Researchers have recently described the puzzling behavior of sea-dwelling worms which can produce one of the loudest sounds ever measured in aquatic animals.

When two sea-worms engage in “mouth fighting”, they produce powerful snapping sounds. Credit: Ryutaro Goto.

Many aquatic animals, including mammals, fish, crustaceans and insects, produce loud sounds underwater. However, this is the first time that scientists have witnessed a soft-bodied marine invertebrate making noises.

“When I first saw their video and audio recordings, my eyes just popped out of my head because it was so unexpected,” said Richard Palmer, Professor of biology at the University of Alberta.

Palmer received the footage from Ryutaro Goto, a Japanese researcher who was looking for some help in figuring out how the snapping sea-worms were producing their weird sounds.

The animals were first discovered in 2017, during a dredging expedition off the coast of Japan. Goto and Isao Hirabayashi, a curator at the Kushimoto Marine Park, were among the first to record the sounds.

Tests found that the sounds are as loud as 157 dB, with frequencies in the 1–100 kHz range and a strong signal at ∼6.9 kHz — that’s comparable to those made by snapping shrimps, which are among the most intense biological sounds that have been measured in the sea

Writing in the journal Current Biology, the Japanese and Canadian researchers explain how and why the loud snapping sounds occur.

According to the researchers, when these worms come close to each other, they open their mouths and snap — something described as “mouth fighting”. This is essentially a territorial display of force which the worms employ to protect their dwellings.

“The real challenge was figuring out how a soft-bodied animal like a worm—which is basically a hollow, muscular tube—could possibly make such loud sounds,” Palmer said.

Palmer says that the snapping sounds are produced by cavitation bubbles due to the extensive array of muscles in the worm’s pharynx.

“It’s like trying to suck a smoothie through a paper straw,” Palmer explained. “When it gets a little bit soft at the end, the tip collapses. It doesn’t take much force to make it collapse, but if you try to suck harder and harder, you build up this immense negative pressure. When the worm finally pops the valve open, it happens so fast that the water can’t fill the space, and the sides of that space collapse together in a point, creating this explosive release of energy in the form of sound.”

This hypothesis has yet to be validated but the researchers hope to conduct an experiment soon.

“It’s just an incredibly cool animal with quite the unexpected behaviour,” Palmer added. “I’ve shown the videos to biologists who study invertebrates and their reaction is always the same: they shake their heads in wonder.”