Introduction
Dik de Boer is a Dutch national who together with his Filipino wife Cora built the first dive resort on Malapascua Island in 1997 (Exotic Island Dive Resort). Tourism was still a long way off at the time, with only a few beach cottages to rent and no existing diving facilities. They began asking the locals about the surrounding waters and were enthralled by stories of sharks and Japanese war wrecks. The stories were enticing enough for Dik and Cora to return in 1997 with a little portable compressor. Whilst exploring the surrounding reefs, they soon realised the enormous potential of Malapascua as a dive destination and laid the foundations to build the first dive resort.
It was during this time that Dik along with his friend Mikael Person saw the first Thresher Shark at Monad Shoal, which has now become a major worldwide attraction. Monad Shoal is now believed to be one of the only places in the world where you can observe at close range these beautiful sharks in only 25m of water, on an almost daily basis. Many film crews from around the world have already visited Malapascua Exotic Island Dive Resort to film these unique sharks, from the National Geographic to Britains Monty Hall. Articles have been published in numerous dive guides including Tauchen, The Diver and Scuba Globe to mention a few and many different travel magazines ensuring the success of Malapascua as a top class dive destination. These infamous sharks are mysterious and beautiful, once sighted you never forget the magnificent tail that swathes through the water like the edge of a knife.
Before we learn about thresher sharks, it would be prudent to have a basic knowledge of sharks in general, especially the family Lamniformes of which the Thresher is a member. Therefore before learning about the special characteristics of the thresher shark here is an introduction into sharks as a cartilaginous fish.
Sharks a cartilaginous fish
Evidence suggests that the cartilaginous fishes to which sharks belong may be the oldest of the jawed vertebrates. Some sharks survived as a group for 200 million years. They are all gone now; no trace of them exists outside of some teeth and scales. Sharks have inhabited the worlds oceans for over 400 million years. They have widespread global distribution and as an apex creature they play a vital role in maintaining the health of ocean ecosystems.
Sharks are likely to be in the first round of marine extinctions caused by human activity. As top predators they are naturally relatively scarce, but also highly vulnerable. Some have gestation periods longer than an elephant, produce only a handful of young and take up to 25 years to mature. Though sharks have swum the oceans since before the dinosaurs, they have never faced a predator as voracious as industrialized humanity.
We still have an irrational fear of sharks, which may explain our lack of will to conserve them. Perhaps because we fear the unknown and so much about sharks is still a mystery. Yet increasingly the well informed are developing a respect for these magnificent predators, some of natures most successful designs. Divers now relish encounters with sharks and will travel to far-flung places simply to have the experience of a shark dive.
2009 was the official Year of the Shark, with governments around the world endorsing safe fishing practices, frowning upon shark fining and other detrimental fishing techniques and generally encouraging communities to do their part in the preservation of our oceans.
Sharks comprise about 7% of living fish species. They inhabit almost every marine ecosystem on earth and are found in all the worlds oceans, as well as in many inland waterways. In evolutionary terms, sharks are one of the most successful families of animals. They have survived extinction events with their diversity relatively intact and may therefore make excellent indicator species in gauging the effects of human activity in marine ecosystems.
So what are cartilaginous fish?
The 4 groups of living cartilaginous fishes are sharks, skates, chimeras and rays. Together they comprise what zoologists call the class of Chondrichthyes. The cartilaginous fishes have an internal skeleton made of cartilage; a flexible, lightweight material, which may be hardened by calcareous deposits but contains no true bone. Sharks keep growing cartilage as long as they live, and have extra mineral deposits in their jaws where they need extra strength. All cartilaginous fish are predators eating an array of food from small zoo plankton, to shell fish to whales.
