Roughly half of all species of ostracods – bean-shaped crustaceans about the size of a sesame seed – can eject clouds of dazzling blue mucus to startle would-be predators. But the males in one group of these “sea fireflies” in the Caribbean use the mucus to create glowing patterns in an elaborate dance to attract mates.
To unravel how this talent evolved from a tool of war to a key part of mating rituals, at the University of California, Santa Barbara, and his colleagues investigated the ostracods’ genetics.
They analysed RNA from 45 species of bioluminescent ostracods from around the world, then built an evolutionary tree from the differences between the genes encoding the RNA. This showed how the species were interrelated, and with the help of fossils, it was possible to estimate how long ago the lineages diverged.
Advertisement
The researchers found that sea fireflies got their glow roughly 267 million years ago, well before the first dinosaurs.
Bioluminescence has arisen in various species in nearly 100 different ways that we know of, including in comb jellies about 350 million years ago, but its emergence in ostracods “is one of the oldest origins of bioluminescence that’s ever been quantified”, says Oakley.
The study confirms that ostracods co-opted their defensive glow charges for reproductive ends an estimated 213 million years ago when they split off from their defensively luminous relatives.
“We were surprised to find that [the transition] was quite a bit older than we expected,” says Oakley. The Caribbean Sea didn’t fully form until a few million years ago, suggesting that the sea fireflies that use bioluminescence to attract others instead of warding them off possibly got their start somewhere else.
“Many fishes are bioluminescent and evolved around the Cretaceous Period. It is amazing that ostracods had already evolved the courtship display by that time,” says at Nagoya University in Japan, noting previous evidence that some bioluminescent fish may steal light-generating powers directly from ostracod prey in their diet by absorbing the molecular ingredients that create the glowing chemical reaction.
at St. Francis College in New York City says it is fascinating to contemplate past oceans “lighting up this huge amount of time ago in a world that’s hard to imagine today”.
Reference: bioRxiv, DOI:
Sign up for Wild Wild Life, a free monthly newsletter celebrating the diversity and science of animals, plants and Earth’s other weird and wonderful inhabitants
