Biology and Ecology
Name Derivation
Feeding
Reproduction and Development
Life history
Growth and Aging
Movement Patterns
Faunal Associations
Human Interaction
Although the whale shark's mouth is wide and capacious, it shows no evidence of the formidable teeth of other sharks. In fact, the whale shark has 300 rows of minute teeth in each jaw, with each tooth less the three millimeters in length and rasp-shaped.
These teeth are the reason for its original family classification,
Rhiniodon. Rhiniodon means rasp tooth; the name came originally from Dr. Andrew Smith of South Africa in 1828. The current term,
Rhincodon, is the unfortunate result of a long-ago printer's error - a "c" typeset instead of an "i".
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The whale shark is a filter feeder.
Although passive filter feeding has been documented and was previously considered characteristic behavior, recent studies now indicate that whale sharks feed primarily at night and at depth.
It is under cover of darkness that the deep scattering layer of planktonic and nektonic prey moves up the water column in the densest concentrations.
For most of the year, at least during the day, the amount of food taken in during subsurface cruising is equivalent to snacking, while the main meal comes after dark in deeper water.
Information gathered, has shown that the correlation between the increased whale shark sightings and mass coral spawning are more than coincidental.
All previous evidence had pointed to passive filter feeding on plankton as the mainstay of the whale sharks diet, but analysis of plankton samples revealed more.
Not only do whale sharks passively filter feed on the superabundance of coral and invertebrate larvae, but also they have a special taste, previously unsuspected, for larger prey.
Scientists now believe that while the whale sharks do consume vast clouds of plankton, it is the concentrations of prey feeding on the plankton that serve to draw whale sharks.
Whale sharks become frenzied upon encountering krill, small, shrimp-like crustaceans, about one third of an inch long.
When they find clouds of krill, whale sharks engage in lunge feeding, and the surface of the water is churned with each rushing gulp.
Smaller pelagic fish species - such as anchovies, sardines, and mackerel - travel together in large schools made up of thousands of individuals. In their quest for food they are often drawn by the planktonic larvae and coral spawn, which they feed upon avidly.
Their excitement as they feed translates into pulsed vibrations, sending out a signal to larger predators, such as tuna, trevally, requiem sharks, bronze whalers and silvertips, as well as whale sharks.
The animal is not dependant on forward motion to operate its filtration mechanism, but rather relies on a versatile "suction" filter feeding method, which enables it to draw water into the mouth at higher velocities than "dynamic" filter feeders, such as the basking shark. This enables it to capture larger, more active nektonic prey as well as zooplankton aggregations, but probably means it can filter a far smaller volume of water.
The whale shark may be more dependent on dense aggregations of prey organisms.
Whale sharks have been seen feeding "passively" (cruising with mouth agape) and also hanging vertically in the water and "actively" feeding by opening their mouths and sucking in prey-rich water.
They have also been observed "coughing" which is thought to be a mechanism employed to clear or flush the grill rakers of accumulated food particles.
Groups of individuals have been observed "actively" feeding at dusk or after dark by ploughing through the surface waters with mouth agape and jaw distended, sometimes also moving their heads from side to side "vacuuming" in seawater rich in prey, or aggressively cutting swathes through schools of prey.
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Whale sharks have not been observed mating - or at least there isn't any photographic evidence - observers have seen claspers (the dual male sex organs) raw and scarred, presumably from recent use.
There have also been reports of whale sharks trailing newborns - like a gaggle of baby ducks - seen from the air and by boats.
Only time and continued research will reveal the answers to these exciting possibilities.
In the family Orectolobiformes, reproduction takes one of two forms.
Nurse sharks reproduce ovoviviparously, which means that the embryo forms within an egg retained in the mother's womb, the yolk supplying the embryo with nutrients is not connected to the mother after the egg case is formed. When the foetus reaches term and has exhausted the yolk sack, the juvenile breaks free of the egg case and is delivered out of the womb via the cloaca.
The zebra shark reproduces oviparously by laying an egg case containing foetus and yolk sack and development occurs wholly outside the mother's body.
From a very recent discovery, the whale shark does indeed give birth to live young - up to an astonishing 300 young from two uteri - there had been considerable debate over whether the whale shark was oviviparous.
This controversy stemmed from a near-term whale shark egg case trawled up from 250 feet of water in the Gulf of Mexico in 1953. The egg case was the size of a football, but uncharacteristically thin walled and without attachment tendrils (essential in attaching an egg case to the bottom) as on other oviparous shark egg cases.
Since no other egg cases had been discovered, it was generally accepted that this particular egg case was a miscarried or aborted foetus.
In July 1995, a female whale shark, measuring approximately 11 meters in length, was harpooned off the eastern coast of Taiwan. The twin uteri of this specimen were found to contain nearly 300 embryos, from 42 - 63cm in length. Fifteen of the embryos were alive and one, measuring 58cm in length, was reared for 143 days in an aquarium in Japan where it developed from foetal through to juvenile stage. This discovery finally confirms that the species is a live-bearer, with and ovo-viviparous mode of development.
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There are no confirmed records of whale sharks between 93cm and 3m, while animals over 3m in length are encountered worldwide.
Most specimens reported in literature are between 4 and 10m in length, but there is a report of a specimen from the Seychelles that measured nearly 14m in length, and a specimen measuring just over 14m was landed in India in 1975. Whale sharks may possibly reach as much as 18m in length although the very large specimen reported from the Gulf of Siam was not accurately measured and therefore total length may have been over-estimated.
Information about size at sexual maturity and longevity is sparse.
