SHARKS - INTRODUCTION
Sharks continue to maintain their reputation as the most feared and impressive fish occupying the oceans of the world. These marine hunters belong to a super-order called Selachimorpha, which is made up of approximately 440 different species. These diverse species can be found in waters all over the planet and are believed to have existed even before the time that dinosaurs roamed the earth, millions of years ago.
Some modern scientists share the commonly held belief that sharks have actually been in existence for some 420 million years. As one of the world’s oldest residents, sharks are certainly all the more fascinating to us, holding some intriguing secrets about the prehistoric world that humans are yet to discover.
Under the umbrella of sharks as a species, there are more than 360 different kinds of these underwater hunters.
The magnificent Great White Shark - Carcharodon carcharias
1. Squatiniformes (Angel sharks – their bodies are flattened, giving them a similar appearance to stingrays)
2. Pristiophoriformes (Saw sharks – so called for their elongated, toothed snout, which they use for slashing at and through their prey)
3. Squaliformes (Dogfish sharks – these include subspecies like the Bramble Shark and Prickly Shark)
4. Lamniformes (Mackerel sharks – these have large jaws and include subspecies such as the Megamouth, Shortfin Mako, Longfin Mako, Great White and Thresher sharks)
5. Orectolobiformes (Carpet sharks – including the Zebra Shark, Nurse Shark, Whale Sharks and Wobbegongs)
6. Heterodontiformes (Bullhead sharks – these are also sometimes known as horn sharks)
7. Hexanchieformes (Frilled and Cow sharks are amongst this order)
8. Carcharinformes (Ground sharks – these sharks have a long snout and a nictitating membrane that protects their eyes. They include well-known species like the Blue Shark, Tiger Shark, Bull Shark, Oceanic Whitetip Shark, Blacktail Reef Shark, Houndshark and Hammerhead Shark, amongst others)
Caribbean Reef Shark - Carcharhinus perezi - shot in the
The different shark species within these orders are extremely diverse in terms of their physical characteristics, distribution, diet and habits. The smallest known shark species is the Lantern Shark, which reaches an average length of only 17 centimetres (or 6.7 inches). The largest is the Whale Shark, which measures an awesome 12 metres (or over 39 feet). The Great White Shark averages an impressive 6.6 to 7 metres (21.6 to 23 feet) and the grizzly Ragged Tooth Shark comes in at about 3.6 metres, or just under 12 feet. With these mammoth hunters gliding through the waters, it is no wonder that sharks have earned themselves a fearsome reputation.
Different shark species tend to prefer different waters (in terms of the prey found therein and the temperatures of the ocean) but, generally, sharks are found all over the world. They may live anywhere between relatively warm, shallow depths and about 2 000 metres (or over 6 500 feet) from the surface, where the waters are dark and icy. Some sharks are even found skating along the ocean floor, camouflaged to blend in with the sand on which they lie in wait for their prey. There are no known sharks found at 3 000 metres or 10 000 feet below the water’s surface. There are a few rare species that have been designed to live in fresh water (such as rivers and lakes), while the vast majority are ocean dwellers. Generally, sharks prefer warmer waters, as they are exothermic creatures (broadly referred to as being cold-blooded, although this is not strictly the case) and rely on the water around them to regulate their body temperature. There are certain species that migrate, while others remain within a defined area for their entire life.
The body of a shark is streamlined to allow these experienced hunters to glide through the water, unhindered by the resistance that the water (which is denser than air) offers. Sharks that skate along the sandy floor of the ocean are generally flattened, allowing them to slide along unnoticed and without unnecessary bulk (since they do not need to exert the same powerful force that is required by sharks that swim against the density of the water).
Scalloped Hammerhead Shark - Sphyrna lewini - photographed
at Cocos Island.
Unlike most other fish, which have both bones and cartilage making up their skeletons, sharks have only a cartilage skeleton. This increases their flexibility and decreases their weight to allow for even faster movement through the water, which is essential for their hunting and general travel.
Sharks have several rows of teeth. As the front row are used to their capacity, they fall out and are replaced by the strong, healthy and brand new row of razor-sharp teeth behind them. These rows of teeth are used only for capturing their prey and tearing chunks of flesh off of the victim. Unlike humans, sharks do not chew their food, but gulp it down as solid hunks of meat.
All sharks are carnivores, meaning that they survive exclusively off a diet of meat and do not eat vegetation in any form. However, although restricted to meat, their diet remains varied. Some species, including the mammoth Whale Shark, swim through the waters with their mouths open, allowing plankton and other small creatures to enter through huge filters. To get an adequate volume of food, this requires these types of feeders to swim like this for most of their lives. The bottom-feeders that skate along the floor of the ocean crush crabs, clams and similar animals that live at this depth and eat these at leisure, while the more ferocious species (such as the Great White and Mako sharks) actively hunt fish, squid, dolphins, seals and turtles. Sharks thrive off prey that has a high body fat content.
