In the Similan, we have 3 main kinds of sharks:
– Reef sharks (blacktip, silvertip and with a lot of luck, grey sharks)
– Leopard sharks
– Whale sharks
Reef sharks are extremely fearful when facing men and divers. This is why they quickly flee our presence, which makes their identification and their study very hard.
The international community is currently focused on the identification and recognition of leopard sharks and whale sharks. That is why we offer you the overview of these 2 sharks in particular.
But do not worry: even if our conferences focus on these 2 types of sharks, you will also get to meet the reef sharks during your cruise!
THE LEOPARD SHARK, ALSO CALLED ZEBRA SHARK
Family: He is the sole member of the Stegostomatidae family and of the Stegostoma genus.
Appearence: Large spindle-shaped body, short snout, small eyes located on the sides of the head, big mouth. The back edge of the first dorsal fin is slightly concave. The pectoral fins are large and triangular for the adult. The body above is of a yellow tone, and white below. Dark spots on the back and a few spots on the sides.
The zebra shark (Stegostoma fasciatum) is a tropical species that can measure up to 3.5 metres. The name comes from the fact that young sharks have stripes before developing dark spots as an adult.
Habitat: The zebra shark can be found from the Indian Ocean to the Indo-Australian archipelago, where it occurs the most. It is an abundant species in temperate to warm waters that preferably lives towards the coast and sometimes in the open sea. More common in shallow waters, from 4 to 40 metres, it enjoys sandy bottoms and coral reefs, favouring spots protected from the current.
Alimentation: it spends part of the day resting on the sea bottom and feeding on crustaceans, molluscs and small fish in nearby coral reefs.
Reproduction: zebra sharks are oviparous, which means that the females lay eggs that have the appearance of a turned-down, more or less rectangular and bulging, envelope of more or less twenty centimetres long. The embryo develops inside the envelope for about 5 months before tearing it to get out. Of about thirty centimetres, the baby wears a black livery adorned with white dots and creamy lines, which has earned the species its common name of zebra shark. Its growth is then rapid, with his weight increasing from 100g to 1kg in the course of three months. During this growth, its livery changes to become beige with black spots, which is the reason why the English also call them leopard sharks.
In the Similan, we can find them around the island of Kho Tachai.
THE WHALE SHARK
Family: The whale shark (Rhincodon typus) is a cartilaginous fish, the sole member of the Rhincodon genus and the only current member of the Rhincodontidae family.
Appearence: The whale shark is currently identified as the biggest fish on Earth that can measure up to 20 meters and weigh up to 34 tonnes. Note that the encounters with individuals of this size are rare, whale sharks usually measuring 4 to 14 meters. Its body is long and massive, and its movements slow and graceful. It has a broad, flattened head and very small, lidless eyes located on the sides of his head. It’s very large and flat mouth can reach 2 meters wide, allowing it to filter up to 2,000 litres of water per hour! Its dentition means a very large number of tiny teeth, but it uses only those located farther down its mouth. In addition to their respiratory function, the five pairs of very long gill slits are used to filter water to separate the food before swallowing.
The skin of the belly of the whale shark is completely white, while the back is grey, darker than most sharks, with lots of light spots and horizontal and vertical yellow and white lines of white or yellow forming a “checkerboard”. These spots allow the identification of individuals by photo-identification: MSDS, a type of identity card, are associated with the animals, and are then used by researchers to recognize and count the whale sharks.
The skin of the whale shark is thicker and harder than any other species in the world, reaching up to 15 centimetres thick. It is covered with dermal dentils, which are its main defence.
The first dorsal fin has a generally triangular shape. The end is rounded in juveniles and becomes more upright and angular with the age. Sides have the same “checkerboard” livery as the body. The second dorsal fin is smaller.
The pectoral fins are powerful and have the shape of a scythe. The upper side is dark and dotted with spots while the underside is white.
The caudal fin is heterocercal (asymmetric), the upper lobe being much larger than the bottom one (the asymmetry becomes less pronounced in adults). This fin is the one that provides the driving force. However, the whale shark is not an efficient swimmer: he uses his whole body to swim and travels at an average speed of 5km/h, a relatively slow speed for a fish.
Habitat: Easy to recognize with its checkerboard livery, the whale shark can be found in open seas and tropical and warm oceans. Its lifetime is estimated between 100 and 150 years, although the oldest recorded specimen was about 70 years old. Although there is no precise data on the total population, the species is considered vulnerable.
Alimentation: It feeds mainly on plankton, algae, krill and microscopic animals that they absorb through their wide mouths, like the blue whale, its equivalent marine mammal. Massive, moving slowly and devoid of aggression, this giant of the seas is perfectly harmless to humans.
The authors do not agree on the mode of reproduction of the whale shark. It seems ovoviviparous, but this has long been disputed. In 1953, an egg 30 centimetres long, 14 centimetres wide and 9 centimetres high, containing an embryo of almost 36 centimetres was discovered in the Gulf of Mexico. The hypothesis suggested that the species is oviparous. However, due to the rarity of the collection of viable eggs, their fragility and the incomplete development of the gills of the embryo inside it, but also due to the presence of umbilical scars observed on smaller living individuals (55 centimetres long), another assumption was made: that the egg found in the Gulf of Mexico was abandoned prematurely and that the whale shark would normally be ovoviviparous.
The type of oviparity adopted by the whale shark is relatively simple and looks very similar to the nurse sharks: their young develop in an egg in the uterus and must be extracted first before being “dropped” by the mother.
The gestation period is not known, but some authors suggest that the whale shark can reproduce every two years, just like the nurse shark. Sometimes it disappears for weeks, plunging more than 1.5km below the surface to rest in the depths. Nobody knows where they mate and calve.
The smallest whale sharks found alive in the wild measured 55 to 59 centimetres long, some sporting umbilical scars. These juveniles were caught in the tropical West Africa in the Central-East Atlantic and close to Central America in the eastern Pacific, near continental waters and in the ocean far away from land, which suggests that their young can be born in the ocean and that calving areas exist there.
In the Similan, we find whale sharks mainly on the sites of Koh Tachai, Kho Bon and to our delight again around the mythical Richelieu Rock!
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