Characteristics unique to the modern cartilaginous fishes include the fact that they do not have a swim bladder, which helps to keep the shark at a certain depth. Therefore if they stop swimming, even while sleeping, they will sink to the bottom and slowly drown. Most cartilaginous fish are ectothermic or cold blooded with a high metabolic rate, yet some species have the capacity for endothermy, which helps maintain a body temperature several degrees above that of the surrounding seawater by conserving metabolic heat with the help of special networks of blood vessels set deep in their musculature. (rete mirabile, Latin for wonder net)
Sharkskin feels like sandpaper because it has small rough placoid scales, also known as dermal denticles. As a result it is often dried and used as a leather product or sandpaper. Placoid scales consist of a basal bony plate buried within the skin and a raised portion that is exposed. Dermal denticles are small tooth-like structures on the skin that give them hydrodynamic advantages as they reduce turbulence when swimming. The placoid scales do not continue to grow once they are fully formed; instead they are replaced by new scales when they wear out
The shape, number and appearance of shark teeth vary considerably among shark species, and can be one of the most important features for species identification.
The nostrils of almost all cartilaginous fishes are located on the underside of their snouts; they only use them for smelling, not for breathing. Smell plays an important role when detecting the scent of a distant food source, e.g. a wounded fish loosing blood. A shark can detect one drop of blood in a million drops of water (25 gallons or 100 liters) and can smell blood 0.25 mile away. Water continually flows through the nostrils, giving the shark olfactory information.
Hearing is much better developed than once thought and also is long ranging as the sound travels almost 4 times faster under water than in air. The obvious source of low frequency sounds are made by sick or wounded fish, and prey in this state is an easy target. Sharks can actually hear sounds up to 0.6 miles away.
Vision is very acute in many sharks. They have been trained to respond to minimum light levels and when compared to human subjects, sharks were about ten times more sensitive than a human tested in the exact same apparatus. Many Cartilaginous fish possess a nictating membrane, a type of eyelid that protects the eye from injury whilst the shark attacks its prey.
Taste comes into play when prey items are taken into the mouth. If the results are not satisfactory the food is spat out again. There is some evidence that they can taste certain fatty substances that are present in seals and whales. Humans are simply too skinny and not tasty enough for them. Most attacks on humans by sharks are almost certainly a case of mistaken identity and the victim is normally spat out as soon as the first bite is taken
Cartilaginous fishes have a comparatively large brain and an impressive set of sensory organs, which are used to find their way through the oceans, locate prey and eventually join up with mating partners. Besides the familiar senses cartilaginous fishes possess a lateral line system, a row of fine pores along each body side and branching on the head, which allows detection of vibrations over a distance of several meters, e.g. the aberrant movements of speared fish.
Sharks have a special network of jelly-filled canals called the Ampullae of Lorenzini, the function of which was not discovered until fairly recently, that detect electric fields This electro-receptive organ lets the shark pick up weak electrical stimuli from the muscle contractions of animals.
Depending on species they have 5-7 pairs of gills, the sub-class Elasmobranch which sharks fall under have 5 sets of gills on either side of the head and gill slits without a bony cover. The gills function is fairly similar to our own lungs. Some sharks actively open and close their mouths, pumping in respiratory water over the gills; while others simply leave their mouths open and keep swimming. This is called ram-jet ventilation. As water passes over their gills, oxygen is absorbed by the blood in the gills and transported from there to the rest of the body. A spiracle which is a tiny opening found on either side of the head behind the eyes allows seabed resting sharks demersal to take in water and pump it through the gill chamber without taking in large amounts of silt or mud.
The shape of a shark, round bodies tapering off at both ends, allows the shark to glide through the water, and is specially designed to help it navigate long distances and manoeuvre around its prey with ease and without using up all their fuel before they get where they want to go. Its several pairs of fins help it navigate through the water,
The fins of sharks are used for stabilizing, steering, lift and propulsion. Each of the fins is used in a different manner. There are one or two fins present along the dorsal midline called the first and second dorsal fin. These are anti-roll stabilizing fins, basically used for balance.Pectoral fins originate behind the head and extend outwards. These fins are used for steering during swimming and help to provide the shark with lift. Pelvic fins are found near the cloaca and are also stabilizers. In males they have a secondary function as they are modified into copulatory organs called claspers. The caudal fin has both an upper and lower lobe that can be of different sizes and the shape varies across species. The primary use of the caudal fin is to provide thrust. The upper lobe of the caudal fin produces the most thrust.
Sharks are born with their own special colours to protect them from predators above and below them in the water. They are generally a dark colour on top, which is called their dorsal side, and a light colour on the bottom, which is called their ventral side.