Some sharks are known for the feeding frenzies that sometimes take place among them. These frenzies occur when a group of sharks come across a large school of fish or a similar assembly of potential prey. They begin to attack the prey from all angles in a crazed, uncontrolled manner. This results in the injury and even death of many fellow sharks, as they snap their powerful jaws without discrimination between prey and fellow hunters. The resulting injuries are sometimes fatal to fellow sharks and those that survive frequently bear massive scars from their encounters.
Whale Shark (Rhincodon typus) from Maldives
As a species, sharks are believed to be relatively intelligent and teachable. They can learn to adopt patterns of behaviour once they are familiar with these and have been shown to recognise certain people and objects time and again. It has not, as yet, been easy to gauge exactly how much sharks are able to learn, since they are elusive, and sometimes vicious, animals. Testing is not easily conducted so much is left to coincidental findings or rare experiences and experiments.
There have been a number of shark attacks on swimmers and surfers, usually resulting in severe injuries and even death. Still, these remain rare, and far fewer people die in shark attacks every year than by other, less common means. In fact, only about 100 shark attacks are reported worldwide every year, with far fewer fatalities resulting. There does exist the problem that, when a shark does attack, its power and hunting skills usually mean that the victim has little opportunity to fight back and the injuries are usually severe, if not fatal. The Bull Shark is the most common predator for bathers as it frequents the same shallow waters in which the people usually choose to swim. This shark also has high population numbers, so there is an increased opportunity for attacks. Other sharks that have been known to attack humans on some occasions include the Great White, Tiger and Oceanic Whitetip sharks.
To avoid shark attacks, it is essential that swimmers and surfers remain within the swimming confines as set out by the lifeguards on duty. If a bather has a bleeding injury, it is necessary that he or she leave the water immediately, as sharks are able to detect the scent of blood from several kilometres away, and misinterpret it as an injured animal that will make for easy prey.
Various beaches and organisations are also attempting to protect their swimmers and surfers by putting up shark nets. In general, these long nets are placed in two parallel lines about 400 metres from the shore, where the water is approximately 10 to 14 metres deep. However, it is important that the animals (as well as other fish, dolphins and turtles) are able to swim over, under or around the nets if they are not to get tangled up in them and die, which is, sadly, what happens in some areas.
Therefore, the nets do not completely block their path to the shore, but merely work in reducing the numbers of sharks that cross their border and go near to where bathers may be.
The nets need to be checked on a regular basis (preferably monthly) to ensure that they are effective and are not trapping other harmless creatures unnecessarily (such as turtles, for example). This requires employees that are experienced to do this.
In some areas, they may need to ban or restrict bathing and surfing at certain times of the day and year and in particular areas for the safety of the people in the water. This may be a permanent measure or might just last for the duration of a particularly high incidence of shark sightings.
Reef Sharks on Caribbean reef
Sharks, like many other animals today, are often victims of hunting at the hands of human beings. In some eastern countries, shark fins are considered to be a delicacy and their cartilage is believed to have medicinal value. This has resulted in the brutal slaying of millions of sharks every year. In order to save the population numbers of these invaluable creatures, large-scale initiatives are required. As their focus, these initiatives have to educate local communities regarding the value of sharks and their role in the ecosystem, and implement legislation to protect the animals and punish the hunters. Often, it is because locals and tourists alike are not aware of the biological implications of upsetting an ecosystem that they carry on supporting such industries.
Therefore, even those that have not been granted an academic education need to be given the opportunity to learn about sharks and why they should be left in their natural environment and protected. This requires an enormous amount of funding, a resource that is rare, hindering such projects time and again.
Another major issue facing the shark and its survival is the matter of environmental damage. Pollution in the form of oil and plastic, for example, is commonly found in the oceans. These substances poison the animals and cause physical injury to those trying to eat it or getting stuck in the actual wire or plastic. The ones responsible for this type of pollution include ignorant individuals, but also major industries that use the waters of the world as a dumping site for even toxic substances. This abuse of the world’s natural resources continues despite its being illegal in many parts of the world and the damage it causes to the fauna and flora involved. Interestingly, in 1991, South Africa was the first country in the world to declare the Great White Shark a legally protected species.
Sharks grow and age slowly and reproduce at a slow rate. This means that the population levels take some time to reach optimal level. Today, we face the threat of the death rate exceeding the rate at which these animals are able to reproduce and recoup their numbers.
In general, sharks do not do well in aquaria, and particularly not in an aquarium in a home. They need to be transported carefully and require specific care, making them delicate animals and inappropriate for domestic captivity.
A pair of Sand tiger sharks, also called Ragged tooth sharks,
cruise the reef in Western Cape, South Africa
There are myths regarding the inability of a shark to contract cancer and a few other illnesses. This has led to them being hunted as they are believed to have anti-arthritic and cancer-fighting properties. As with so many of these types of myths, this is simply not true. Sharks are, in fact, able to get cancer, as well as other viruses, parasites and diseases.
Sharks are gracious; magnificent in form and impressive in power. They continue to be threatened by hunting and pollution, yet remain one of the most fascinating and humbling creatures to encounter.