In the dense darkness of midnight in the Caribbean, the displays look like strings of tiny, glowing pearls. Some light up in zigzag patterns, others appear in a horizontal trail or streak upward toward the surface. The luminous beads linger for a few seconds, then fade to black, like bashful underwater ghosts.
But Emily Ellis, an evolutionary biologist who studies these displays, knows what they really are: blobs of bioluminescent mucus vomited up by crustaceans called ostracods. Males use the vomit to impress potential mates.
"It sounds nasty," she acknowledged, laughing. "But I guess the females really like it."
And the outcome of this display has ramifications far beyond the ostrocods involved. In a study published this week in the journal Current Biology, Ellis and fellow evolutionary biologist Todd Oakley report that bioluminescent courting rituals appear to promote diversity, increasing the number of species within groups that perform them.
"This is some of the clearest evidence to date of a sexually selected trait" — a trait that's dictated by whom creatures want to mate with, as opposed to environmental pressures like predation or climate — "leading to changes in species diversity number," Oakley said.
Ellis and Oakley, both of the University of California at Santa Barbara, have spent years in the Caribbean studying bioluminescent ostracods. The creatures come in dozens of species, each with its own particular light-up display pattern: up, down, zigzag, swirls.
"When you're there watching this display it's spectacular," Ellis said. "You can have up to nine species all in the same area displaying at similar times. I don't know how the females do it, but they're really good at figuring out who is their correct male."
Ellis and Oakley had a hunch that the courtship displays might be driving this diversity. Sexual selection, like many other evolutionary pressures, often favors creatures with the most particular (and peculiar) adaptations.
"It drives them to extremes," Ellis explained, leading to birds with brilliant plumage, frogs with bellowing croaks, deer with massive antlers. But she and Oakley think it can also act as a wedge, driving a once uniform population to develop into two distinct species.
The duo applied for a grant to study whether the rate of diversification really did increase after courtship arose but were told that they needed to broaden their study. Bioluminescent courtship evolved only once in the ostracods, so whatever conclusion they reached wouldn't have much statistical significance.
"So we thought, 'Let's see if this is a pattern in other bioluminescent animals,'" Ellis said — and it was.
In the end, she and Oakley calculated the rates of species accumulation (the rate at which new species arise within a particular branch of the evolutionary tree, minus the rate at which they go extinct) for 10 bioluminescent lineages, including tiny beetles, glowing sharks, terrifying-looking dragonfish, familiar fireflies. When they compared their finding with the species accumulation for comparable sister lineages, they found that the number for the courting group was much higher. This was also true when they compared the accumulation rate for the courting group with the rate for its overall branch on the evolutionary tree. Often, the accumulation rates for the courting groups were an order of magnitude greater than those of their counterparts.
To ensure it was sexual selection and not just bioluminescence that was driving this diversity, they compared species accumulation rates for courting creatures with those of groups that had bio-luminescence but used it for camouflage or hunting. The ones who used their light-up capabilities for mating still came out on top.
"We knew for quite a while that within a species, sexual selection will lead to diversification, but it's been debated in the literature whether this extends outside a species," Oakley said.
It's difficult to study the phenomenon across various phylogenetic groups because many sexually selected traits are species specific, and because it's sometimes difficult to isolate sexual selection from other evolutionary forces.
But with bioluminescence, he and Ellis could study a phenomenon that evolved multiple times in diverse groups, and that allowed them to draw general conclusions like this one: Courtship rituals — even ones that involve vomit — can send speciation into hyperdrive, leading to some of the wild and weird creatures we see today.
Guys, take note.