Whale Shark: Ecology and Biology

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Ecology and Biology

 

Physical Description

Whale sharks have a flattened head, a large mouth, and a dark blue, grey or brown body covered in white spots and stripes. Scientists believe this body marking acts as camouflage to protect these animals (particularly juveniles) from predators. These spots and stripes also serve as unique “fingerprints” for each shark, and researchers can use these markings to identify one whale shark from another. Whale shark skin is covered with small tooth-like scales called dermal denticles that serve as protection, and also improve swimming by reducing drag. Like all other sharks, their skeleton is made of cartilage, the same lightweight material that makes our ears and nose. Male and female whale sharks are identified by the presence or absence of “claspers”, a pair of appendages located on the underside of the shark.

 

Size and Maturity

Whale sharks are the largest fish in the world. They are believed to grow to up to 20 meters, reach maturity at more than 9 meters long, and live to be a century old. In 1987, the largest whale shark on record was reported at a Taiwanese fishery, measuring 20 meters in total length and weighing 34 tons. The second largest shark measuring 18·8 meters in total length was found at an Indian fishery in 2001.

Recorded whale shark embryos range in size from 35-64 cm long. The smallest, free-swimming whale shark was found in the Philippines, measuring only 46 cm in total length (about 18 inches).

 

Mating and Reproduction

Very little is known about the mating and reproduction of whale sharks. The mating behavior, breeding ground locations, gestation period, and birthing intervals of whale sharks are virtually unknown.

Pregnant females, adult females, and small whale sharks (less than 3 meters in total length) are rarely encountered. Adult females have been sighted in the Sea of Cortez (Mexico), the Galapagos Islands (Ecuador), India, Taiwan and the Philippines. Pregnant females have reportedly been spotted in Baja California (Mexico) and the Galapagos islands.

In 1995, a 10.6-meter female was caught in Taiwan that was pregnant with more than 300 pups ranging from 35-64 cm in total length. This is the largest litter recorded of any shark. Out of the over 300 pups, 15 were full-term, measuring between 58-64 cm in total length, and appeared ready to be born. This confirmed that whale sharks keep their eggs inside their uterus until they hatch, giving birth to live young. Genetic samples taken from this litter suggest that all of the pups had one father.

Similarly, very little is known about where whale sharks reproduce. Newborn whale sharks have been recorded in Pakistan, Philippines, Taiwan, and Gulf of Mexico, suggesting that these places are potential nursery grounds.

 

Feeding and Diet

Whale sharks are one of the three filter-feeding species of shark. They feed on a variety of plankton and nekton, such as copepods, crab larvae, fish eggs, thimble jellyfish, a variety of krill, and small deep-sea fishes.

The feeding technique of whale sharks is similar to a vacuum cleaner — filtering by sucking and gulping — but may vary depending on the amount of food in the water. They feed by swimming forward with their mouths slightly or fully open to strain food from water passing through their gills, by gulping water on the surface while swimming, or by gulping water while vertically suspended in water. When feeding near the surface, the shark’s mouth, top of the head, first dorsal fin and tip of the caudal fin (tail) are usually visible. Feeding whale sharks are often seen in coastal waters, but there is little data about their feeding behavior and diet in the open seas.

 

Aggregations and Distribution

Whale sharks are found globally in tropical and warm termperate seas. They are found in both oceanic and inshore waters, and may form predictable seasonal aggregations in some areas.

In 2009, the biggest aggregation was found off of the Yucatan Peninsula, Mexico, where researchers observed 420 individuals in a single aerial survey within an area of 18 square km. Other well-known whale shark aggregation sites are in the Gulf of Mexico, North and South Gulf of California, the Galapagos Islands, Mozambique, Maldives, Seychelles, Western Australia, Thailand, Philippines, Djibouti, Red Sea, Arabian Gulf, Belize, and Honduras. Juvenile male whale sharks (6-8 meters) dominate most of these aggregations.

 

Horizontal and Vertical Movements

Whale sharks can dive to more than 1,000 meters. The deepest dive ever recorded by a whale shark in the Bohol sea is 1,300 m (>4,000 ft), while the deepest dive ever recorded by a whale shark is 1,720 m (over 5,000 ft).

Whale sharks may also migrate long distances and make trans-oceanic journeys. In 1998, a satellite-tagged whale shark in Salay, Mindanao (Bohol Sea) swam through the South China Sea to southern Vietnam,travelling a distance of 4,567 km in 73 days. Another tagged whale shark in the Sea of Cortez (Mexico) journeyed westward towards Hawaii and the North Pacific Ocean, covering over 12,000 km in 37 months. A whale shark tagged and photographed in Taiwan in 2012 was identified in Southern Leyte, Philippines (P-545) in 2013. It traveled over 1,600 km in 11 months, and was the first matched whale shark between two Southeast Asian countries. More than 700 whale shark individuals have been identified in the Philippines to date.