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Species:

Habitat: , gaping at everything

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 . 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 , not least because it rarely comes , and spends most of its time over 100 metres down. , and less than half of those have ; a specimen caught in 2009 . 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 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 . Only two other sharks do this: the similarly weird-looking , and the largest living fish, the .

Two new studies try to get to the bottom of : 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.

Lights, electricity, action

of the University of Western Australia in Crawley is not so sure. He thinks the megamouth’s maw holds so much water that, if it simply swam into a cloud of plankton with its mouth open, they would be pushed aside rather than drawn in. He suspects that it uses suction when it finds large numbers of plankton that are worth the effort to obtain.

Like many deep-sea predators, they may lure in their prey with light. The first specimen apparently had a luminescent strip along its upper jaw, but no one has managed to find one since.

Kempster and his colleague think that, like many fish, the megamouth can sense weak electrical fields. Its head is covered with several patches of tiny forward-facing sensors. They suggest that the shark lures plankton in with its luminous upper jaw, then uses its electrical sense to decide when they are close enough – at which point it engulfs them.

Journal references: Tomita and colleagues, ; Kempster and Collin,

Read previous Zoologger columns: The hairy beast with seven fuzzy sexes, Australia’s truly glamorous camper, Jet-propelled living fossil with a problem, The sharpest mind in the farmyard, Invasion of the crazy incestuous ants, The fish with no stomach for its prey, Well-fed black widows promise safe sex, The butterfly that sleeps its way to safety, How to get elected in a termite democracy, Away in a vermin-infested manger, Child clones shape-shift to escape hunters.