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AN INTRODUCTION TO SHARKS BY PROJECT OCEAN VISION
Sharks belong to the class chondrichthyes or cartilaginous fishes. They are jawed fish with paired fins and two-chambered hearts. The class is divided into two subclasses: Elasmobranchii or elasmobranchs - the sharks, rays and skates - and Holocephali - the chimaeras, sometimes called ghost sharks. Elasmobranch means that they have arch-shaped gills. Unlike the bony fishes (teleosts), chondrichthyes' skeletons are made of cartilage and instead of bony scales, they have skin that is covered in dermal denticles, like small teeth, making their skin very rough and producing a highly efficient hydrodynamic surface.
Sharks mouths and nostrils are located on the underside of the head (exceptions to this are: frilled sharks, megamouth sharks and devil rays, which have 'terminal' mouths - this is the most obvious distinguishing feature between manta rays and devil rays). See the video to the right. Males have a pair of claspers between their pelvic fins; these are used for internal fertilisation. Their teeth are not attached to their jaws, but instead are embedded in their gums. This means that their teeth can be replaced continually throughout their lives; a shark may go through 20,000 individual teeth. Each species of shark has teeth adapted to suit its feeding habits.
Manta Ray showing position of mouth.
A good way of assessing the success of an evolutionary group is to look at its diversity, longevity and distribution. There are around 350 species of sharks and rays (estimates vary) in our oceans and rivers all around the world. Some shark species are extremely primitive and have remained more or less unchanged since the days of the dinosaurs. The first known sharks appear in the fossil record some 350 million years ago. A list of the main shark families is at Appendix 1. By this reckoning, the elasmobranchs are a highly successful group - at least until we came along.
In this marine conservation article I shall concentrate on just the sharks. This is simply an introduction to sharks and illustrates the threats to their survival and why they are vulnerable. More detail and some discussion on individual species will come in future articles.
SHARKS AND THEIR REPUTATION
Shark families include the ocean’s prime predators and this has earned them an unjustified reputation for being dangerous, mindless killers – perhaps they are if you are a fish, but generally not if you are a human. In fact very few of those 350 or so species are dangerous to man and none actively targets human meat for food.
Deciding which species of shark are dangerous is more complicated than you may think because the statistics need some interpretation. Very few observers would consider, for example, a nurse shark to be particularly dangerous, but actually quite a few people are bitten by them; not because they are aggressive (quite the opposite), but because so many people try to handle them. The same is true for blacktip sharks, but in my experience you’d be very lucky ever to get close enough to one in the wild; they are very shy and retiring.
Technically, any shark over about 1.8 metres in length (about 6 feet) can be a danger to man, purely because of its size, teeth and strong jaws. That said, we know that the largest shark (indeed the largest fish) in the seas today, the Whale Shark, is mild mannered and placid – although they too have teeth. Whale Sharks cruise the oceans and eat little that is bigger than plankton. The size of these sharks makes them fearless and easy to approach - sometimes to their disadvantage! See the footage of Whale Sharks in our Expedition Ocean Vision 2 Video Diary.
If I had to name the species of shark that could be considered dangerous, I would have to list five:
Sharks attack humans very infrequently; it is very easy to get the threat way out of proportion. While there are somewhere in the region of 50, maybe 100, shark attacks around the world each year, only around 5, maybe 10, are fatal (statistics from the International Shark Attack File, 1990-2008). You have about a one-in-two-million chance of drowning in the sea or dying at the beach; you have about a one-in-twelve-million chance of being attacked by a shark. Consider that more people die of bee stings, twice as many are killed by lightning in the USA alone and many hundreds of times more are bitten by dogs. And it would be wrong to think that all shark attacks involve horrific injuries; an attack may simply be a bump or a rasping with the upper teeth. Most important, humans kill many thousands of times more sharks than the other way round. Actually, millions of times more, but we’ll come to that later.
