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Cartilaginous Fish - Sharks, Rays and Skates
"Cartilaginous fish" - not a very awe-inspiring title is it? And yet many of the species that cause most excitement for divers belong to this group. Cartilaginous fish of the class Chondrichthyes include sharks, skates and rays, all of which have a skeleton made entirely of cartilage. They are some of the oldest species on the planet; sharks have been around for 400 million years, appearing long before the emergence of dinosaurs.
Here we introduce some of the species you might see when diving with Master Divers.
Whale Shark (Rhincodon typus)
This beautiful slow moving shark is the largest fish in our oceans today, reaching a maximum of 20m and weighing over 12 tonnes. It can often be seen at Chumphon pinnacle just off from Koh Tao, but has also been spotted at shallower dive sites. The migratory route of the whale shark is still relatively unknown, however it is predicted the shark comes to the waters surrounding Koh Tao to feed. The shark is a filter feeder obtaining small plankton from the water, by sucking water over its gills and extracting the plankton using gill rakers (small bony projections). Whale sharks unlike most fish are ovoviviparous, giving birth to live young after incubating eggs internally.
Leopard Shark (Triakis semifasciata)
Leopard sharks have distinctive markings resembling a leopard which gives the species its name. It can reach up to 2m in length and are usually found near the ocean floor as they are bottom feeders, consuming crustaceans, mollusks and small fish. This shark is viviparous, giving birth to live young.
Grey Reef Shark (Carcharhinus amblyrhynchos)
The grey reef shark can frequently be seen at Chumphon Pinnacle off Koh Tao, and is the most common shark in these waters, they are commonly found in groups as they are a social species. The shark is almost entirely grey excepted for its white underside and can reach sizes of up to 2.5m. The species predominantly predate squids, various crustaceans and reef fishes. Grey reef sharks are viviparous animals giving birth to live young with up to 6 pups in a single litter.
Blue-spotted Sting Ray (Taeniura lymma)
This common ray can reach up to 2m including its tail and is often located on sandy patches sheltered by rocks and corals. It has a venomous spine on its tail for protection and actively hunts for invertebrate prey including molluscs, crabs, shrimp and worms.
Manta Ray (Manta birostris)
This incredible ray can reach a wing span of up to 8m and is the largest ray in our oceans today. The ray is a plankton feeder, filter feeding the small organisms out of the water column; cephalic horns lie either side of the ray’s mouth and are two long lobes which act like a funnel to the plankton. They are generally located in areas above the reef where currents bring larger quantities of planktonic organisms to them. Manta rays are ovoviviparous, giving birth to live young after incubating eggs internally; they have been known to give birth to their young as they leap clear of the waters surface.
Environmental Issues:
Shark Finning: Sharks all over the world are being hunted in absolutely enormous numbers. Once the fins have been butchered from them, the body is tossed back into the water. The shark is often still alive, and it can take days for the butchered shark to die. This is by far the biggest problem facing sharks today, and it's growing. Check our page on shark finning for more information, and learn how you can help to put an end to it.
Pollution: The main issue here is mercury poisoning. Whilst some sources claim that the high levels of mercury now being found in sharks and fish comes from natural sources, the concensus is that it comes from burning coal. The minute quantities of mercury found in coal are released, and through various means, find their way ultimately to the ocean. Due to the considerable volume of coal burnt on earth every day by power stations, the amount finding its way into the sea is unsustainable. Unlike other poisons, once mercury finds its way into your body, it cannot be processed, so it remains in the body. As your life conrinues, and you absorb more and more mercury, the levels finally become more toxic than your body can deal with, resulting in insanity and death. Mercury was rubbed into the felt of a top hat by its maker in order to achieve a fine glossy finish. Mercury is easily absorbed through the skin, hence the term "Mad as a hatter!": the milliner concerned is clearly suffering from mercury poisoning, and it was common enough to coin the phrase. Sharks die in significant numbers due to mercury poisoning.
Public Opinion: Sharks aren't seen in the same way as other endangered species. When most people hear the word 'shark', they think 'Jaws'. This novel by Peter Benchley (and the subsequent film) was key in creating a false opinion of sharks in the minds of almost the entire western world as merciless killing machines. This utterly false impression is responsible for the deaths of huge numbers of sharks, because people generally just think "Good, one less shark!", leaving shark finners and the like free to exploit these beautiful creatures by murdering them in large numbers.
Yes, it's true that people do get bitten by sharks (though far less than get attacked by dogs, cats, or even birds each year), but, as when a dog bites, it's usually because people don't know how to behave around sharks and they frightened them. Even frightened, most sharks will simply swim away as quickly as possible, not turn around and attack you.
Fortunately, Peter Benchley has at least acknowledged his contribution to shark population by introducing 'The Peter Benchley Award for Shark Conservation' in an attempt to reverse the trend.
The number of sharks in our waters has declined by 90%, and many species are now endangered.
Diving with Master Divers, you'll have the opportunity to see sharks in their natural habitat if you want to. Please, take what you learn from doing this and tell your friends - spread the word that sharks don't deserve to die because of some silly stories told 30 years ago...
