The whale shark, Rhincodon typus, is the largest known fish species, growing up to 61.7 feet (18.8 meters). Despite its immense size, this shark is a gentle filter feeder, consuming microscopic organisms and small fish. This specialized feeding strategy sets it apart from many other large marine creatures.
Understanding Filter Feeding
Filter feeding is a common aquatic feeding method where an animal strains suspended food particles from water. This process involves drawing water into the mouth and passing it over a filtering structure that captures small organisms while allowing the water to exit.
Many aquatic animals, from tiny zooplankton to massive whales, employ some form of filter feeding to obtain their sustenance. The efficiency of this method allows these animals to thrive on abundant, yet individually small, food sources.
This feeding strategy contrasts sharply with the predatory hunting methods of most other large marine animals. Instead of pursuing individual prey, filter feeders access a vast food supply by processing large volumes of water. The specific structures and mechanisms used for filtration vary widely among different species, reflecting diverse evolutionary paths to exploit this food source.
Whale Shark Gill Structure
The whale shark has a broad, flattened head with a large mouth positioned at the front. This wide mouth, which can span up to 5.1 feet (1.55 meters) across, is the initial entry point for water and food. Behind the mouth, whale sharks have five large pairs of gill slits on each side, used for both respiration and feeding.
Inside the pharyngeal cavity, the whale shark’s filtering apparatus consists of around 20 filtering pads. These dark, sieve-like structures are believed to be modified gill rakers. Gill rakers are projections from the gill arches that prevent solid material from passing through the gill slits. In whale sharks, these filter pads completely occlude the pharyngeal cavity, forming a mesh-like barrier with openings averaging 1.2 millimeters in diameter.
How Filtration Works
When feeding, a whale shark employs different methods to draw in water and food. One method is ram filtration, where the shark swims forward with its mouth open, pushing water and food into its mouth. Another method is active suction feeding, where the shark opens and closes its mouth, drawing in large volumes of water even while stationary. This suction method allows the whale shark to pull in more plankton.
Once water containing plankton and small organisms enters the whale shark’s mouth, it flows over the filter pads, which act like a sieve. Food particles, such as plankton, small fish, and crustaceans, are trapped by the reticulated mesh. The filtered water then exits through the five large gill slits on the sides of the head.
Food separation occurs through cross-flow filtration. In this method, water flows nearly parallel to the surface of the filter pads, rather than directly through them. This allows denser food particles to continue towards the back of the throat for swallowing, while filtered water is expelled. This minimizes clogging of the filter pads, though whale sharks have been observed “coughing” to clear any build-up.
Unique Gill Adaptations
The whale shark’s gill design is an effective adaptation for its filter-feeding lifestyle. Their filter pads are not simply coarse strainers; they possess a spongy or mesh-like structure. This enables them to retain even very small prey, down to about 1 millimeter in diameter, while allowing water to pass through. The precise spacing and structure of these pads are important for capturing the diverse array of plankton and small nektonic life that make up their diet.
The large size of the whale shark’s mouth and the extensive surface area of its gill filtering system allow it to process immense quantities of water. A whale shark can filter thousands of cubic meters of water per hour, ensuring a steady food supply. This capacity, combined with their ability to switch between ram filtration and active suction feeding, makes their feeding mechanism versatile for exploiting dense concentrations of prey. The arrangement of cartilaginous vanes supporting the pads also directs water efficiently across the gill filaments, contributing to their feeding efficiency.