Australasian Science: Australia's authority on science since 1938

Why Do Whale Sharks Get So Big?

Credit: Mark Meekan

Credit: Mark Meekan

By Mark Meekan

Whale sharks have evolved to become the world’s largest fish as a consequence of feeding on vast amounts of tiny prey in the cold ocean depths.

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Although whale sharks are acknowledged as the world’s largest fish, there are debates around just how big they can get. Consensus has settled on a maximum size of around 18 metres total length – about the same size as an articulated bus that you might hail on a city street. The largest whale shark that was ever captured weighed 34 tonnes, but its length was only estimated rather than directly measured. This is the case for most other sightings of the largest animals – size has usually been estimated by reference to objects of known length, such as boats or snorkelers.

Given their immense size, it’s a paradox that whale sharks feed on tiny prey. Studies of their gut contents and faeces show that they target tropical krill and mysids, shrimp-like animals that have body lengths about the size of a fingernail. They also feed on small fish and crab and fish eggs, with the sharks arriving in large numbers in coastal areas where fish schools are spawning or where land crabs make seasonal migrations to release eggs at the water’s edge.

Whale sharks focus on this tiny prey because they are extremely abundant, so food is readily to hand, but because the size of each individual prey is very small, whale sharks must gather immense numbers to power their growth and sustain their metabolism. They do this by filter-feeding, drawing large volumes of water across...

The full text of this article can be purchased from Informit.