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.

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