The Shark Skeleton
We have seen that the skeleton of an elasmobranch is made of cartilage. The cartilage may be partially calcified (with calcium phosphates and carbonates) particularly in the vertebral column, but this is not true bone. Shark cartilage contains an active ingredient that is thought to inhibit tumour growth; sharks rarely develop cancer. With no hard bones, red blood cells are produced in the kidneys and a special organ called the epigonal. White blood cells are created in the shark's spleen and the spiral valve within the intestine.
A shark's cranium is a single, compact, cartilaginous block that encloses the brain, olfactory, and auditory capsules. The upper and lower jaws are loosely attached to it – the upper jaw is not an integral part of the “skull” and can be moved independently.
Roughly 85% of a shark’s weight is muscle. Sharks have two basic types of muscle, red and white.
A shark's red muscle is relatively slow-contracting and requires a rich supply of oxygen (it is aerobic and contains myoglobin, an oxygen-carrying pigment similar to haemoglobin). Red muscle has tremendous stamina and is used for cruising.
Shark swimming muscles are composed of W-shaped blocks called myotomes. Contractions propagate smoothly along the shark's body and produce the majority of a shark's propulsive force through the caudal fin. In cross-section, shark swimming muscles form bundles of concentric rings that meet at the midline of the body underneath the lateral line. The bundles above the shark's lateral line are termed epaxial (back), those below hypaxial (flank). The arrangement of muscle fibres in a shark is quite different from that in a teleost. In a bony fish, muscle fibres tend to loop back on themselves, in sharks the muscle fibres attach to tough longitudinal struts composed of connective tissue. These struts act as an attachment point along the shark's flanks, compensating for sharks' lack of ribs.
The Shark's Digestive Tract
The shark's oesophagus is short and wide, barely discernible from its stomach. The U-shaped stomach leads to the gut and, in many species, the spiral valve. The spiral valve is the lower portion of the digestive tract. It is internally twisted or coiled to increase the surface area, which increases nutrient absorption. After the spiral valve, the digestive tract leads to the rectum and to the cloaca. The cloaca is a common opening for the urinary, digestive, and reproductive systems.
The Shark's Circulatory System
A shark's heart is a two-chambered S-shaped tube with an atrium and a ventricle. It is small in proportion to body size and, therefore, sharks have relatively low blood pressure. The action of swimming helps to circulate blood throughout the shark's body. Blood is pumped by the heart through the afferent branchial arteries (ventral aorta) to capillaries in the shark's gills where the blood is oxygenated. The blood then flows through efferent branchial arteries (paired dorsal aorta) then through the tissues of the body, and back to heart through the veins.
Fast-swimming sharks, such as great white sharks and mako sharks, have a body temperature that can be quite a bit higher than the surrounding water (up to 8°C higher). The heat is due to the modified circulatory system associated with the red muscle in these shark species. As red muscle functions, it generates heat; this warms the blood circulating through the red muscle. Due to the close proximity of the arteries and veins, heat passes from warmer veins to cooler arteries within the shark's body, rather than dissipating to the cooler environment. This heat exchange system retains heat in the red muscles, improving efficiency.
Shark's Gills and Respiration
Sharks, like other fish, use their gills to extract dissolved oxygen from the water through a system of capillaries. Sharks have 5, 6 or 7 pairs of gill slits located on the sides of their heads, but unlike bony fish, they do not have gill covers. Water must flow continually across these slits in order for the shark to breathe. Filter feeding sharks (such as the whale shark) expel the filtered water through their gills.
Some sharks also have spiracles, which are special gill slits located just behind the eyes, which can supply oxygen directly to the eyes and brain through a separate blood vessel. Generally speaking, pelagic shark species must keep swimming to maintain a flow of oxygenated over their gills, but some species can pump water in through their spiracles and out through their gills allowing these shark species (such as angelsharks) use this extra respiratory organ to breathe while at rest on the sea floor. It is also used for respiration when the shark is feeding. Thanks to these extra gills, Tiger sharks have been found 'sleeping' in sea caves in the Gulf of Mexico, completely stationary for long periods of time.
