What does the shark say? First shark recorded making noises

More than 1,000 species of bony fishes make sounds to communicate with each other, but scientists thought sharks were the strong, silent type.

Now, that reputation has been blown straight out of the water, with the first ever documented case of a shark deliberately producing sound.

The New Zealand rigs (Mustelus lenticulatus) made clicking sounds when they were handled by divers. Based on the high-frequency nature and short duration of the sounds (about 48 milliseconds), they are probably made by forcefully snapping the rigs’ teeth together. Don’t tell their dentists!

Bony fishes produce sounds for communication – including courtship, spawning, defence against predators, and unfriendly encounters – and use lots of different mechanisms to do so. The most important involves the swim bladder, a gas-filled organ used to modulate buoyancy, and various types of drumming muscles which cause it to vibrate.

But elasmobranchs – a group which including cartilaginous fishes such as sharks, skates and rays – do not have swim bladders and were thought to lack the ability make sounds, despite being capable of hearing.

This long-held view has been challenged in the last few years, with a skate and several species of ray shown to produce short clicks when disturbed by divers.

To investigate whether sharks also have this reaction, marine scientists studied 10 juvenile rigs which were caught in the Kaipara Harbour of New Zealand’s North Island.

The rig grows to between 70-150cm long and is preyed upon by larger sharks and marine mammals. It has blunt, flattened teeth that form mosaic-like plates on the upper and lower jaws, which it uses to crush crustaceans.

“The species is endemic to inner shelf areas, coastal waters (up to 1000m depth) and shallow estuaries around New Zealand,” write the authors of paper describing the research, which is published in the journal Royal Society Open Science.

The rigs were transferred individually from holding tanks to a large experimental tank equipped with a hydrophone to record underwater sounds. They were then handled by a submerged diver.

“Rigs handled for 20s produced significantly more clicks during the first 10s than during the subsequent 10s, and both in the presence and absence of body movements,” the authors write.

“Rigs were not observed to produce clicks during feeding or while free-swimming in the tank. This may suggest that the initial handling triggers a stress or startle response, resulting in increased click activity. As rigs become accustomed to the handling, the behavioural response likely diminishes, leading to fewer clicks over time.”

The purpose of the clicks, however, remains unknown. The authors suggest that “…they could either be an incidental (physiological, mechanical) byproduct of a startle response or a meaningful biological signal (e.g. alarm, warning, defence).”