Every day, troves of hungry marine organisms — shrimp, jellyfish, squid and bony fish — migrate from the ocean's murky depths to the surface. For the first time, marine biologists have captured the sound of their collective lunch outing. May it be the sonic backdrop to all of your future, jellyfish-induced nightmares. Cynics will point out that this sounds more like a struggling air conditioner than an epic call of the wild. But even if it isn't the most musically interesting soundbite, this recording is ecologically fascinating. Worldwide, an estimated ten billion tonnes of scaly and tentacled critters make the daily trek from the ocean's mesopelagic zone — 200 to 1000m beneath the surface — to the phytoplankton-rich surface, moving at night to avoid the prying eyes of hungry predators. By dawn, the animals are headed back to the safety of deep ocean darkness.
It's an enormous migration event that plays a key role in the marine food web and planetary carbon cycle. And aside from a handful of biology nerds, few human beings will ever pause to consider it. You can now count yourself among those lucky few.
The low-frequency hum (300 to 900 Hertz) produced by this fishy diaspora is roughly three to six times louder than background noise, occurs for one to two hours daily, and can be picked up with sophisticated audio equipment. Scientists aren't yet sure who's contributing to the plankton-fuelled groan, or what purpose, if any, it serves. It could be an ecological dinner bell, signalling the start or end of the feasting frenzy. Or the sound could serve as a warning against predators. If nothing else, it's reminder that the oceans are a weird and fascinating place — and that we've still got plenty of mysteries to solve in their murky depths.