The basking shark, Cetorhinus maximus, is the second-largest fish in the ocean, surpassed only by the whale shark. Reaching lengths of 7.9 to 12 meters (26 to 39 feet) and weighing over 5 tons, its immense size often leads to questions about its behavior. This filter-feeding shark is frequently observed swimming slowly at the surface with its enormous mouth wide open, prompting many to wonder if it can truly close its mouth. Despite its imposing appearance, it poses no threat to humans.
Structure of the Basking Shark’s Mouth
The basking shark can close its mouth, though it is often seen open due to its specialized feeding method. Its mouth is uniquely adapted for filter feeding, featuring large, hinged jaws that can open up to 1 meter (3.3 feet) wide. Unlike predatory sharks, basking sharks do not have large, biting teeth; instead, they possess hundreds of very small, hooked teeth, typically only 5 to 6 millimeters long. These teeth are arranged in multiple rows and play a minimal role in feeding, functioning more like a sieve.
The most distinctive features of its mouth are the numerous dark, bristle-like structures called gill rakers. These structures line the inside of the gill arches, supported by greatly enlarged gill slits that nearly encircle the shark’s head. The entire mouth structure is designed to maximize water intake, forming a wide funnel that efficiently channels water towards the filtering apparatus. When not actively feeding, or to conserve energy and reduce drag, the basking shark closes its mouth.
How Basking Sharks Feed
A basking shark’s mouth is usually open due to its passive filter-feeding behavior. It swims slowly, typically around 3.7 kilometers per hour (2 knots), with its mouth agape to strain microscopic organisms from the water. Its diet consists primarily of zooplankton, especially small crustaceans like copepods, along with small fish and fish eggs.
As the shark moves, water flows into its mouth and passes over the specialized gill rakers. These comb-like structures trap the tiny food particles, which are then swallowed, while the filtered water exits through the five large gill slits on the sides of its head. This efficient system allows a basking shark to filter an astonishing volume of water, estimated to be around 2,000 tons per hour. The shark periodically closes its mouth, every 30 to 60 seconds, to dislodge and swallow the accumulated plankton.
Basking Sharks and Human Interaction
Despite their immense size and wide-open mouths, basking sharks are not dangerous to humans. They are docile creatures whose diet consists solely of plankton, posing no threat of biting or attacking people. Encounters are typically accidental, often occurring when they feed at the surface.
While generally tolerant of human presence, caution is advised due to their sheer size and rough skin, which can cause abrasions upon contact. Basking sharks are categorized as Endangered on the IUCN Red List, reflecting a significant decline in their global populations, estimated to be between 50-79% over the past century. They face ongoing threats such as accidental capture in fishing gear (bycatch), entanglement in ropes, and collisions with boats.
Conservation efforts are underway globally to protect these animals, including bans on targeted fishing and the establishment of protected marine areas. Observing these creatures respectfully and from a distance contributes to their conservation.