Whale Sharks

whale shark anatomy

The species was distinguished as Rhincodon typus in April, 1828 after the harpooning of a 4.6 m (15 ft) specimen in Table Bay, South Africa. Andrew Smith, a military doctor associated with British troops stationed in Cape Town, described it the following year. The name "whale shark" directly refers to both the fish's large size and whale-like feeding behavior. It's grouped in the same order as bamboo, zebra, wobbegong and nurse sharks but one of only 3 shark species (the whale shark, the basking shark, and the megamouth shark) that filter feed, straining their food from the water column. The whale shark is mostly found in tropical and warmer oceans and lives a pelagic lifestyle, with seasonal feeding aggregations in many parts of the world. Mexico is lucky to have many of these sites in its waters, with Isla Holbox, Isla Mujeres and the bay of La Paz being the most popular for tourism. Whale sharks gather in these areas to feed on plankton blooms, large concentrations of microscopic life including crustaceans and algae. These organisms are invisible to the human eye but occur in such large numbers that they become a viable food source.

Whale shark anatomy

whale shark gillsLike most sharks they have large dorsal, pectoral and tail fins. Whale sharks have 5 pairs of gills, a very wide head and large mouth which they use to filter large amounts of water. Although they still have 300-350 rows of teeth the teeth are very small, less than 2mm and not used for feeding. Instead whale sharks have evolved a very efficient filtering system which allows them to reach such large sizes. The largest scientifically recorded example was 12.65m(41.5ft) long, measured 7m(23ft) around the thickest part of the body and weighed an estimated 15-21,000kg (33-46,000lbs). It was captured off Baba Island, near Karachi, Pakistan on the 11th of November, 1949. Each whale shark has it's own unique pattern of spots, much like finger humanprints. The pattern of spots around the gill area is unique to each individual allowing researchers to identify individual sharks. Whale sharks are typically bluish-gray on their backs and sides with a pattern of white spots and lines and a white underside. Juveniles can have much brighter colors and it's thought that the pattern acts as a UV "sunblock", necessary because the whale shark spends so much time on the surface. The skin of a whale shark can measure up to 4 inches thick and has extraordinary regenerative capabilities.

Feeding Behaviors

whale shark feeding on the surfaceThe whale shark is mostly found in tropical and warmer oceans and lives a pelagic lifestyle, with seasonal feeding aggregations in many parts of the world. Mexico is lucky to have three of these sites in its waters, Isla Holbox, Isla Mujeres and the Bay of La Paz. In all three sites whale sharks gather to feed on plankton blooms, large concentrations of microscopic life including crustaceans and algae. These organisms are very small but occur in such large numbers that they become a viable food source.
To capture this small prey the whale shark utilizes one of two feeding methods; Ram filtration or active suction feeding. A system of 10 filter pads, which are modified gill rakers, allows the whale shark to efficiently filter this very small food. As the water passes across these filter pads food is separated and swallowed while water is expelled through the gills. This filtering system can get clogged from time to time and it is common to see whale sharks "coughing" to clear it.

copepodStudies have shown that the planktonic life consumed by whale sharks in La Paz is dominated by copepods. Copepods are very small animals, typically 1-2mm in length and exist in every type of aquatic environment, including places such as a simple rain puddle. In the ocean environment copepods exist in vast numbers and when the right conditions are present can multiply rapidly and create what's known as a plankton bloom. These blooms can last just a few days or months depending on the environmental conditions. Whale sharks are opportunistic feeders and will also feed on schools of spawning fish and their gametes (eggs and sperm), squid, algae and other marine plant material and sometimes larger fish that are also feeding on the same planktonic life.

Ram filtration

The most commonly observed method of feeding here in La Paz. This method is simply the whale shark swimming horizontally with its mouth open and filtering large amounts of water. Whale sharks will feed in this manner when the concentration of plankton is lower and distributed over a larger area. They may swim with the mouth fully or partially open and/or opening and closing the mouth repeatedly. 

Active suction feeding

Typically occurs when there are more solitary patches of dense plankton, in this case the whale shark is vertical and opens and closes its mouth to suction water in, filter the food and expel it out of the gills. You are very lucky if you get to observe this feeding method as the whale shark is not moving anywhere and allows for a more relaxed approach.

 Life Cycle

Little is known about the life cycle of the Whale shark. There has never been a recorded birth, instance of mating or whale shark found smaller than 31cm, 12 inches. We do know that whale sharks are ovoviviparous (also called ‘aplacental viviparity’). This means the female gives birth to live young, called pups, which have developed from eggs hatched within her uterus. Evidence indicates the pups are not all born at once, but rather the female retains sperm from one mating and produces a steady stream of pups over a prolonged period. They reach sexual maturity at around 30 years and their lifespan is an estimated 70 to 100 years. Whale sharks are solitary creatures, while they may gather in large groups they do not socialize like whales and dolphins. You can identify males by the presence of claspers, found on the underside of the whale shark.

Senses

Whale sharks have the five senses that we have, touch, sight, hearing, smell and taste. They use these senses to find food, avoid predators and throughout their normal activities. Whale sharks also possess a sixth and seventh sense, two highly developed systems that have allowed them to be so successful in the world.

Vision

Whale sharks have two very small eyes located on either side of the head. Their vision is very poor but are sensitive to low light conditions, similar to house cats. They do not have eyelids but can rotate, close and pull them back in their head for protection.

Sense of smell and taste

Researchers believe that the whale shark uses its sense of smell to locate the most protein-rich waters. This behavior can be observed in La Paz as we see the whale shark alternate between feeding and swimming. The nostrils are located on the upper lip, on either side of the mouth. 

Hearing

Not much is known about specifically about the whale shark, sharks in general have a strong sense of hearing and are more attuned to lower frequencies.

Touch

Whale sharks can feel touch just as we do, while some sharks may bump with their nose to feel potential food or prey whale sharks do not. In general whale sharks will react negatively to touch, typically swimming deeper or away from the situation.

Sixth Sense: The Lateral Line

Whale Sharks can sense vibrations in the water using an organ called the lateral line. The lateral line is a fluid-filled vessel that runs from head to tail in a depression between the second and third lateral ridge. Many small pores open up on the skin, detecting the intensity and direction of vibrations in the water. In predatory sharks this system enables the shark to detect injured fish that are thrashing around in the water. Whale sharks use this system for orientation, when you are swimming with them it’s important to move slowly and deliberately as to not disturb them.

whale shark nostrilSeventh Sense: The Ampullae of Lorenzini

Whale sharks can sense the electrical fields generated by us, other animals and the earth’s electromagnetic field. In most sharks this ability is used to hunt, in the case of whale sharks it’s mainly used for orientation and navigation. Whale sharks migrate between different feeding areas throughout the year and use this sense to navigate the world's oceans.

 

For some more information about whale sharks or to see how you can get involved in their conservation check out these sites.

IUCN

Wildbook for whale sharks