They’re called umi-hotaru-“sea fireflies”-and in the first half of the 20th century, they caught the eye of Princeton University biochemist E. In Japan, dried ostracods are popular as curiosities because they glow when rehydrated. Compared with other animals with complex mating rituals-songbirds, say-they may more readily yield clues about the forces that generate biological diversity. The mechanics and biochemistry of their light flashes are relatively simple, and many species of ostracods overlap in small areas. Ostracods are an “elegant system” for doing so, he says. “The ability to ask interesting questions about evolutionary patterns across multiple species is a powerful tool,” says Christopher Cratsley, a behavioral ecologist at Fitchburg State University who works on fireflies. In just the past 2 years, researchers have figured out how to grow ostracods in the lab, a development that will allow them to dissect the molecular mechanisms of evolution in a way once possible only in more conventional lab animals such as nematodes and fruit flies. With modern genetic tools, they’ve been using these creatures to investigate the factors that wedge species apart, including sexual selection, driven by female preferences geographic isolation and genetic drift-the accumulation of random genetic changes. Today, thousands of dives later, they believe those signals have driven Caribbean ostracods to diversify into more than 100 species. But in the Caribbean, and only in the Caribbean, as Morin and colleagues discovered, those bright blue dots can double as mating calls. In most of the world’s oceans, ostracods do this for defense-to startle and distract would-be predators. Only seagoing ostracods are bioluminescent, and it’s not their bodies that glow. “They are very cute but also sort of bizarre-like a cross between a crab and a tiny spaceship,” says Timothy Fallon, an evolutionary biochemist at the University of California (UC), San Diego. No bigger than a grain of sand, ostracods abound in fresh and saltwater.
Most recreational divers don’t dive at night, and those who do tend to use lights, which prompt the creatures to switch off for the evening.Ī long exposure captures ostracods in motion on a Bonaire reef, driven in part by currents. Male ostracods only display for about an hour, shortly after sunset on moonless nights in warm Caribbean seas. Now a professor emeritus at Cornell University, Morin has spent the past 4 decades working with a small, dedicated group of colleagues to unravel the mysteries of what they describe as the most spectacular natural wonder that most people will never see. The realization changed the course of Morin’s career. The ostracods lit up in specific patterns in space and time, much like the courtship flashes of fireflies that light up summertime meadows. After multiple dives, he discerned that the flashes weren’t random. When he shone his flashlight through the water, he saw scores of ostracods flitting across its beam. When de Riville examined the sparkling water with his microscope, he discovered that the “small stars” were tiny crustaceans now known as ostracods.Ĭenturies later, in 1980, marine biologist James Morin was scuba diving just after sunset in the Virgin Islands when he noticed bright blue dots blinking on and off several meters away. The sea “was covered over with small stars every wave which broke about us dispersed a most vivid light, in complexion like that of a silver tissue electrified in the dark,” he recounted in his journal. In the 18th century, the French naturalist Godeheu de Riville was sailing across the Indian Ocean when he came upon a remarkable sight. A version of this story appeared in Science, Vol 377, Issue 6609.
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