈥� says Kim. 鈥淚t was really warm, and there were no polar glaciations.鈥� Then, as now, marine species faced the challenge of rising temperatures. But here again, sharks seem to fare just fine. Some may even get a boost. Last year, for example, was reported in the waters of the Outer Banks, a strip of islands off North Carolina. 鈥淏ull shark nurseries used to only be as far north as North Florida,鈥� says marine biologist David Shiffman at Simon Fraser University, Vancouver. The species rears its young in relatively warm waters, and as ocean temperatures have climbed, they have expanded their range.

In hot water

Other shark species seem to positively relish being in hot water. When ocean engineer Brennan Phillips at the University of Rhode Island off the Solomon Islands, one thing he didn鈥檛 expect to find was sharks. The water spewing out of hydrothermal vents can exceed 400掳C. Yet, there, swimming through the murky depths, his team spotted hammerhead and silky sharks. The press had a field day (), but for Phillips the discovery hints at something deeper.

Submarine volcanoes have been relatively constant features in the oceans since long before sharks evolved, he says. If today鈥檚 sharks are at home here, their ancestors might have been able to rely on volcanoes as other habitats came and went. The idea needs further exploration but is lent support by the discovery last year of skates .

Sharks are adaptable in many ways, but of course individual species aren鈥檛 immune to extinction. Even the largest shark that ever lived, megalodon, which may have grown up to 17 metres long and dined on whales, vanished eventually. It became extinct about 2.6 million years ago, having survived for 20 million years.

Catalina Pimiento at Swansea University, UK, thinks she knows why. While studying megalodon fossils along the Panama Canal, she from juveniles, indicating what she thinks are ancient nurseries in sediments that were once part of a near-shore marine environment. Such habitat vanished around the time that megalodons went extinct. 鈥淲hat happened was the sea level was oscillating violently,鈥� she says. These rises and falls would have destroyed megalodon nursery habitats, and the species along with them.

Mass extinctions will have put paid to many other species of shark, and reshuffled the list of those around. In fact, the first such event that sharks lived through left the group with its most notable characteristic. This was Earth鈥檚 second mass extinction, which began about 375 million years ago, probably driven in part by massive glaciations.

鈥淪harks are survivors but there is one thing they can鈥檛 easily swim away from: us鈥�

Before this cataclysmic event, sharks mostly had bony skeletons, says Lauren Sallan at the University of Pennsylvania. Afterwards, most had soft, cartilaginous skeletons, just as they do today. What鈥檚 more, post-extinction sharks were tiny 鈥� usually less than 10 centimetres long.

The second extinction changed the world, and sharks changed with it. Still, they had traits that enabled the group as a whole to weave through the cataclysm. And their flexibility means that they remain resilient. 鈥淧eople ask me: 鈥榙oes climate change affect sharks鈥�?鈥� says Shiffman. 鈥淭he short answer is: 鈥楩or the most part, not really.'鈥� But there is one thing sharks can鈥檛 easily swim away from: us. Overfishing is the main risk they face today.

Increasingly, there are ranging from international conventions to local laws. In parts of the world, however, populations continue to be harvested, as they have been for centuries, for their fins, considered a delicacy in some countries. Meanwhile, as fishing becomes more intensive and deep-sea trawling more common, growing numbers of sharks are netted accidentally.

Release back into the ocean isn鈥檛 always enough to save them, as the plight of the . This species is agile, but also fragile, says Shiffman. 鈥淭hey just stress out really quickly.鈥� When caught by fishing nets, the stress of the ordeal alone is often enough to kill them. But the problem can be reduced, he says, with the use of modified fishing gear that traps only the intended target fish, and by restricting fishing to times of day when the sharks aren鈥檛 intermingled with those fish.

The toll that fishing takes isn鈥檛 helped by the fact that many of today鈥檚 sharks are relatively large compared with their forebears, says Sallan. Bigger sharks take longer to reach the age at which they can have pups. And if humans are faster than the animals can reproduce, then their populations will inevitably nosedive.

Today, about by the International Union for Conservation of Nature as threatened. As a group, sharks possess special powers that have made them extinction-proof. But overfishing is a new challenge that some species won鈥檛 survive unless we change our ways.