‘The world’s biggest fish’
Despite their huge size, whale sharks are docile, filter feeders that cruise the world’s oceans looking for plankton. These gentle giants are famous for their annual gathering at WA’s Ningaloo Reef, but outside these waters very little is known about this threatened species, which is considered as globally vulnerable.
Are Whale Sharks a whale or a shark?
Whale sharks are a type of shark, not a whale. This is because they breathe through their gills, and have cartilage instead of bone. They are the largest known living fish species with the largest confirmed length of 12.65m (41.50 ft) and a weight of more than 21.5 tons.
Whale sharks inhabit all tropical and warm-temperate seas. They are primarily pelagic meaning that they live in the open sea but not in the greater depths of the ocean. Seasonal feeding aggregations occur at only several coastal sites including the Ninagloo Reef.
Whale sharks have a mouth that can be 1.5m wide containing up to 350 rows of tiny teeth and 10 filter pads, which it uses to filter feed. They have 5 large pairs of gills and appear grey with a white belly, with a skin of up to 10cm’s thick which is thicker and tougher than any other species. The markings on their skin are pale yellow spots and stripes, which are unique to each individual. We can use these patterns to correctly identify and record sightings of whale sharks worldwide. A juvenile shark will have a larger upper tail fin than its lower fin, while adult tails become somewhat moon-shaped. Male whale sharks can be distinguished by the presence of two claspers near the pelvic fin, which are absent in females.
The whale shark is one of only three known filter feeding shark species. They have sensory cells in the nasal grooves above the mouth, which help the shark detect food in the water. It feeds on macroalgae, plankton, krill, Christmas Island red crab larvae and small nektonic life, such as small squid or vertebrates. It also feeds on small fish and the clouds of eggs and sperm during mass spawning of fish. Their movement patterns are appear to be linked with spawning of coral and plankton blooms, which explains their presence in Ningaloo following the mass spawning of coral each year in these waters.
Whale sharks feed either by ram filtration, where the animal opens its mouth and swims forward, pushing water and food into the mouth, or by active suction feeding, where the animal opens and closes its mouth, sucking in volumes of water that are then expelled through the gills. In both cases, the filter pads serve to separate food from water. These unique, black sieve-like structures are presumed to be modified gill rakers. Whale sharks have on occasion been observed “coughing”, presumably to clear a build-up of particles from the filter pads.
Whale sharks reach sexual maturity at around 30 years. Mating behaviors of whale sharks haven’t yet been observed, but it is believed that the eggs remain in the body and the females give birth to live young, which are 40-60cm long. Evidence has indicated that pups are not all born at once, but rather the female retains sperm from one mating and produces a steady stream of pups over a prolonged period.
The population of the whale shark is unknown and the species is considered vulnerable by the IUCN. Ongoing research will help us learn more about these amazing animals. Australia plays an important role in the conservation and protection of this migratory species. Ningaloo Reef Marine Park represents one of the few strongholds in the world for this magnificent, gentle giant of the deep.