Water enters the shark's gill chambers through the mouth or spiracles and exits through the gill slits. Blood in the gill filaments absorbs oxygen from the incoming water. Gill rakers, cartilaginous projections on the gill, support the gill structures and protect the delicate filaments from particles in the water that might damage them.
The Shark Liver
A shark's liver is made of two large lobes that concentrate and store oils and fatty acids. The liver functions as an energy store and aids buoyancy. A shark's liver is relatively large, 5 to 25% of its total body weight and takes up to 90% of the body cavity. Unfortunately for the shark, the liver may yield many litres of valuable oil; this has been one of the reasons for shark hunting over the centuries.
The Shark Brain
Sharks (and rays) have very large and complex brains (varying from species to species), but very little is known about them. Sharks's brains to body mass ratio is higher than most other fish and is comparable to other vertebrates, including some mammals. The sharks with the largest brain-to-body weight ratio are the active sharks (such as the dusky shark and the scalloped hammerhead shark). The sluggish bottom-dwellers (like angelsharks) have relatively smaller, less complex brains.
Within the shark brain the cerebellum is largely responsible for movement; most sensory information is processed in the hindbrain, integrated in the tectum and coordinated in the forebrain; the olfactory lobes are the centres for the sense of smell and the diencephalon regulates hormones and certain behaviors.
Sharks' electric sense perception is centred in the ampullae of Lorenzini, small tubes that are extremely sensitive to electrical signals in the water. All animal muscles produce and emit electrical signals when they work and sharks can detect these emissions. These organs are sensitive enough for hammerhead sharks to detect the small electrical signals emitted by their prey whilst it lies motionless below the sand; sharks are able to detect these fields as small as 5 millionths of a volt per centimetre in the 1-8 Hz range. However, since bioelectrical fields are generally so weak, sharks use their electrosensory systems at relatively short-range.
A shark’s vision is about seven times better than human vision, allowing them to see 20 metres or more underwater, in good conditions. They have a structure in their eyes called the tapetum lucidum that increases the sensitivity of the sharks' eyes to light, allowing them to see in the lower light levels associated with deeper water. However, their eyes are mainly adapted to see motion and cannot resolve shapes very clearly. Some sharks have evolved a nictitating membrane on the lower eye lid that protects the eye from damage by prey species during feeding.
Sharks possess a pair of nares, nostril-like holes that are located just under the leading edge of the snout. Each nare is divided by a nasal flap into two openings. Water is channeled into the in-current aperture and having passed over the feather-shaped olfactory lamellae, exits the olfactory sac through the ex-current aperture. The olfactory lamellae is a series of folds on the interior surface of the olfactory cavity that increase the effective surface area improving the shark's sensitivity to chemicals in the water. The olfactory bulb in sharks' brains performs the interpretations of the received data and is a large component in the brain. Sharks can detect molecules at concentrations of only 40 parts per billion.
While sharks are able to detect sound through an organ in the utriculus portion of the inner ear, they primarily rely on small hairs in the ears to feel changes in water pressure. The ears of sharks are located inside the head. By connecting the inner ear to the surrounding water by a number of ducts, the shark has a sensitive mechanism for interpreting sounds under water and direction finding the source. And since sound travels much further and faster underwater, sharks are able to interpret sounds from many kilometres.
A shark's lateral line is essentially a pressure sensor. As any solid object moves through the water it creates pressure waves. Sharks both detect that movement and sense direction. Humans have no sense comparable to the lateral line.
The male shark has a pair of claspers that are a part of their pelvic fins and used to fertilise the female internally. Female sharks produce young in one of three ways, according to species. Sharks are either oviparous, ovoviviparous or viviparous.
