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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.

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 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 accumulate 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.

First, they swim slowly, moving at speeds of 0.8–1.0 m/s. This is very slow for such a large fish, but it minimises the cost of locomotion per unit of horizontal distance covered. This means that they expend very little energy travelling between patches of food.

Second, as they move up and down through the water column searching for prey, whale sharks use their slight negative buoyancy to their advantage by making long, slow gliding descents. By simply opening their mouth on the descent, they can filter water without the need to actively swim forwards.

Finally, whale sharks have very asymmetric dive patterns, with long, shallow descents followed by very steep ascents to return to the surface. This provides 13–23% more time to search and feed during the energetically inexpensive descents.

Together, these strategies increase foraging efficiency by 22–32% compared with horizontal swimming.

While whale sharks are highly evolved to exploit their planktonic prey, their prey have also evolved to try to avoid predation. Each day at dawn in the open ocean, the small fishes, shrimps and other plankton on which whale sharks feed descend and congregate in waters 300–500 metres deep. Here they form the “deep scattering layer”, so-called because it scatters acoustic signals and forms a distinctive layer in sonar scans of the ocean depths. By occupying waters that are cold, dark and lower in oxygen during the day, these animals limit the time that their predators can access them. In the early evening, the plankton rises towards the surface, accumulating around the thermocline 100 metres deep.

Whale sharks follow the same migration pattern, descending to 300–500 metres to search for their prey in the daytime. However, the temperatures they encounter at these depths, which can be 10°C cooler than surface waters, limit the time that they can spend searching for prey, and this is a particularly critical issue for an animal that feeds using the same organ (the gill) that must oxygenate the blood. Filtering not only catches prey, it also has the potential to cool the blood and the rest of the body tissues very quickly.

Whale sharks deal with the problem of their prey hiding in cool, deep water in two interlinked ways. The first is a behavioural response. Whale sharks bask on the surface to warm their bodies both before and after diving. During the day, dives to deeper waters are interspersed with long periods of relative inactivity in surface waters, where up to 90% of their day may be spent. The cooler the water temperatures they encounter at depth, the longer these sharks require at the surface to recover. At night, when the plankton rises to the thermocline in shallower water, whale sharks don’t venture into deeper waters, and they rise to the surface relatively rarely.

The second way that whale sharks deal with the problem of access to plankton in deep water is through their body structure. Massive blocks of poorly vascularised white muscle lie along the vertebrae and central nervous system, whereas active red muscle forms only a very thin layer on the outer dorsal surface just below the skin. This means that the central core of the body has relatively little blood flow while the active, highly vascularised tissues that are involved in routine swimming – and will thus cool rapidly – are kept away from the nervous system. Effectively, these huge blocks of muscle, pre-warmed at the surface by basking, act as a heat store, slowly dissipating warmth to the rest of the body.

This approach to thermoregulation, called “gigantothermy”, increases in efficiency with very large body sizes, providing the likely answer to the question of why whale sharks have evolved to grow so big. They have not been alone in developing this body-size strategy to stabilise temperatures – the same approach evolved in large reptiles including leatherback turtles, and in the more distant past by the largest of the dinosaurs, the massive sauropods.

Ultimately, the maximum size of whale sharks is an optimal balance between the cost of gathering a small, abundant but patchy prey and the need to access this food in the cool, deep waters of the open ocean. Today, one of the challenges facing whale sharks is the potential for this equilibrium to be distorted by the anthropogenic processes of global warming and acidification, which are changing the physical structure and food webs of the world’s tropical oceans. How this might alter the maximum sizes that these animals can attain in the future is as yet unknown.

Mark Meekan is Principal Research Scientist at the Australian Institute of Marine Science, and is based at The University of Western Australia.