A singing fish: it glows green during courtship and looks like Boris Johnson’s hardship face

If, at midnight, you stick your oar into the water of the lagoon near Kallady Bridge in Batticaloa, on Sri Lanka’s east coast, you might hear fish sing. It sounds, according to Prince Casinader, who was a former local MP moonlighting as a journalist, “like a man idly playing on the keys of a piano. Bass notes and treble notes. Or like someone rubbing a finger around the rim of a wet wine glass.”

For a while, people were uncertain about whether the fish really existed. In a 2017 Trip Advisor review of singing fish tours organised by a local hotel, the tourist writes, “The Sound was there and we listened by placing the ear-end of the Oar … in water. And in one spot we heard it even without listening through the oars. It was an amazing experience.” The hotel’s owner responds: “Even though a lot of people know Batticaloa as the ‘Land of the Singing Fish’ most of them including locals think it is a legend and no truth to it. I heard it first when I was around 12 almost 30 years ago and after the long disruption due to the civil war people forgot about it and it slipped into distant memory.”

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A few years earlier, a group of citizen scientists who call themselves the Science Navigators, set out to see if they could record the fish – they had found musical notation of a recording of the fish taken in the 1950s and since lost, so the fish were surely real. They dropped a waterproof microphone into the lagoon and there it was: deep whomps and reedy notes. Like frogs, but less croaky. You can listen from the bridge, too, if you press your ears against its iron pillars.

In an interview for a documentary about the fish, one of the Science Navigators, cardiologist Arulnithy Kanagasingam, says, “I have no knowledge of music. So what I can hear is a good sound, it’s not a disturbing sound.” He says that they have seen references to similar singing in California.

There, according to the Christian Science Monitor, houseboat residents heard the sound and thought that it was produced by navy experiments or aliens. Fortunately, it was “plainfin midshipman”, a species of toadfish that glows green during courtship and looks like Boris Johnson’s hardship face. The fish produce the sound by “vibrating a gas-filled bladder within the abdomen”.

In 2014, scientists discovered that fish, like people, raise their voices when their environment gets noisier. In other words, as Emily Anthes writes in one of my favourite science articles of all time, they shout. The scientists were investigating the effects of the noises humans have introduced to water – ferries, cargo ships, sonar and drilling – on the creatures that live there. In order to test how fish were adapting to the brouhaha, an ecologist and her doctoral student placed blacktail shiners, a type of minnow, into a tank. The male shiners are known to growl. When white noise was played in the tank, the fish, instead of moving closer, growled more loudly.

I thought about shouting fish and singing fish recently while watching an opera – La Traviata – performed on the water in Sydney harbour. My husband and I had bought tickets using the government vouchers given to the good people of the state of New South Wales to encourage us to “Dine and Discover” in the name of post-pandemic recovery. The opera had offered to double the value of the vouchers (give a man $25 and he will watch a simple movie. Teach a man to double his voucher money etc etc). Speaking of post-pandemic recovery, scientists (and fish) are hoping the economic devastation has left the seas quieter.

The singers performed on a tilted square-shaped stage, like a large paper napkin suspended on the water, over which hung a giant chandelier. They sang of “love, the pulse of the whole world” and “cruel fate” as the moon moved from stage left to stage right and bats squeaked and chirped – more loudly than usual, I suspect – in the giant fig trees overhead.

• “The Nature of … ” is a column by Helen Sullivan dedicated to interesting animals, insects, plants and natural phenomena. Is there an intriguing creature or particularly lively plant you think would delight our readers? Let us know on Twitter @helenrsullivan or via email: helen.sullivan@theguardian.com