Mating dance of sea fireflies is ‘the coolest fireworks show that you’ve ever seen’

Nicholai Hensley has spent countless hours standing waist-deep in pitch-black waters off the coast of Panama, watching thousands of tiny sea creatures perform dazzling displays of bright blue light.

The creatures — each the size of a sesame seed — belong to a recently discovered species of ostracod, also known as sea fireflies, whose males lure potential mates with a synchronized dance powered by iridescent mucus.

“It looks phenomenal,” Hensley, an evolutionary biologist at Cornell University in the U.K., told As It Happens guest host Peter Armstrong.

“I like to say it’s the coolest fireworks show that you’ve ever seen — but probably have never seen.”

Hensley and his colleagues describe that fireworks show in detail in a new study, published this week in the journal Proceedings of the Royal Society B.

Ostracods are itty bitty crustaceans with tiny pink bodies ensconced in clear shells.

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“We like to affectionately think of them as shrimp inside of a clamshell,” Hensley said. 

“They have these large shells that come off their back and cover their face, and they end up walking around like little tanks, actually. They’re very armoured little guys.”

Sexy blue mucus

When it’s time to find a mate, the males stop walking and start swimming, and employ their sexiest feature — the ability to secrete glowing snot.

“They swim up into the water, out of the sand, hundreds at a time, and they will start to secrete this blue ball of mucus into the water,” Hensley said.

“It’s almost like they’re sneezing or vomiting it up. And they can actually leave that ball in the water there and it will glow for a long time, and then they’ll swim along and they’ll do it again.”

The ladies seem to dig it, though the specific contours of their desire remain a mystery.

“The females will orient themselves towards light, but we don’t know about what makes one display better-looking than another, or why some males might be preferred over others,” Hensley said.  

“We have no idea what the females are looking for.”

A new — and unique — species

Other kinds of ostracods do this too, Hensley said. But the species in this study — called entraining grassbed downers — is special.

“With this new species that we’ve documented for the first time, we have hundreds of thousands of males who actually synchronize their display,” Hensley said.

“They all swim into water at the same time, and then they start creating their patterns. And as soon as one creates the pattern, other males will start to create this pattern. So you get this huge wave of blue light kind of reaching out from the starting point.”

They always do this, he said, during “nautical twilight” — the brief period after the sun sets, but before the moon rises. 

That’s because they need total darkness. The researchers have observed the ritual both in the wild and in a lab, and any time they introduce even the smallest source of light, the sea fireflies refuse to perform. 

“That guarantees that they’re going to be the brightest thing in the oceans for females to see,” Hensley said.

Scientists James Morin and Todd Oakley —  the latter of whom is a co-author of the study —  first observed entraining grassbed downers when they happened upon one of their light shows while snorkeling off the coast of Panama in 2017.

“We were both kind of swearing into our snorkels in amazement,” Oakley, a professor at the University of California, Santa Barbara, told the New York Times.

Morin, professor emeritus of ecology and evolutionary biology, called it a “remarkable experience.”

“They really jump out at you. I’ve worked with ostracods for years and this species is spectacular,” he said in a Cornell University press release.

The ocean isn’t as dark as you might think 

Canadian zoologist Mark Edward Siddall, who wasn’t involved in the research, called it a “terrific study” and “one that is profoundly data rich and exquisitely analyzed.”

Siddall has previously studied bioluminescent Bermuda fireworms in the Caribbean, which also light up to attract mates.

“This is a horse of a very different colour,” he told CBC in an email. “What they have demonstrated is a dynamic bioluminescent synchrony in these crustaceans operating like [a] football stadium wave or a murmuration of starlings, but for the purpose of sex.”

It’s also been theorized that ostracods use their glowing goo to trick predators, he said. If they leave behind little ostracod-shaped blobs, whatever’s hunting them might gobble up their leavings instead. 

“It’s … startling to see bioluminescence get co-opted by sexual selection in a manner that probably increases the chance of the very predation that was its original raison d’être,” Siddall said. 

Maxime Geoffroy, a fisheries researcher at Memorial University’s Marine Institute in St. John’s, says bioluminescence in the ocean is common — even more so than scientists previously believed. 

But we’re still learning about why it happens, he says, and how different species make use of it. 

“[This study] brings a bit more knowledge about what the role of bioluminescence is — at least for that species,” Geoffroy, who also wasn’t involved in the research, told CBC.

“It opens a can of worms as to how we can apply these observations to other species.”