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 buoy-
ancy 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 plank-
tonic 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, accu-
mulating 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 struc-
ture. 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 swim-
ming – 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 acidifi-
cation, 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.MAY/JUNE 2017 | | 35“... these strategies increase foraging
efficiency by 22–32% compared with
horizontal swimming”.
frolova_elena/Adobe