Whale sharks: Giants of the oceans

When looking at a whale shark (Rhincodon typus) in its full dimension it is easy to forget that it is in fact is not a whale but a fish. With a maximum body length of 14 meter and a weight of up to 12 tons the whale shark is the largest living fish in the ocean. It belongs to the category of carpet sharks (Orectolobiformes) and its distribution area reaches around the globe from tropical to subtropical waters.

Its colour varies from a greyish to a brownish shade with white, spotted lines on his back and a white bottom side without patterns. Each individual has a different pattern on the back which makes it possible to identify an individual whale shark by looking at the spotted lines on its back, like a fingerprint. Its mouth goes along the whole side of the head and is equipped with around 3600 small teeth. The mouth functions as a funnel to filter microorganisms from the water when it is open.

Zooplankton is the main source of food, which is why whale sharks can be found in waters with seasonal plankton. To catch its prey a whale shark is swimming for hours with an open mouth and sucks in up to 6000 l of water per hour. The water is then pressed out again through a sponge-like filter at its gills and the plankton stays inside the whale shark and can be digested.

Even though whale sharks can be easily spotted due to their size, there is not much known about these gentle giants, especially about their social and mating behaviour. The capture of a pregnant female in 1995 along the coastline of Taiwan revealed that whale sharks are ovoviviparous. This means that they are developing eggs which hatch inside the female’s uterus and are born as living juveniles. The different stages of development inside the mother’s body lead to the conclusion that the whale shark can actively influence the birth date of the offspring and therewith increase the chance of their survival.

Whale sharks start reproducing between the age of 10 and 30 years. Due to the slow reproduction rate and overfishing the whale shark is highly endangered. A sign for the protection of these animals was set by environmental activists and NGOs in 2010 in Dubai. They protested against the capture of a 4-meter-long male whale shark which was kept in the biggest aquarium in the world in the Atlantis hotel. The management of the hotel gave in to the public’s pressure and the whale shark was released back into its natural habitat in the Persian Gulf.