In 1976 a research boat was moored off the coast of the Hawaiian island of Oahu. When the crew winched up the anchors they discovered a 4.5-metre-long shark tangled up in the cables. It looked like nothing they had ever seen.

It was seven years before the animal was officially described and named. Taking their lead from press coverage of the discovery, the scientists suggested calling it the megamouth shark. It's an appropriate moniker: the shark's mouth can be over a metre across, or a fifth of the animal's length.

Since then the megamouth has proved an elusive beast, not least because it rarely comes shallower than 12 metres, and spends most of its time over 100 metres down. Only 50 have ever been seen, and less than half of those have made their way to scientists; a specimen caught in 2009 wound up being cooked. Nevertheless we are building up a picture of how it lives, including answers to the most important question: what it does with that gaping maw.
Open wide

It looks like something H. P. Lovecraft would have dreamed up, but the megamouth shouldn't keep you out of the water. It feeds entirely on tiny animals called plankton, as do humpback whales. Only two other sharks do this: the similarly weird-looking basking shark, and the largest living fish, the whale shark.

Two new studies try to get to the bottom of how the megamouth shark goes about eating: a tricky business, because nobody has ever seen them feed. Taketeru Tomita of the University of Tokyo, Japan, and colleagues have looked at the mechanics of its jaw to find out whether it actively sucks plankton in, or just swims through clouds of them with its mouth open and hopes for the best – a technique called ram feeding.

They reasoned that sucking prey into your mouth is hard work, so animals that do it need to have stronger jaws that those that don't. One telltale bit of anatomy should be the ceratohyal cartilage, which is attached to a shark's tongue. During suction this cartilage pulls the tongue downwards, creating more space in the mouth and helping to suck in water.

In shark species that use suction feeding, the ceratohyal cartilage is short and stiff, whereas in ram-feeding species that do not rely on its strength it is longer and flexible. Tomita looked at the ceratohyal cartilage of a megamouth shark preserved in a Japanese museum and found that its length and stiffness fitted neatly into the ram-feeding group, not the suction-feeding group. So, he says, the megamouth cannot suck in its prey and must be a ram-feeder. edit post