Reference section 2 - Whale shark Biology
and Ecology
Introduction
The whale shark (Rhincodon typus) is a filter feeding shark and the largest living fish. Despite its large
size (up to 18 m), seasonal occurrence in coastal waters in some areas, and popularity with tourists
most aspects of its biology and ecology are poorly known.
This course aims to provide tour operators with enough of the available information about whale
sharks to field questions from guests. This section therefore focuses on the most asked about aspects
of the whale shark - where the whale shark fits into the animal kingdom, where it lives, what are the
names and functions of its body parts, what it eats and what eats it.
Such is the extent of our lack of knowledge of the whale shark that most questions about its life could
be legitimately answered: ‘’we don’t know’’! As such this section also outlines inferences or
suggestions based on what might be known from studies of other closely related species. These
suggestions should not be presented by the tour operator as fact but rather used to introduce their
guests to reasonable possibilities about this majestic shark and hopefully adding to a sense of wonder
and inviting further discussion.
Included in this section you will find:
1.
2.
3.
4.
5.
6.
Taxonomy and Classification
Distribution
Physiology and Anatomy
Diet and Feeding
Reproduction and life cycle
Predators
KEY POINTS
The whale shark is the largest fish in the world but despite its size and popularity very little is
known about its biology
This section outlines the facts and some of the theories regarding whale shark biology and
ecology
1. Taxonomy and Classification
The common name of the whale shark (a fish) can cause confusion. It refers to the fact it shares
some characteristics with baleen whales (warm blooded mammals), namely its large size and use of
filters to catch tiny prey.
The key feature that makes sharks and rays different from all other animals including mammals,
reptiles, birds and even other fish is that their internal skeleton is made of cartilage instead of bone.
There are a number of other structures and adaptations that are exclusive to sharks:
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Cartilage skeleton – This is lighter and more flexible than bone
A covering of dermal denticles – These protect the skin from damage and parasites and
improve hydro-dynamics so the shark can move faster and more quietly.
They have several sets of replaceable teeth – teeth are continuously being replaced and
therefore are always sharp
5 to 7 gills used to extract oxygen from water
Did you know? Humans have cartilage in our bodies too. That’s what the flexible part of our nose and
ears are made from.
These features have made sharks very successful animals. They have been dominant species on
Earth, pre-dating dinosaurs by 220 million years. Today there are 440 different species of sharks
ranging from smallest, the lantern shark (a deep seas species which is about 17cm long) to the
largest, the whale shark. Sharks live in many different geographic and vertical habitats and some like
the bull shark and river shark can survive in fresh, brackish, and salt water.
The physical appearances of the 440 shark species vary substantially. Key distinguishing features
like the number of dorsal fins or number of gills and the presence of an anal fin have been used to
group species together into different taxonomic families and orders (i.e., closely related families).
For example the mackerel sharks (order Lamniformes) include seven families including Laminae (i.e.,
great white, mako, salmon and porbeagle sharks).
The whale shark is in the order Orectolobiformes (carpet sharks) and is the only species in the family
Rhincodontidae. It doesn’t have any very close relatives but it does share some features with other
carpet sharks like the nurse shark and the zebra shark. For example they all have two dorsal fins and
their mouths are positioned at the very front of their bodies – in front of the eyes. The zebra shark also
has clear ridges along its back like the whale shark and the nurse shark has similar nostrils.
There are two other large filter feeding sharks ,the basking shark and megamouth shark, but they are
in the mackerel shark order and are not closely related to the whale shark.
KEY POINTS
The whale shark is a true shark. It has a skeleton made of cartilage, rows of replaceable teeth, and skin
covered in denticles instead of scales.
Sharks are very successful animals and appeared about 220 million years before dinosaurs did. There are
over 440 different species that live in diverse aquatic habitats
Sharks have been split into eight orders. Whale sharks are in the order of carpet sharks
(Orectolobiformes).
Whale sharks are not closely related to any sharks, though they do share some features with other carpet
sharks including a mouth at the front of the head, prominent lateral ridges, two dorsal fins and nostril
barbells.
