A

lthough 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 specialised plates covered in tiny hairs

that lie above the gills. To feed, the shark simply opens its

mouth and slowly swims forward, allowing the food to accu-

mulate in the throat before swallowing. This is called ram-filter

feeding, however sharks can also take advantage of dense schools

or accumulations of prey by gulping large mouthfuls of water.

Although the plankton and meso-zooplankton that the

whale sharks target is abundant, this does not necessarily mean

that it is easy to catch. If you have ever tried to tow a plankton

net through water by hand, you will appreciate how much

energy it takes to filter-feed. Water is very dense, in fact so

dense that fish spend 10–30% of their energy just breathing.

So it’s not surprising that whale sharks have evolved very

cost-efficient patterns of foraging to minimise the energy that

they expend filtering food. Our recent work at Ningaloo Reef

deployed accelerometer tags on whale sharks, and found that

they have evolved at least three strategies to reduce their energy

output.

34 ||MAY/JUNE 2017

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.

Why Do

Whale Sharks

Get So Big?

zeamonkeyimages/Adobe