Whale Shark: A Whale or a Shark?
Whale sharks are considered the largest fish (and shark) in the sea, weighing up to 60 tons and reaching lengths of 40 feet or more. Despite their large, intimidating size, whale sharks are usually docile and gentle, sometimes even allowing divers to latch on for a ride. They generally roam the ocean alone, but can sometimes be found gathered in large numbers in areas with abundant plankton food. Because of this, the distribution of whale sharks indicates the presence of plankton, and therefore the overall health of our oceans.
Behaviour and Characteristics of the Whale Shark
Whale sharks have a flattened head and a blunt snout above its mouth which has short barrels (slender, whiskerlike sensory organs near the mouth of the whale shark, where the taste buds are housed and used to search for food in murky water) protrude from its nostrils. Whale sharks are also most commonly identified through their unique physical appearance; their backs and sides are gray to brown with white spots scattered among pale horizontal and vertical stripes. Its belly, like most fish, is white, and it has two dorsal fins which point opposite relative to its body and ends in a large dual-lobbed caudal fin (the tail).
Whale sharks have an affinity for temperate and warm waters (between latitudes of 30 degrees N and 35 degrees S), and so they tend to be found along the equator. They are primarily pelagic (tend to live in the open sea, but not in the greater depths of the ocean), however, they can be found in both shallow and deep coastal waters and the lagoons of coral atolls and reefs.
Whale sharks are considered to be mainly 'filter feeders' - a method of aquatic food consumption by which the animal takes in much small prey at once by simply opening their mouth and taking in whatever happens to be there, and filtering out the undesirable items. This is the opposite of 'specialised feeders', who actively seek specific kinds of prey. However, it is suggested that whale sharks use three feeding methods. The first method is the one mentioned - passive feeding, where they swim slowly with their (usually) five feet wide mouth open, capturing plankton from the water; this method is normally referred to as "cross-flow filtration". The second method is called 'vertical feeding', whereby the whale shark floats vertically, with minimal forward movement whilst using a suction method to draw prey (plankton mostly) into their mouths. The third method is commonly referred to as 'active feeding', and it is where the whale shark filter-feeds while swimming steadily, which allows them to draw water into their mouths at higher velocities. This method of feeding is also referred to as 'ram-filter feeding'.
They may change their feeding behaviour according to prey abundance (and type of prey) in order to maximise the 'relationship between energy expended with caloric intake'. For example, passive feeding does not require much energy, allowing the shark to graze while searching for higher density food. Vertical feeding is a method whereby smaller patches of food can be consumed with lower energy costs, than active feeding, for example. Active feeding, however, is typically only encountered in high prey density areas.
Side note: they are referred to as 'whales' because of their size, but they're not actually whales, they are indeed sharks.
The Status of Whale Sharks
Currently, whale sharks are considered vulnerable as their population is declining. This is due to many reasons, mainly because they are highly valued for their meat, fins, and oils, all of which remain a threat to the species (particularly by unregulated fisheries). Like many fish, they are victims of bycatch, which is the unintentional capture of non-target species in fishing gear. It is unarguable that laws and regulations must be passed in order to protect species that are on the verge of extinction, such as the whale shark.