Watch the mucus-filled, synchronized mating dance of bioluminescent ‘sea fireflies’

When bioluminescent ostracods or ‘sea fireflies’ mate, they perform a courtship dance complete with glowing blue mucus. The males sway together in sync while basking in the light of the shiny slime. This mating ritual is detailed for the first time in a study published November 29 in the journal Proceedings of the Royal Society B.

[Related: Surprise! These sea cucumbers glow.]

Ostracods are tiny crustaceans that are about the size of a sesame seed. They are found in a variety of fresh and saltwater environments, from deep ocean depths to shallow seas to rivers, lakes, and estuaries. The dancing, shrimplike species in the study is called the entraining grassbed downer and was observed in the Caribbean Sea near Panama. 

During this dance routine, male EGDs create their distinct patterns of bioluminescence to attract females. They secrete packets of protein from a specialized gland. The females respond by angling themselves to these bright blue luminous displays and swimming towards the males. According to study co-author Nicholai Hensley, a Cornell University evolutionary biologist who specializes in animal behavior, the other males will then join in a synchronous light display and repeat the same pattern in the water during each dance.

The study found that this very coordinated swim also doesn’t happen randomly. The mating dance sequence only occurs after sunset at nautical twilight, when the moon isn’t bright in the night sky. The team found their level of precision and coordination very surprising. 

“This precise timing leads to the unexpected phenomena of huge waves of light that cascade across the grass bed, with hundreds of males displaying at the same time,” Hensley tells PopSci. “Amazingly, this is very similar in appearance to the fireflies most people are familiar with, where some species are also synchronized. But ostracods and fireflies last shared a common ancestor 500 million years ago, when most animal life was evolving beyond looking slightly more than worm-like.”

Ostracods are special because they evolved their bioluminescence and bioluminescent behaviors completely independently from other animals that act like them. “They are also spectacular little animals, whose whole world escapes notice by 98 percent of people unless you know where to look,” says Hensley.

Luckily, some of Hensley’s collaborators knew where to look and had some luck on their side. In 2017, James Morin, a professor emeritus of ecology and evolutionary biology at Cornell and Todd Oakley, a professor at the University of California, Santa Barbara were diving near the Smithsonian Tropical Research Institute’s Bocas del Toro island research station in Panama. When Morin turned on his dive light to test it out, hundreds of ostracods responded with their own light. According to Morin, there are more than 100 species of signaling ostracods in the Caribbean alone.

[Related: These newly discovered bioluminescent sea worms are named after Japanese folklore.]

“What’s really remarkable about EGD is the duration, the brightness and the density,” Morin said in a statement. “It was a remarkable experience. They really jump out at you. I’ve worked with ostracods for years and this species is spectacular.”

With this discovery, Hensley and study-co-author Trevor Rivers from the University of Kansas set up some preliminary experiments to determine just what the animals were responding to when a light flashed on them. They found that the EDGs are very sensitive to both the time and intensity of a light. 

“It’s how they coordinate their own signals with one another,” says Hensley.

The courtship ritual with snotty light likely evolved about 20 million years ago. However, why the males perform these gyrations is still a mystery. The team only knows that these displays are for attraction purposes, and are still figuring out the other functions. It’s possible that the males are competing with one another for attention, which leads to what Hensley calls a “giant free-for-all.” They also may be cooperating to make a brighter display that could attract more females. The team plans on testing how these displays look to females and measuring their behaviors to better understand this mating dance.

“There’s a whole world filled with new questions and unexplored ideas out there if you pay attention to the little details around you,” says Hensley. “Get out there, pay attention, and take chances, make sure to seize the moments of the rare opportunities that come your way. You can’t predict where it will lead, but you can be sure you learn something along the way.”