2. Distribution
Whale sharks live in all tropic and warm-temperate seas except evidently the Mediterranean. It is
thought to be primarily pelagic (prefer an open-ocean habitat) but seasonal feeding aggregations do
occur at several coastal sites throughout the tropics. The most well documented sites are in the Gulf
of Mexico, Gulf of California, Belize, Honduras, Western Australia, the Galapagos, New Zealand,
Philippines, Indonesia, Madagascar, Mozambique, Kenya, India, Pakistan, Maldives, Seychelles,
Indonesia, and Thailand.
Whale sharks have been found offshore and close to land, entering lagoons or coral atolls, and near
the mouths of estuaries and rivers. Its range is generally restricted to about ±30° latitude although
whale sharks have been seen at higher latitudes on occasion off the coast of South Africa and
eastern North America.
Although whale sharks exist mainly in areas where the sea surface temperature is between 25˚C and
30˚C tagging studies have shown they are capable of diving very deep in excess of 1600 metres
where they tolerate temperatures as low as 3˚C.
Whale sharks are highly mobile animals, capable of swimming thousands of miles. Despite often
being described as such, it is yet to be seen whether whale sharks are truly migratory. Migratory
animals move between particular areas on regular seasonal or annual schedules as opposed to
simple movements within more defined home ranges, or immigration or emigration.
KEY POINTS
Whale sharks are found throughout all tropic and warm temperate seas.
There are about 18 to 20 countries where whale sharks are known to occur at particular seasons.
Whale sharks spend time in offshore and near shore marine habitats.
Whale sharks can dive deeply (up to 1600m or more) and tolerate very low water temperature (down
to 3˚C)
Although not necessarily migratory the whale shark is a highly mobile animal capable of traveling
thousands of miles annually.
3. Physiology and Anatomy
The largest whale shark that has been directly measured was reported to be 12.65 metres (41.50 ft)
long and a weighed more than 21.5 tonnes (47,000 lb), though few other reliable measurements have
been made Some claims of of sharks up to 18m long have been made though none validated.
The body surface coloration is mostly grey dorsally (above) and white ventrally (below). Three
prominent ridges run along the sides and the skin is marked with a checkerboard of spots and stripes
which may function as camouflage. The patterns of spots are used in some areas to identify sharks
from photographs, amethods that assumes that they are unique to an individual and that they do not
change with age.
The skin of whale sharks is relatively thin, a couple of millimeters, and is covered in tiny teeth-like
scales called denticles. These denticles provide a tough, hydrodynamic surface layer. Below the skin
is a layer of fatty tissue that can be 10-15cm thick and is probably important for insulating the shark’s
muscles and vital organs and perhaps a nutrient and energy store.
Whale sharks have two dorsal fins and two lateral pectoral fins. The upper lobe of the tail (i.e.,
caudal) fin of juvenile sharks is larger than the lower lobe compared with adults which have a more
crescent-shaped tail.
Spiracles
Spiracles are the small circular openings just behind the whale shark’s eyes. These openings appear
in many species of sharks and are actually the remains of a pair of gills that grown apart from the
other five pairs of gills. The function of the spiracles in whale sharks is not known. In some species
of bottom-dwelling sharks (e.g. nurse sharks) spiracles are used in respiration, creating a flow of
water that delivers oxygen to the gills when the animal is at rest. Large pelagic or open ocean sharks
like the great white shark do not have spiracles and have to keep moving to create this flow of
oxygenated water, over the gills.
The function of the spiracles of whale sharks might be intermediate as whale sharks are more closely
related to bottom-dwelling sharks though they also spend substantial time swimming throughout the
water column. Consequently, the spiracles might function when whale sharks are occasionally
stationary but not feeding (when water is passing over the gills).
Mouth
The mouth of this large filter feeder is also relatively large and can be up to 1.5 metres wide. It
contains between 300 and 350 rows of tiny (3mm) scale-like teeth which look like a rasp close-up and
account for the species original latin name Rhinodonte, which means rasp-tooth. These teeth do not
appear to have function in feeding.