It is speculated that whale sharks do not reach sexual maturity until they are over 30 years of age and that they may have a life span of over 100 years.
Two large female sharks captured off the Indian coastline, which measured 8-9m in length, were both found to have immature ovaries.
Sexual maturity in both sexes may not occur until the sharks are over 9m in length. Data on sex ratio is also very limited. Of 31 specimens reported from India, 17 were male and 14 were female. A 1:1 sex ratio was reported for the 297 embryos removed from the pregnant female in Taiwan, with 150 females and 147 males. . On the basis of current information it is not possible to say whether sexual segregation, of either a behavioural or geographical nature, occurs.
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Virtually nothing is known about growth rates or ageing in whale sharks.
Growth curves for sharks have generally been derived from age estimates based on growth zones (bands or ridges) in calcified structures such as vertebral centra.
The growth of whale sharks has been opportunistically studied from animals held in captivity in the Okinawa Expo Aquarium in Japan. Preliminary results indicated that one pair of growth zones was deposited per year in captivity.
How this relates to growth in the wild can only be determined by examination of growth zones in vertebral centra samples collected from dead specimens.
Studies of this nature are hampered by small size, as very few dead animals are available for collection of hard tissues.
Researchers from the Natal Sharks Board in South Africa have recently been retaining vertebra samples from specimens that occasionally strand along the Kwa-Zulu Natal coastline.
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In general, occurrences of whale sharks appear to be sporadic and unpredictable, which is partly a reflection of the lack of knowledge about the animal's habitat and ecology.
The animals are generally encountered singly but aggregations of over a hundred have been seen, which suggests that schooling activity does occur.
They are usually observed on or near the surface and at times have been seen apparently basking.
Whale sharks are now known to be highly migratory and undertake trans-oceanic migrations of over 6500km in the Indian Ocean and similar movements in the Pacific and Atlantic oceans.
Their movements are probably related to in increase in local productivity such as plankton blooms and invertebrate spawning events. Different locations seem to be preferred at various times of the year and they may undertake either fairly localized migration or alternatively large-scale trans-oceanic movements, governed by the timing and location of production pulses and possibly by breeding behavior.
These animals could move offshore to deeper waters and may only be seen when they come inshore and into surface waters to exploit periodic increases in plankton productivity, such as those around the time of mass coral spawning.
There are several reports in print of seasonal aggregations of whale sharks in Indian coastal waters from December to April, in the Seychelles during July/August and November/December and off the West Coast of Mexico from Cabo San Lucas to Acapulco between March and August.
Significant numbers of whale sharks have also been reported from inshore waters off southern Mozambique and the northeastern coast of South Africa from October through to March.
Aerial surveys and other methods for monitoring movement patterns can assist assessing large-scale migratory patterns throughout the Indian Ocean region.
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Whale sharks are often associated with schools of pelagic fish that are probably feeding on the same prey organisms.
There are many references to sightings of whale sharks in association with several tuna species, bonito, mackerel and schools of small baitfish such as sardines and anchovies.
Globally, whale sharks are reported to associate with numerous species including hammerhead sharks, tiger sharks, manta rays, remoras, pilot fish and golden trevally.
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There has been virtually no sustained scientific research on this species and it has been the target of only limited commercial fisheries in the past.
The flesh is soft and bland and has a very high water content, with levels of up to 68% and 75% being reported.
Whale shark meat is sought after in Taiwan, where it is described as being like "tofu" and sells at around US$2.50 to US$6.50 per kilo.
Whale shark is also served in restaurants in Taiwan.
Currently, whale sharks from coastal fisheries fetch up to US$7 per kilo (uncleaned) at Taiwanese fish markets and a 1995 survey of seine net and harpoon fisheries along the east and northwest coasts of the island estimated the annual catch at 272 individuals.
Seasonal (January to May) fisheries exist in the Philippines, where an estimated 95 sharks were taken at four sites in the Bohol Sea during the 1996 season.
These fisheries have been increasing in recent years and there are indications that they may be unsustainable and that the whale shark population is under threat.
Sudhakara Rao recorded the landing of 40 whale sharks over a four-day period in 1982, from a harpoon fishery off the Veraval Coast in India. After removal of the liver the carcasses were discarded, as there was no local demand for the flesh.
In the past small harpoon fisheries have been reported from India, Pakistan, Indonesia and Iraq.
Whale sharks are occasionally taken accidentally in gill and purse seine net fisheries off the coast of India. Occasionally some of the flesh was eaten, either fresh or salted and dried, and the liver oil was utilized for water-proofing wooden fishing boats and other appliances, for the manufacture of shoe polish and as a treatment for some skin diseases.
The processing of whale shark fins and fin rays has been reported from India and a recent WWF-India/TRAFFIC-India report on the trade in sharks and shark products gave details of whale shark fishing at two locations on the west coast.
Whale sharks are taken at Verbal and Okha, mainly for fins, livers and meat, with the fresh or frozen meat being exported to Taiwan, Korea and Singapore.
The species is often used as a fish "aggregator" or indicator of waters rich in plankton and plankton-feeding fish that will, in turn, attract more valuable species such as tuna.
Potential damage to fishing gear from entangled whale sharks causes them to be avoided in other areas.
Whale sharks have also been kept in captivity at the Okinawa Expo Aquarium in Japan. Currently the Okinawa Expo Aquarium in Osaka has live whale sharks on display.
There have been few cases reported of whale sharks inadvertently ramming boats, but generally the sharks are more at risk from being accidentally struck by vessels whilst basking or feeding on the surface.