THE SHARKS LIFE CYCLE
The average life span of a shark is about 20 to 30 years, although some may live for very much longer; in the case of the piked dogfish, 70 years.
Sharks must reach maturity before breeding and gestation periods can be as much as 22 months. Litter sizes vary, but are generally fewer than 20 pups of which fewer still will survive to sexual maturity. These factors mean that shark populations grow very slowly and any reduction in populations will take long time to recover.
THREATS TO SHARKS
Over-fishing and accidental by-catch are two major causes of rapidly diminishing shark numbers. By far the most abhorrent practice is that of shark finning. Due to their anatomy (described earlier), shark meat becomes tainted with urea soon after death unless refrigerated. So shark meat is low value and not worth transporting to market. Shark fins, however, are an expensive delicacy in Chinese cuisine; one kilo of dried shark fin can retail for $500 or more. Sharks are caught, most frequently using longlines (the most significant cause of losses in shark populations worldwide). The fins are cut from the shark and the carcass is discarded. The shark is usually still alive when it is tossed back into the water; the finless sharks, unable to swim, sink to the ocean bottom and die.
Shark finning is widespread and largely unmonitored. The practice has increased over the past decade due to the increasing demand for fins for Chinese shark fin soup and traditional medicines. Shark specialists estimate that over 38 million sharks are killed for their fins annually. Think about that number: 38 million sharks a year. It is a billion dollar industry.
Because of their life cycle, shark populations are highly vulnerable to finning, over-fishing and other threats such as nets, pollution and reduction of prey species. Using the Red List definitions (see below), many shark species are now endangered, some critically.
The Red list Definitions are:
As a good illustration of the effects of human pressures on sharks, I have included a pdf version of a report entitled Large-Scale Absence of Sharks on Reefs in the Greater-Caribbean: A Footprint of Human Pressures.
A WORD ON CONSERVING SHARKS
It would be simple to state how shark populations can be preserved, but very much harder to limit or halt the practices that are devastating shark populations. Hard economics, ignorance and callous disregard for simple ecology are difficult challenges to overcome. There are many organizations (Appendix 2, below) that are attempting to tackle the issues affecting sharks. At a personal level the best approach is knowledge, support to conservation organizations and not supporting the shark trade, deliberately or accidentally (see Appendix 3) - do not buy shark products.
The General Authority for Fish Resources Development in Egypt ordered a complete ban on shark fishing in the Egyptian Red Sea in 2005 (decree number 484) - one of the few places in the world that has a total ban on shark fishing.
Shark conservation is the subject of a future article. Meanwhile I have recently been sent a very useful link to an excellent site, which is well worth a visit: http://www.travbuddy.com/world-travel-guide/travel-sharks-safety. I am also very thankful to Brendan for sending another very useful link to this site: http://caribbeanhotels.com/islands/creatures-of-the-caribbean-sharks/.
Now, below is a short video by world famous shark conservationist Lesley Rochat from YouTube.
WARNING: This video contains brutal scenes showing the finning of live sharks. Some people may find this upsetting or offensive - you should. Everyone will certainly find it shocking. At least, you should; unless you like the idea of having your limbs cut off whilst you are alive and then being thrown into the sea to drown.
For more on sharks, read our article, Diving with Sharks.
APPENDIX 1: List of Shark Families
APPENDIX 2: Shark Conservation Organizations
Adopt a Shark
Australian Marine Conservation Society
Canadian Shark Research Laboratory
Monterey Bay Aquarium
Project Aware (PADI)
Shark and Coral Conservation
Shark Conservation Society
Shark Research Institute
South African White Shark Research Institute
The Shark Alliance
The Shark Trust
Whale Shark Project
White Shark Trust
APPENDIX 3: Shark Products
Find out about shark products at these sites:
Shark on the Shelf
FAO Corporate Document Repository
Climate of Fortune Article – success story…
|Shark Video from the Underwater Channel|
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