Gills
Whale sharks have five large pairs of gills which serve two functions They extract oxygen from the
water to support metabolism and also filter out small prey from seawater.
The gills are modified with 20 filter pads (two for each gill). These filters have tiny 1.5 millimeter gaps.
Water entering the mouth is forced past the holes in the filters and out through the gill slits. Any
organisms larger than 0.5 millimeters is sieved out by the filters and swallowed by the shark.
Eyes
There are two small eyes located near the front of the wide, flat head. The forward, lateral placement
of the eyes allows the sharks to see forward, backward and above and below.
The eyes appear quite well evolved though vision is likely rather poor and limited to detecting
relatively close (i.e., < 3 m) movements.
The eyes can roll and retract into the sockets when something approaches very closely if the shark is
exposed to a flash from a camera strobe.
Nostrils
The nostrils are relatively small and not very well developed. However, their position at the front of the
head above the mouth and far apart suggests that they may have an important function in detection of
substances in the water. The large, well developed internal sensory structures (olfactory capsules)
connected to the nostrils support this suggestion. Similar findings of an acute sense of smell in its
close relative the nurse shark suggests the detection of chemicals dissolved in seawater may be
important in locating concentrated prey.
Other Whale Shark Senses
The hearing of whale sharks has not been studied. Anatomically, their inner ear is the largest of any
animals, perhaps simply because of the species large physical size or perhaps related to adaptations
for low frequency hearing.
Similar to some other sharks, whale sharks have anatomical structures that function to detect
vibrations (lateral line) in water and small electrical currents (ampullae of Lorenzini) produced by
other animals. These structures may have similar function in whale sharks though they have not
been investigated yet.
KEY POINTS
Largest whale shark measured was 12.85m although they might grow as long as 18m.
Whale shark skin is thin, though tough, with a thick fatty layer below it. It is covered by teeth-like scales
called denticles
Whale sharks’ bodies are covered by spots and stripes that are being used to identify individuals,
conditioned on assumptions that patterns are unique to individual sharks and that they do not change
with age or size.
The whale shark has 300 rows of very tiny rasp-like teeth that apparently have no function
The gills of whale sharks function to extract oxygen and to filter prey from seawater.
Spiracles are the small holes behind the whale sharks eyes which in some closely related sharks are
connected by channels to their gills and provide some ventilation when the sharks are stationary.
The small eyes are on the side of the head, apparently to maximize range of vision. Visual detection is
probably limited to movements very nearby (<3m).
The two nostrils are at the very front of the shark above the mouth and might function to detect (i.e.,
smell) chemicals dissolved in seawater.
How important each different sense is to the whale shark is unknown but they probably use a
combination of vision, smell, hearing and detection of electrical currents.
4. Diet and Feeding
Whale sharks mostly eat tiny planktonic animals (e.g. euphausiids, copepods, fish and squid eggs or
larvae) though they have also been observed to ingest small fishes like sardines and anchovies.
Whale sharks are able to feed in two different ways:
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ram filter feeding,which involves swimming along with their mouths open and feeding on prey
that happen to be caught as water passes through the gill rakers.
suction filter feeding,where the whale shark can actually remain stationary in the water and by
opening and closing its mouth can actively suck large quantities of water and prey in to its
mouth. When the mouth closes the water is forced out through the filter pads and any prey is
sieved out. Suction feeding allows the whale shark to feed on more mobile prey like small fish
which would usually be able to escape from a ram filter feeder.
It has been suggested that the whale sharks gills are more suited to suction filter feeding than they
are to ram filter feeding because the filter pads on their gills have smaller gaps and work best when
water is being physically forced through rather than just flowing through them. The structure of the
gills in other filter feeding sharks (e.g. basking sharks) appear to have contrasting structures with
larger gaps that appear to be only suitable for ram filter feeding.
