The following article ‘Sharks Demystified’ was originally published in the shark issue (105) of Asian Diver Magazine:
Requiem for a Requiem- Sharks Demystified
Sharks, like snakes and spiders, are one of those creatures that have the ability to instil dread in even the most rational of people. Unlike those other groups, which are always slithery and crawly, sharks are a hugely diverse group and not all are the stereotypical “Jaws” you might imagine. Obviously a handful of species have been responsible for attacks on people but the remaining 450 or so species are entirely innocent and unfortunately persecuted with the same fervour as more their dangerous cousins.
The sleek lines, large pointed fins and big toothy mouth of the great white shark are the most well-known and feared fish in the ocean. It is the largest predatory fish currently in existence, although a closely related fifteen meters goliath went extinct only 1.5 million years ago. The sleek predatory, or requiem, sharks that roam the open ocean are honed in every way to catch their favoured prey but they represent only a small subset of the huge diversity within the group. Many sharks are in fact small, bottom living and inconspicuous.
There are some 450-500 species of sharks known to science and only twelve have caused fatal attacks on humans. Sharks inhabit many ecosystems from the deepest ocean trench to shallow coral lagoons; they have even been found 4,000 km up the Amazon River. Their diet can be equally diverse; whale sharks feed on tiny plankton, the cookie cutter shark bites a perfect circle of flesh out of large fish and dolphins and tiger sharks have even been found with car tires and licence plates in their stomach. There are some true creatures of science fiction within the group. Hammerhead sharks with their stalked eyes, horned sharks that have a spine in front of the dorsal fin and thresher sharks with a tail as long as the body used to whip and stun prey. Not to mention the elusive and cartoon-like megamouth shark, known from only forty animals, the most recently collected specimen became a curry in the Philippines!
A Long Lineage
Sharks split from the other fishes 100 million years before the dinosaurs even walked the Earth. As a result of millions of years of evolution there are some very fundamental differences between today’s bony and cartilaginous fish. The bony fishes are the most species rich vertebrate group with up to 30,000 members that include the smallest fish (a tiny carp measuring 0.8 cm) and the fastest fish (sailfish reaching 110 km/hr). The obvious difference between bony and cartilaginous fish is their skeleton, which is not calcified in cartilaginous fish and therefore more flexible compared to relatively brittle true bone. Sharks belong to the cartilaginous group and have many other unique features to their morphology. The skin of sharks is made of millions of tiny tooth-like scales known as dermal denticles, which are both tough and help streamline the fish. The same principles behind denticles have actually been applied to professional swimming suits as they reduce drag in the water.
Sink or Swim
Another unique feature of sharks is their buoyancy control. It is not only divers that must remain neutrally buoyant in the water column. Sharks and bony fishes have approached this quandary from different perspectives. The bony fish use similar principles to a BCD and have an organ known as a swim bladder that is basically a balloon filled with air. Air within the swim bladder is added to or expelled depending on whether the fish wishes to go shallower or deeper. You can even see this happen occasionally when a school of fish quickly ascend. This isn’t fish flatulence but air being released from the swim bladder!
Sharks have a different solution to buoyancy control and use a combination of a buoyant liver and morphology to keep their heavy bodies from sinking. The liver of a shark is full of oils, which are positively buoyant and thus help towards making the animal float. The liver can make up 25% of its weight. In addition to the liver, the shark’s fins and body contours also are aligned to push it upwards in the water. The snout is angled slightly, pushing the head upwards, and tail shape is designed to help provide lift. If you look at the tail of a shark the upper lobe of the fin is virtually always longer than the lower lobe. The longer upper lobe helps to push more water downwards, and therefore the shark’s body slightly upwards as it swims.
After learning a few aspects about the biology of sharks it is fascinating to then apply this knowledge to some of the sharks we might see on a dive. The bottom living sharks common to our area such as leopard, bamboo, wobbegong and epaulette are all content sitting on a rock rather than constantly swimming around. Their body has evolved to suit this life style and this is evident in the length of their tail. As I mentioned, the top lobe of a shark’s tail provides lift and the bottom living sharks need a lot of this to get going. The lobe is hugely extended in these species to help lift them up. They are quite negatively buoyant so without the extra lift generated from the long upper lobe of the fin they would have trouble getting off the bottom.
A longer upper lobe is not limited to bottom-dwelling sharks. All but a few sharks have these asymmetrical fins that help to counteract their sinking bodies. The upper lobe of some sharks, such as reef and tiger sharks, also helps them with manoeuvrability and allows for sudden bursts of speed. The varied prey that these sharks feed on has driven the evolution of their tail fin. A couple of notable exceptions, which actually have almost symmetrical tail fins are great white and mako sharks. These are active sharks that rely on speed rather than agility in the water to catch their prey. The enlarged lower lobe of the tail fin provides them with the additional thrust to catch fast moving prey such as tuna and marine mammals.
The teeth of sharks are another fascinating feature of their anatomy. As you might expect each species has teeth that are perfectly shaped to suit their diet, and to some extent it is possible to infer the diet from the shape of the tooth. Grey nurse sharks, although they look fearsome, thanks to rows of needle-like teeth, prey only on small fish. The teeth are suited to pierce the fish and would be completely unsuitable for biting chunks out of bigger prey. The teeth of tiger sharks on the other hand are adapted to allow them to tear chunks out of large prey such as a whale carcass. They are oblique and serrated allowing them to feed on a great variety of prey including stingrays, seals and turtles.
It is a common myth that sharks will asphyxiate if they stop swimming. This stems from the fact that oxygen transfer between seawater and the blood in the gills is not as efficient as is in bony fish. Sharks need a constant stream of fresh water to flow over the gills for them to extract oxygen. Many open ocean and reef sharks therefore must keep swimming to quench their demand for oxygen, but obviously many other species have found a way around this problem. If you notice a white tip reef shark sitting on the sand it will look as if it is panting, but it is actually keeping a flow of water over the gills providing all the oxygen in needs whilst at rest.
Most people are more familiar with the open ocean sharks than the smaller inshore and reef-living species. These smaller sharks are fascinating in their own right and should not be overlooked by divers. One group of bottom-living species found in our area are the epaulette sharks. The word epaulette is derived from the French meaning “little shoulder” and refers to the black spot that is found in many members of the group, just above the pectoral fins. There has been a recent flurry of new species described in the genus Hemiscyllium, to which the epaulettes belong. The island of New Guinea has revealed a host of species that were only discovered as expeditions went to document the wild shores of the little known island. Since epaulettes do not travel far in their lifetime, and will not cross areas of unsuitable habitat, they easily become isolated in bays and gulfs. Once populations become separated from each other in this way, they gradually, over many generations, change to suit the conditions where they are living and become a separate species. Examples of this are the Milne Bay species of eastern Papua New Guinea that has a brown honeycomb-like pattern, whereas the Raja Ampat epaulette from western New Guinea is pale with occasional spots.
Unfortunately sharks are facing their greatest challenge for survival in the 400 million years of their existence. Shark finning is the term used for the fishing technique where sharks are caught, usually by long line, and their pectoral, dorsal and tail fins removed and body discarded. The insatiable demand for fins to be used in shark fin soup is decimating their populations globally and estimated to claim the lives of 70-100 million sharks per year. Without protection the current trend of disappearing sharks will continue until they are no more.
A great mythology surrounds this misunderstood group. Divers soon realise that sharks are graceful, fascinating and harmless. Perhaps less well appreciated is the amazing diversity found within the group and the threats faced by them. These are animals on the brink of extinction and with them we would loose a huge array of astounding fish.