How one of the smallest fish makes a sound as loud as a firecracker

One of the world’s smallest fish, measuring just 12 millimetres long, can make a sound registering more than 140 decibels – about as loud as firecrackers being set off.

Danionella cerebrum is a tiny species of fish that is native to small streams along the Bago Yoma mountain range in Myanmar. It has the smallest known brain of any vertebrate in the world.

“We study this fish primarily for neuroscience, because they’re so small and transparent,” says Verity Cook at Charité – Berlin University Medicine in Germany. But the fish are so loud that, even when just walking past their tanks, you can hear audible pulsing sounds, she says.

To investigate how the tiny fish make these sounds, Cook and her colleagues gathered groups of three or four fish into a small tank and took high-speed videos of them.

When the fish produce sound, the rib that sits adjacent to their swim bladder – the organ that helps to control their buoyancy – gets pulled by a specialised muscle into a piece of cartilage with a small indentation.

“There’s tension built up in this contraction,” says Cook. “When that is very rapidly released, [the cartilage] strikes the swim bladder, which produces the drumming sound.”

Based on analysis of gene expression, these specialised muscles are more resilient to fatigue compared with other muscles in the fish, which allows them to produce lots of hits in quick succession.

The reason why these fish make such a loud noise is unclear. D. cerebrum normally live in murky, turbid waters, so they might have evolved this behaviour to help locate each other when visibility is poor, says Cook.

“But the fact that it’s only males that make sound suggests that it’s to do with aggressive behaviour towards other males or mating behaviour with females,” she says.