Basking sharks have enormous mouths because they feed by swimming forward with their jaws wide open, filtering tiny plankton from massive volumes of seawater. Their mouth can stretch up to a metre wide, and a large adult can process around 130,000 gallons of water per hour. Since their prey (mostly copepods and other zooplankton) are just millimetres long, the only way to get enough calories is to strain an extraordinary amount of ocean through an equally extraordinary opening.
How Ram Filter Feeding Works
Unlike whales that gulp or suction-feed, basking sharks use a method called ram filter feeding. They simply swim forward with their mouth agape, letting water rush in and pass over their gills. The water exits through five huge gill slits on each side of the head, while food particles get trapped inside. There’s no pumping, no gulping. The shark’s forward motion does all the work, which is why a bigger mouth means more water captured per second of swimming.
During surface feeding, basking sharks cruise at roughly 0.85 metres per second, about 1.9 miles per hour. That’s actually 24% slower than their normal cruising speed with the mouth closed. Opening that cavernous jaw creates significant drag, so the sharks deliberately slow down to avoid burning more energy than they’re taking in. It’s a careful energy trade-off: swim fast enough to filter plenty of water, but slow enough that the drag cost doesn’t cancel out the meal.
The Gill Raker Filtration System
The mouth is only half the system. Inside, each of the shark’s gill arches carries between 1,000 and 1,300 bristle-like structures called gill rakers, some reaching up to 10 centimetres long. When the mouth opens, a complex set of muscles contracts and erects these rakers so their free ends point toward the incoming water, forming a dense mesh that traps plankton while letting water pass through to the gills.
The captured plankton doesn’t just sit on the rakers. The tissue at their base secretes a layer of mucus that tangles and holds tiny organisms in place. When the shark closes its mouth, elastic fibres pull the rakers flat against the gill arches, squeezing the mucus-and-plankton mixture back into the mouth cavity where it can be swallowed. Then the mouth opens again and the cycle repeats. It’s an elegant, nearly passive system that lets the shark eat continuously while doing little more than swimming in a straight line.
Why a Bigger Mouth Captures More Food
Zooplankton are not evenly distributed in the ocean. They cluster in patches and dense layers, often near the surface. A basking shark feeding through one of these patches has only a limited window to collect as much food as possible before the concentration thins out. The wider the mouth, the greater the cross-section of water entering per second, and the more plankton captured in a single pass. For an animal that can reach 12 metres in length and needs thousands of calories daily, a small mouth would simply never take in enough.
This is the same basic logic behind other filter-feeding giants. Whale sharks, manta rays, and baleen whales all have oversized mouths relative to their body plans. But the basking shark’s approach is arguably the most minimalist: no active suction, no lunge feeding, just a gaping jaw and forward momentum. That simplicity makes mouth size the single most important variable in how much food the shark can collect.
Jaw Structure That Allows Extreme Gape
A mouth that opens a full metre wide needs a skeleton that can accommodate it. Like all sharks, basking sharks have a skeleton made entirely of cartilage rather than bone, which is inherently more flexible. The upper jaw hangs from the braincase by a combination of cartilage supports at the back and long, stretchy ligaments at the front. These ligaments allow the upper jaw to drop away from the skull far more than a rigid bony connection would permit.
The lower jaw connects in a similar fashion, creating a hinge system with remarkable range of motion. When fully extended, the entire front of the shark’s head becomes a wide, circular opening. The positioning is distinctive too: the mouth sits at the very front of the head, ahead of the eyes, rather than underslung like in most sharks. This terminal mouth placement lets the shark drive straight into plankton patches and scoop water from directly ahead, maximizing the efficiency of its forward-swimming feeding style.
Energy Balance of Filter Feeding
The 24% speed reduction during feeding hints at how costly it is to swim with a metre-wide hole in the front of your body. Researchers studying basking shark swimming speeds found that feeding sharks moved at about 0.85 metres per second compared to 1.08 metres per second when cruising with the mouth closed. Interestingly, these observed feeding speeds were 29 to 39% slower than mathematical models predicted they should be, likely because surface drag effects and the sheer resistance of an open mouth take a bigger toll than theoretical calculations assumed.
This means basking sharks are making a constant calculation, trading speed and energy for food intake. They tend to feed only where plankton density is high enough to make the drag cost worthwhile. When concentrations drop below a certain threshold, they close their mouths and move on, sometimes diving to deeper layers or traveling long distances to find the next productive patch. The big mouth is an investment that only pays off in the right conditions, which is why basking sharks are so strongly tied to seasonal plankton blooms in temperate waters.