In the wake of a fluorescent sea turtle recently captured on video come two marine eels that glow neon green.
The first bright green fluorescent fish ever recorded in the wild, their discovery dates back to 2011, when a tiny eel photobombed David Gruber off Little Cayman Island in the Caribbean.
For Gruber, who was photographing biofluorescent corals at the time, the finger-length eel couldn’t have been more illuminating. (See "Rainbow of Fluorescent Corals Found—Why Do They Glow?")
“This big lightbulb went off in my head,” says Gruber, a National Geographic Explorer and marine biologist at City University of New York. “I hadn’t expected a fish to be glowing as brightly as the coral.”
That discovery sparked a new investigation into biofluorescence in marine animals. So far Gruber's work has helped identify more than 180 species of biofluorescent fish, uncovering the stunning extent to which marine vertebrates manipulate light to create color in their uniformly blue environment.
Whereas bioluminescent animals produce their own light through chemical reactions or by hosting light-emitting bacteria, biofluorescence is created by absorbing natural light and re-emitting it as a different color.
Now, Gruber, working with other scientists, has got to the bottom of that first glowing eel he saw four years ago—though it wasn’t easy.
“These eels are so reclusive and so difficult to find,” he says. "That image that was captured accidentally was the first time that these eels have ever been photographed in the wild.”
Published November 11 in the journal PLOS ONE, Gruber and colleagues' new study identifies two species that glow: The plain false moray (Kaupichthys hyoproroides) and another false moray eel that’s completely new to science and is yet unnamed. It's hard to know which species was in the 2011 photograph.
Mating Under the Full Moon
During a later expedition, Gruber and colleagues caught two specimens of the eels—one of each species—at Lee Stocking Island in the Bahamas.
Molecular analysis of the eels’ muscle tissue showed that the secret to their green hue is a previously unknown family of fluorescent proteins, says Gruber, whose work was supported by the National Geographic Society/Waitt Grants Program.
At some point in the eels' evolutionary history, these newfound proteins migrated from the animals' brains to their muscles, where they started to fluoresce. (Also see "Like The 'Glowing' Sea Turtle, These Animals Also Light Up.")
At that point, the eels' bodies “intensely selected” these proteins, meaning the fluorescence was being harnessed for a specific function, Gruber says.
What that role is isn’t yet known, but he suspects the reclusive eels use their brightness to link up when it’s time to reproduce.
“They have to come out and mate at some point,” he says. “I hypothesize that they come out to mate on full moon nights,” when the blue light of the moon activates their green glow.
Michael Miller, of Nihon University’s Laboratory of Eel Science in Kanagawa, Japan, agrees that biofluorescence may be a way for “the eels to see each other at night and form spawning aggregations.”
“And if most other predators can’t see that kind of light, then it would be a great mechanism to enhance spawning success and avoid predation,” Miller, who wasn’t involved in the new study, says via email.
Miller is also impressed that the study team managed to find any Kaupichthys eels at all. “They are so rarely seen or collected,” he says. It may even be that the glowing eels have evolved to "hide primarily in green-fluorescing corals,” he suggests.
Little Eel, Big Impact
Gruber describes the ocean as “this massive blue filter” that blocks other colors of the light spectrum. The glowing eels provide a model for how fish and other visual creatures use biofluorescence “to create other colors in their blue world.” (See pictures of other animals that glow.)
Matthew Davis, a professor of biology at St. Cloud State University in Minnesota, says the light shows produced “are as amazing and beautiful as the peacock’s tail or the poison dart frog’s skin.”
But until the elusive Kaupichthys eels were spotted, no one had been looking for biofluorescence in marine vertebrates.
Subsequent discoveries have “transformed our conceptions of the likely importance of fluorescence to the evolution of marine vertebrates,” according to Davis.
And all thanks to that accidental little eel, wherever it's hiding.