Sharks are often perceived as fearsome ocean predators, equipped with rows of sharp teeth designed for hunting and tearing prey. While this image holds true for many well-known species, a fascinating group of sharks defies this common perception. These unique sharks possess highly specialized feeding adaptations that allow them to thrive on some of the ocean’s smallest organisms, using distinct feeding structures and sophisticated mechanisms to sustain their massive bodies.
Understanding Specialized Shark Dentition
While some sharks are perceived as “toothless,” filter-feeding sharks are equipped with numerous, often tiny, and highly modified teeth. These teeth are not used for biting or tearing but rather play a role in the overall filtration process or are vestigial structures. For instance, whale sharks can have over 300 rows of minute teeth that do not assist in feeding.
Instead, these sharks feature unique internal mouth and gill structures adapted for their specific dietary needs. These adaptations allow them to efficiently process vast quantities of water to extract microscopic food particles. The presence of these specialized structures, rather than the absence of teeth, defines their unique feeding strategy.
Giants of the Ocean: Sharks with Unique Feeding Structures
Three prominent species exemplify sharks with unconventional feeding structures: the whale shark, basking shark, and megamouth shark. Each has evolved distinct anatomical features to facilitate their filter-feeding lifestyle.
The whale shark, the largest fish in the world, can reach lengths exceeding 60 feet and weigh over 20 tons. Its massive mouth, positioned at the front of its broad, flat head, can open up to 5 feet wide. Inside its mouth are specialized filter pads, sometimes described as modified gill rakers.
The basking shark, the second-largest fish, typically grows up to 33 feet long and is found in cooler waters globally. It has an enormous, gaping mouth that can expand to resemble a hoop-like structure when feeding, and its gill slits are exceptionally large, almost encircling its head. Its feeding mechanism relies on bristle-like gill rakers.
The megamouth shark is a rare and elusive species, first discovered in 1976, and can grow up to 18 feet. It is characterized by a broad head, rubbery lips, and a remarkably wide mouth that can open up to 4 feet. Despite its large mouth, it uses it for straining food particles.
The Mechanics of Filter Feeding
These ocean giants employ sophisticated methods to strain their microscopic meals from the water. The primary mechanism involves specialized structures called gill rakers, which are bony or cartilaginous projections from the gill arches. These rakers act as a sieve, trapping tiny organisms as water flows through the shark’s mouth and over its gills.
Filter-feeding sharks utilize two main techniques: ram feeding and suction feeding. Ram feeding involves swimming forward with an open mouth, allowing water to continuously flow in and over the gill rakers. The basking shark is a classic example of a passive ram feeder, cruising slowly near the surface with its mouth agape to filter zooplankton, particularly copepods. A large basking shark can filter over 330,000 gallons of water per hour.
Whale sharks can use both ram filtration and active suction feeding. For ram feeding, they swim with their mouths open, pushing water and prey into their mouths. They also employ suction feeding, actively opening and closing their mouths to suck in volumes of water containing plankton, small fish, and even fish eggs. This active gulping allows them to target denser concentrations of food.
The megamouth shark’s feeding strategy is still being researched, but it is thought to use low-velocity, high-volume suction or engulfment feeding, expanding its pharynx to draw in prey-laden water. Unlike the basking shark, it does not rely solely on continuous forward motion. Whale sharks have also been observed “coughing,” which is believed to be a mechanism to clear their filter pads of accumulated particles.