Godeheu de Riville was sailing across the Indian Ocean in the 18th century when he saw a strange sight. He said in his journal that the sea was covered with small stars and every wave dispersed a most vivid light. The ostracods were found when de Riville examined the water with his microscope.
James Morin was scuba diving in the Virgin Islands in 1980 when he saw bright blue dots. He shone his flashlight through the water and saw a lot of people. He found out that the flashes weren't random. The ostracods lit up in different ways in space and time, similar to the flashes of firefly that light up summer meadow. The realization made a big difference in the course of his career.
The most spectacular natural wonder that most people will never see is what a small group of colleagues have been unraveling for the past 4 decades. On moonless nights in the Caribbean, male ostracods only display for about an hour. Most recreational divers don't dive at night, and those who do tend to use lights, which causes the creatures to switch off for the evening
ostracods are abundant in fresh and saltwater. They are cute but also weird, like a cross between a crab and a tiny spaceship.
Seagoing ostracods are the only ones that glow in the dark. They spit out mucus. ostracods do this for defense in most of the oceans. There are bright blue dots in the Caribbean and they can double as sex calls. Thousands of dives later, they think those signals have led Caribbean ostracods to become more than 100 different species.
They use modern genetic tools to investigate factors that wedge species apart, including sexual selection, driven by female preferences, geographic isolation, and genetic drift. In just the past 2 years, researchers have figured out how to grow ostracods in the lab, a development that will allow them to study evolution in a way that has never been done before.
The ability to ask interesting questions is a powerful tool. He says that ostracods are an "elegant system" for doing that. Many species of ostracods overlap in small areas due to their simple mechanics and biochemistry. They may be able to give clues about the forces that create biological diversity.
ostracods are popular as curiosities in Japan. The first half of the 20th century saw the emergence of umi-hotaru, which caught the attention of E.Newton Harvey. The basic chemistry of bioluminescence has evolved 100 times. Organisms frombacteria to fish to insects use it to communicate with each other. It is a means of attraction for many.
The light show is performed underwater. When the water is dark enough, male ostracods take off from the reef where they spend most of their time. Male and female swimmers swim towards the flashes.
Sea firefly researchers in scuba gear position themselves on the sea floor just after dark, using red lights to find their way. It is an unnerving experience. There are snapping shrimp and parrot fish on coral at night. The waters are calm.
Biologists used a night vision monocular attached to a VHS camera to take pictures. The images were blurry. One of the first sea firefly graduate students is now an evolutionary ecologist at the University of Wisconsin, Madison. Even though the video equipment has improved, researchers still use mechanical pencils to write on pieces of pipe. They keep their place by using a rubber band that rolls down a finger. They take the creatures with a net after swimming to the flashes.
They sort through their catch and look at their ostracods under a microscope in the lab. Sometimes a subtle difference in the shape and size of the reproductive organs is all that distinguishes one species from another. More than 20 different species have been named so far.
The behavior of courting ostracods is complex, and it was discovered by the early 2000s. They last from one second to another. The ostracods move in a variety of ways, from up, down, or on a slant, creating strings of flashes that range in length from less than one meter up to 30 meters. There were new species and new behaviors everywhere. Nicholai Hensley is an evolutionary biologist at Cornell.
Gerrish recruited nearly all the other sea firefly researchers to study the sea firefly in a project that spanned five sites in the Caribbean between 2015 and 2019. They developed an underwater camera system that records both visible and IR light at the same time. The researchers were able to see the blue flashes. The only way to see the ostracods without being blinded by the flashes is to use IR. It made us capable of documenting the displays in the field.
In the presence of oxygen, the enzyme luciferase excites a three-amino-acid molecule called luciferin to release a photon of light.
Separate storage compartments and dedicated rows of secretory nozzles isolate the light-generating luciferin from the enzyme luciferase until a flash is needed.
When the light organ squirts out luciferin and luciferase inside a ring of mucus, the two molecules react and make the mucus light up blue.