Whale sharks evidently only use the suction method when they are feeding on very high
concentrations of prey. Consequently, some think that the sharks might actively search for specific
areas or seasonal events where high densities of food are more likely and that this might explain why
whale shark aggregations are seasonal in most areas. In areas where there is no clear whale shark
season (e.g., the Maldives) other factors like the lunar cycle might have stronger influences on the
number of sharks observed. These are also natural triggers for spawning and plankton blooming
events.
Information collected from tagging studies has shown that whale sharks are constantly diving and
ascending spending only short periods of time near the surface and diving as deep as 1600 metres,
evidently to search for the areas where there are high densities of prey in the water column.
KEY POINTS
Whale sharks feed on tiny prey including plankton, fish and squid eggs and larvae, and small shoaling
baitfish.
They use two different feeding methods: ram and suction filter feeding
Suction filter feeding evidently enables them to feed on more mobile prey
There are two other filter feeding sharks, the basking shark and the megamouth shark
Whale sharks apparently seek out high densities of prey and as a result they are often seasonal visitors
to specific locations
Whale sharks constantly dive and ascend, perhaps to actively search for areas of high prey density
5. Reproduction and life cycle
No one has ever witnessed whale shark courtship, mating, or birth. It is now thought that whale
sharks give birth to relatively large litters of live young (i.e., ovoviviparity), though this is based on
observations of one pregnant female.
During mating it is thought that male whale sharks inseminate females with their claspers (sexual
organs ( located between the ventral pelvic fins). The presence of claspers is the easiest way to
determine the sex of a whale shark, though they can be hard to identify in very young males.
Females are thought to be capable of storing a male’s sperm and fertilizing many different eggs,
retaining the egg cases until they hatch inside their uteri until they hatch. It is not known how long this
process takes (gestation period) no one knows nor how often it occurs. One female whale shark
captured off Taiwan had 300 embryos in her uterus in various stages of development.
Whale shark offspring or ‘pups’ are thought to be about 55–60 cm in length at birth and weigh about 1
kg and grow to about 1.4 m long and 20 kg within the next four months.
Whale sharks like most sharks mature very slowly, perhaps not maturing sexually until around 8 to 9
metres long and 25 to 35 years old and may live for up to 70 to 100 years, though there is very little
evidence to support these suggestions.
Did you know? Scientists are attempting to estimate whale shark age by counting the number of
growth rings in the vertebrae (back bones) of dead whale sharks - much like one would count the
rings on a tree stump. The problem is currently they can only make a good guess as to what one
growth ring represents in units of time.
Another mystery is that groups or aggregations of whale sharks in most hot-spots are composed
primarily of adolescents with very few females and virtually no adults. In the Indian Ocean for example
(e.g., the Seychelles, Maldives, Mozambique and Western Australia) over 60% of the sharks
observed are adolescent males.
The Galapagos Island archipelago in the Pacific Ocean is the only site where large mature female
sharks are regularly seen.
Remember! We must keep in mind that humans can only encounter whale sharks with any regularity
near the surface and near the coast. It may simply be that females prefer a slightly different habitat to
the males - one that is out of reach of humans - deeper down or further out to sea.
KEY POINTS
No one has ever reported whale shark reproductive behaviour.
Males inserts one of its claspers into the female, females store sperm and fertilize many eggs.
Whale sharks give birth to live young – they are Ovoviviparous
Pups are about 60cm when they are born
WS do not reach sexual maturity until approximately 9m or 30 years old and possibly live until 70 -100 years
old
The vast majority of whale sharks encountered by humans around the world are juvenile males
6. Predators
The whale sharks best defence is its large size and as such adult whale sharks do not have many
natural enemies. There are however reports of great white sharks and killer whales attacking adult
whale sharks. New born whale sharks are much more vulnerable and specimens have been found in
the stomachs of blue sharks and swordfish.
KEY POINTS
Whale sharks do not have many natural enemies due to their size
Great white shark and killer whale attacks on adult whale sharks have been reported
Juvenile sharks are more vulnerable. Young sharks found in stomachs of smaller sharks and swordfish