There are many different forms of reproduction in cartilaginous fishes, however all have internal fertilization. The males of cartilaginous fishes have a pair of claspers, which are located at the inner margin of the pelvic fins. One or rarely both of these intermittent organs are inserted into the genital opening, cloaca, of the female during copulation. Some species are oviparous, or egglayers, and some are ovoviviparous, hatching the eggs within the female and giving birth to live young. Still others may be viviparous, with the young developing in utero, similar to mammals, with the yolk sac developing into a yolk placenta providing nutrients to the embryo. Most sharks, including the thresher shark, give birth to live young.
An introduction to the thresher shark
The Thresher shark is from the family Lamniformes, otherwise called makeral sharks This family of sharks is amongst the fastest in the ocean. The genus and family name for the thresher shark derive from the Greek word Alopex meaning fox. In some European countries they are known simply as the Fox Shark.
There are three types of Thresher shark, the Common Thresher Shark, Alopias Vulpinus, the Pelagic Thresher Shark, Alopias Pelagicus, and the Bigeye Thresher Shark, Alopias Supercilious. Pelagic Thresher Shark, this is the smallest of the Threshers growing to a maximum length of 3mts.
The common shark, is the biggest of the Threshers growing to 5 or 6mts. The Bigeye Thresher Shark, grows to a length of 4 to 5 mts All three types have a counter shaded body Each three species have specific dorsal colouring. The Common Thresher Shark has a mostly dark green dorsal colour, the Bigeye is mostly brown and the pelagic Thresher has a blue dorsal colouring.
The Thresher shark has an almost dolphin like quality to it with its somewhat pointed cone shaped head and a short stubby body. At the end of this body is the characteristically elongated caudal fin, reaching up to a third (33%) of the length of the entire shark. This extremely long scythe shaped caudal fin is used as a weapon to stun and corral prey and also to propel the shark out of the water. This stunt is calledbreaching similar to dolphins and whales; the shark leaps from the water and slams down repeatedly on one side to remove annoying parasites.
On either side of the body are the long narrow and slightly curved, extremely strong pectoral fins. These are used for direction whilst swimming and can be lowered to slow the shark down when they wish to be cleaned. A small anal fin can be located on the underside and 2 dorsal fins, the first being much larger than the second are located on the upper back body.
The relatively small eyes are set backwards of the conical head and have no nictating membrane As the Thresher sharks are predominantly deep water sharks they are very sensitive to light, The use of strobes and camera flashes is not recommended as they simply scare off the sharks. Common with the Lamniforme shark family the Thresher shark has 5 asymmetrical gill slits that are set behind the eyes. Threshers have to have a constant supply of water washing over their gills in order for them to stay alive.
Unlike most sharks that have a large terminal mouth and use it to hunt, the thresher sharks have a small mouth set slightly below and backwards of the eyes. They have 24 29 rows of small yet razor sharp curved teeth; with smooth edge cusps found both on the top and bottom jaws. They regenerate 2 sets of new teeth every 10 days during their younger years.
The skin of the Thresher shark is covered in very strong pointed placoid scales, also called dermal denticles. The Thresher sharks are ovoviviparous: give birth to live young. The eggs hatch inside the uterus and develop into embryos. Each shark has two uteruss therefore they tend to give birth to 2-4 shark pups, ranging in size from 3-5 feet. There is no placenta to nourish the pups. The pups are oophagous and eat any unfertilized eggs and the dominant pup in each embryo will eat the smaller weaker pups, therefore controlling birthing numbers.
Both sexes are slow to mature with the female reaching sexual maturity between 7 13 years and at a size 3.5 4mts and the male between 8 - 14 years and at a size 3 3.5mts. They are essentially piscivorous sharks, feeding mainly on small schooling pelagic fishes such as mackerel corralling them with its large elongated tail and crustaceans and squids.
They have been known to approach shorelines, shallow water outlets and bays for feeding also, again using their long tails to herd schooling fishes. The Thresher shark hunts its prey in a different manner from other sharks, it uses its long elongated tail to corral and then repeatedly stun fish until they become unconscious
