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A small international team of marine biologists has observed the first known instance of a shark intentionally making sounds. In their study in the journal Royal Society Open Science, the group reveals that several of the sharks made clicking noises when they were handled, and they suggest ways the noises might have been made.

Prior research has suggested that sharks are not able to produce sounds due to the lack of swim bladders, which is what fish use to produce sounds. Also, scans of shark anatomy have shown that they have no other organs or structures that appear capable of producing sounds. So, it came as a surprise to researchers when the juvenile rig sharks they were handling in a test tank produced noises. Rig, or spotted estuary smoothhound sharks, are native to New Zealand and instead of rows of sharp teeth, they have a mouthful of plate-like teeth, useful for cracking open crustaceans.

The researchers were conducting experiments in a large water tank involving shark hearing. Part of the work involved behavioral training, which necessitated grabbing the sharks and holding onto them for a few minutes. It was during these grabbing sessions that they began to hear clicking sounds coming from the sharks, all of whom were juveniles.

To find out if it was deliberate, the team put several of them into separate tanks and grabbed them as they had done earlier, this time with underwater microphones capturing the sounds. They found that all the sharks produced the sounds when handled. However, they noted the clicking sounds slowed and eventually ceased after a while, presumably as the sharks grew accustomed to such treatment.

Some species of rays and skates are known to make similar sounds when disturbed, and the researchers suggest the clicks in this case are made by the sharks rubbing their teeth together. Such clicking sounds, they theorize, could serve as a means to momentarily confuse predators, such as fur seals. They also note that it is unlikely the sounds are calls for help from others of their kind, as most of the clicks' frequencies were higher than the range of shark hearing.