The group began to tease apart relationships. To sequence the "transcriptome," or set of expressed genes, for Caribbean ostracods, a team of evolutionary biologists at UC Santa Barbara used samples from the Pacific and Indian oceans and compared them with the transcriptome of other ostracods. They used the number of genetic differences between species as a "molecular clock" to determine when each species originated.
The ability to generate light evolved about 200 million years ago, according to a preprint posted on 13 April. This defense mechanism was co-opted by evolution once about 150 million years ago. After the Isthmus of Panama separated Caribbean ostracods from their Pacific kin 3 million years ago, it was assumed that this behavior was a new innovation. It is unclear why ostracods did not spread to the Pacific before the barrier was formed. In murky Pacific waters, visual signals don't work.
One of the biggest questions in evolutionary biology is what drives the creation of new species. One of the candidates is female choosiness. Sex selection can drive the evolution of brighter colors or bigger horns in males, but it has not been proven that it can split a species into two. Some of the first proof may be provided by his studies of bioluminescence. They compared the rates at which new species form in ostracods that use their flashes as courtship displays with the rates at which they don't. Sexual selection should lead to the emergence of new species more often. They do in Caribbean ostracods, as well as in insects, fish, and octopuses with bio-luminescent displays.
Sexual selection can drive diversity. She and others had observed that when multiple ostracod species live close together, individual displays become more distinct than when the same species are found on their own.
Gerrish and her graduate student Nick Reda found evidence that one species might be pushing another to split into two. Most males of the species Photeros annecohenae flash as they flash, painting their string of glowing dots in a consistent direction. This tendency seems to be increasing, suggesting that enough females prefer this behavior to cause it to grow. In one bed where Gerrish has been working, 50% of the males now exhibit this odd behavior, and in an adjacent bed that percentage has grown to 70%, she and Reda plan to report in a paper later this year. This population is more likely to be isolated as this display becomes more common.
There are hints that other evolutionary forces are at play. She, Reda, and colleagues plan to report later this year on the genetics of multiple populations of P. annecohenae. Some of the differences are thought to be caused by random genetic drift. It appears that geographic isolation contributes. When storms create short breaks in the reef, isolating populations on either side, genetic differences are more than expected.
ostracod researchers are looking into how evolution shaped the features of the animals. The ostracod genes for luciferase are the ones that add oxygen to a molecule that makes light. Every new species we look at has a new genes. The activity of the ostracod luciferase can affect the brightness, duration, and other features of each flash. The findings show how novelty can lead to behavioral changes that can lead to new species.
ostracods are notoriously difficult to culture and such studies would be easier in the lab. Researchers don't know what most species eat because the animals are picky about temperature and light. Their slow life cycle means a long wait to see if the animals will breed in a specific area.
The Vargula tsujii is a California species that is close to the Caribbean ostracods but does not use its glow for sex. The team tinkered with the aquaria, the water flow, and the number of ostracods per tank for more than 2 years. The researchers were able to get a reproducing population in 2020. They want to do the same with Caribbean ostracods. Gerrish says we are poised to run in a bunch of different directions to learn more about these creatures.
Evidence for a key idea about the evolution of novelty has been found using lab-grown ostracods. At a meeting this winter, his graduate student Lisa Mesrop reported that ostracods rely on genes similar to those that are active in venom in centipedes and wasp. ostracod bioluminescence was a novel application for an existing network of genes when it first appeared.
The same mechanism for regulating light-generating luciferin has been found by another student in the lab. She has found that both sets of organisms add sulfur to its chemical structure. Lau suggests that the same mechanism is at work in other bioluminescent organisms.
Researchers hope to sequence the entire genome when ostracods reproduce in the lab. The ostracod genome is more complex than ours and it is difficult to sequence from a single specimen. Genetic engineering is not yet possible in most multicellular bioluminescent organisms. He wants to know why ostracods have so many genes. He hopes to learn more about light production and function by modifying these genes.
Researchers might be able to track down the enzymes that make the drug in the future. An opportunity to figure out what's really necessary to make bioluminescence could be provided by comparing those enzymes with their counterparts in other bioluminescent organisms. It was one of the many insights de Riville could never have imagined when he was a child.
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