[♪ INTRO] While we’re pretty clear on the general idea of evolution, believe it or not, we still have plenty to learn about its specific mechanisms.

In particular, there are still open questions around speciation, where one species diverges into two. And small, bioluminescent marine organisms called ostracods might hold the key to answering that question.

If you go out just after sunset on a moonless night in the Caribbean, you might encounter these tiny crustaceans. Each one is no bigger than a grain of sand, but they produce their own blue light, which they blink on and off like fireflies. They do it by vomiting up little globs of glowing blue mucus, which light up for a few seconds before fading to black.

While that sounds kind of gross, the light displays are key for male ostracods in attracting potential mates. This behavior is unique to the Caribbean ostracods. In other parts of the ocean, ostracods, both male and female, use their glowing blue vomit for defense, using it to distract potential predators.

Light displays aren’t the only thing Caribbean ostracods are doing differently. Because ostracods in this particular region are split into a staggering number of species. Researchers describe this based on the relative proportions of the ostracods’ body parts, and by the size and shape of their reproductive organs.

And, though not all have been formally described yet, researchers say they have found more than 100 species, all occupying roughly the same habitat. This is really strange, because normally, you wouldn’t expect to see so many similar species living in the same place. To understand why, we have to think about what a species is.

Historically, we’ve often defined species as groups of animals that both can and do reproduce with each other and produce fertile offspring. Species usually form when a population becomes somehow cut off from others of its kind and can no longer interbreed with them. One gene pool becomes two, and different changes start to accumulate in each.

But the Caribbean ostracods aren’t isolated from each other, at least not physically. They live in the same place and share the same resources. But they’re still made up of hundreds of populations that don’t interbreed.

So the question is, what’s stopping them? Scientists think that the mating displays might have something to do with it. These displays might actually be leading to the formation of new species.

They think this might be due to a phenomenon called sexual selection. This is a phenomenon that was first described by Charles Darwin himself, and nowadays is often talked about in species with courtship displays. Basically, it’s the idea that the traits that are passed onto future generations are dependent on what females of a species look for in eligible mates.

For example, if females of a species choose a mate based on the brightness of his feathers, males of that species will pass on genes that lead to brighter feathers over time. This is related to natural selection, the idea that traits that get passed on in species are the ones that make an organism more likely to reproduce in the first place. But while natural selection is a well established phenomenon, we still have some questions about sexual selection, especially whether it actually drives the creation of new species out there in the real world.

Ostracods are a particularly elegant way to test this phenomenon. Since they don’t live for very long, it’s possible to study multiple generations in the lab. And since their light displays are fairly simple as far as mating displays are concerned, it’s not hard for researchers to map them out and find the differences between them.

Of course, to do all this you’d have to be able to breed ostracods in the lab, something we’ve only learned how to do recently, and are still perfecting. But now that we can, we can look into what’s driving ostracods in the Caribbean to split off into new species what seems like every other week. To do that, researchers have been looking at the ostracods’ RNA.

Since cells use RNA to generate messages of the genes they’re actively using, the transcriptome, or sequence of a species’ RNA transcripts, is a shortcut to finding meaningful differences in DNA. They looked at those differences, plus the morphology data they already had, to figure out which groups were different enough from each other to count as a different species. They then compared records from Caribbean ostracods to ostracods in other parts of the world.

They even looked at the fossil record, to look at ostracods that are now extinct, and put together a sort of family tree to see how closely these different species are related to each other. They found that ostracod groups that use light for mating displays tend to form new species faster than groups that don’t. That lends some support to the idea that’s it’s all about sexual selection, because while all these species are bioluminescent, it’s the ostracods with mating displays that are diverging.

See, when a lot of ostracods live together, their mating displays are much more elaborate than when they live alone. That helps them stand out from the crowd, to better attract mates. But as these groups form more and more elaborate displays, they start isolating themselves into groups.

And the more isolated each group is, the more likely a new species is to form. The next step in this research would be to sequence the full ostracod genome. That means mapping out all of the ostracods genes, not just the ones each species is currently using.

This isn’t trivial, though. The ostracod genome is longer than our own. But by doing this, researchers hope to compare the genomes of different ostracod species, to figure out exactly how these species diverge.

That can help us figure out whether sexual selection really can make new species, answering a long-standing question about one of Charles Darwin’s evolutionary theories. These results could be applied not just to ostracods, but also to other forms of life. And if you want to keep learning about other bioluminescent forms of life, then you should watch the Bizarre Beasts video on New Zealand glowworms.

You might just get sucked into a new show created by some of the same people who make SciShow! You can learn about all kinds of Bizarre Beasts from frogs that can’t land their own jumps to lizards with legs in the front and a wormy behind. Join hosts Hank Green and Sarah Suta to explore what makes these animals so weird to us.

And if one, or all, of them particularly strikes your fancy, you can take it home through the Bizarre Beasts pin club! Profits go toward our community’s efforts to decrease maternal mortality in Sierra Leone. The links for the channel and the pin club are in the description below! [♪ OUTRO]