Scientists uncover secret behind beautiful glowing Bermuda fireworms

A new study published in the journal PLOS ONE confirms that Bermuda fireworms, unique bioluminescent sea creatures first documented by Christopher Columbus, get their glow on in a way that's entirely unlike the more well known firefly or jelly fish.

The Bermuda fireworm, known formally as Odontosyllis enopla, is found in Bermuda and down through the Caribbean. It was first documented by Columbus and his crew in 1492 as "looking like the flame of a small candle alternately raised and lowered" when their ships were about to make landing in the Caribbean islands. 

But it wasn't until the 1930s when scientists connected the approximately four-centimetre-long fireworms and their unique mating ritual to that early record.

Now a team of researchers from the American Museum of Natural History in New York has identified the enzyme behind the worms' glowing mechanism.

Precisely timed mating ritual

That glow is key to making the mating magic happen during summer and autumn months when female fireworms come up from the bottom of the sea to spawn at precisely 55 minutes after sunset on the third night after the full moon. 

The female worms swim in tight circles, secreting a bright blue-green luminescence that attracts the males, who zoom comet-like in the females' direction, also giving off that same glow, said Canadian zoologist Mark Siddall, a curator at the American Museum of Nature History and one of the study authors.

There's "a little explosion of light" as the worms release their eggs and sperm into the water, he said.

The glow improves the worms' odds of reproductive success by ensuring males and females release their reproductive cells in the same place.

Earlier research from the 1970s had attempted to isolate what makes the worms glow, but that work had incorrectly attributed it to a kind of photo protein, said Siddall.

Photo proteins are what make some jelly fish glow, for example.

But the Bermuda fireworm gets its glowing reputation from a type of enzyme called a luciferase — which are the principal drivers of bioluminescence across the natural world.  

It's still one of the most beautiful biological things I've ever seen.- Mark Siddall, American Museum of Natural History

These enzymes require a high energy surface known as a substrate, as well as the presence of oxygen, in order to release light.

"The difference is photo protein glows all the time whereas luciferases turns off and on, more or less," said Siddall.

The luciferase that powers the fireworms is different to the one that lights up fireflies and, in fact, is unique among the enzymes of its kind discovered so far.

Siddall first came across the worms in 1995 while teaching at the Bermuda Institute of Ocean Sciences. 

A colleague took him and a group of students out to view the worms at the right stage in a lunar cycle that summer. "It really impressed me when I saw it and it's still one of the most beautiful biological things I've ever seen."

But it would be another 20 years before Siddall would collect samples of the worms for this study. He and his wife travelled to Bermuda over Halloween in 2015. Then, three days after the full moon at 55 minute past sunset, "I got in the water up to my chest and I waited and the worms came out. I plucked them right out of the water and preserved them in something that would retain their RNA for later."

RNA stands for Ribonucleic acid, and along with DNA and proteins, it's one of three major biological macromolecules essential for all known forms of life, according to the website for the RNA Society in Bethesda, Md.

All the better to see you with, my dear

The information gleaned from those few fireworms allowed Siddall and his colleagues not only to identify the genes being expressed while the worms were glowing, but also to investigate some other mysteries about the tiny animals.

These include the worms' four eyes growing larger during spawning, likely to further improve their odds of finding each other in the dark, said Siddall.

Also during mating cycles, an organ similar to the kidney gets used to store and release gametes — eggs in the females and sperm in the males.

This way they're not devoting body space to making gametes all the time, he says.

Further research on the glowing worms could explore how the animals have evolved with a seemingly risky trait of making themselves more visible to predators.

"Generally speaking, advertising your presence is not what you want to do," said Siddall.