The basking shark, the second-largest fish in the ocean, can reach lengths of up to 12 meters (39 feet) and is frequently observed cruising near the surface. Its most striking feature is an enormous, cavernous mouth, often seen wide open. This posture leads many to believe the basking shark cannot close its mouth, a common misconception.
Can the Basking Shark Close Its Mouth?
Despite its imposing appearance, the basking shark can close its mouth. Flexible jaws and powerful muscles surrounding the mouth allow it to control the opening and closing.
The shark’s mouth does not remain perpetually agape. When not actively feeding or after gathering enough plankton, the basking shark closes its mouth to swallow the accumulated food. This demonstrates that the open-mouth posture is a behavioral choice linked to its feeding strategy, not an anatomical limitation. A closed basking shark mouth can even resemble the streamlined profile of a great white shark.
The Science of Filter Feeding
A basking shark’s frequently open mouth relates to its unique feeding strategy, known as passive filter feeding or ram feeding. This involves swimming forward with its mouth wide open, allowing vast quantities of water to flow through. The shark’s mouth can extend up to a meter (3.3 feet) wide, creating a large intake area for its microscopic prey.
As water enters the mouth, it passes over specialized structures called gill rakers, which are bristly, cartilage-like projections. These gill rakers act as a sieve, efficiently trapping tiny organisms, primarily zooplankton like copepods, along with small fish and fish eggs. The processed water then exits through the shark’s large gill slits, which nearly encircle its head. An adult basking shark filters up to 4 million pounds (2,000 tons) of water per hour to collect enough sustenance.
When the Mouth is Closed
Basking sharks close their mouths for a variety of practical reasons beyond simply swallowing food. When not actively engaged in feeding, such as during periods of rest or when migrating through areas with low plankton concentrations, keeping the mouth closed offers several advantages.
Closing the mouth reduces hydrodynamic drag, allowing the shark to swim more efficiently and conserve energy. This streamlined shape also enables them to achieve higher speeds and even breach the water’s surface, a behavior observed with their mouths closed. Mouth closure also protects their delicate gill rakers from damage and helps to clear any debris that might accumulate during feeding. This demonstrates that while filter feeding is central to their survival, mouth closure is a natural part of their daily life.