Herring communicate in the dark by emitting high-frequency bursts of gas from their anal ducts, a phenomenon scientists literally named Fast Repetitive Tick.

How a herring makes a tick underwater

Herring have a gas-filled swim bladder, the organ that helps them control buoyancy. They can release gas from it through a duct that opens near the anus. As gas escapes, it produces rapid, high-pitched clicks called Fast Repetitive Ticks, or FRTs. The sound is not a voice-like call: it is a brief series of pulses made by moving gas.

Because the clicks are so high in frequency, many are beyond the range of human hearing. Underwater, however, sound travels through the surrounding water, where nearby fish could potentially detect it.

A signal with an unresolved purpose

Herring often make FRTs at night, when a school’s members cannot rely as much on sight to keep track of one another. That timing has led researchers to consider whether the bursts help fish stay in contact with nearby schoolmates. In a dense school, even a simple signal could help individuals remain aware of their neighbors without requiring each fish to identify or answer another.

But timing alone does not establish what the sounds mean. The bursts might have another function, or serve more than one. Scientists are still investigating whether other herring respond to them and what information, if any, the sound conveys.

Why a possible roll call matters

A school is not a single coordinated organism: each fish has to react to its immediate surroundings. If FRTs help herring sense nearby companions in darkness, the sound could support group behavior without being a conversation in the human sense. The striking possibility is that a familiar-looking school may rely on a signal people can barely hear—and whose purpose is still being worked out.