Glowing sharks light up underwater worlds

Our deep underwater environments may be shrouded in darkness, but they also play host to some 180 species of fish and sharks that can emit brilliant neon colours.

Now, part of that cryptic world of biofluorescence has been catalogued by scientists with the American Museum of Natural History.

The researchers wanted to document the variety of biofluorescent underwater organisms — creatures with proteins in their skin that can make them appear in striking oranges, greens, yellows and reds when the animals pass under blue light.

"We've long known about biofluorescence underwater in organisms like corals, jellyfish and even in land animals like butterflies and parrots, but fish biofluorescence has been reported in only a few research publications," said John Sparks, one of the lead authors of the report published in the journal PLoS ONE.

"This paper is the first to look at the wide distribution of biofluorescence across fishes, and it opens up a number of new research areas."

Although it may seem like underwater environments are black because water absorbs most of the visible light spectrum, fish actually live in a mostly blue world. Researchers say biofluorescent fish absorb that blue light and re-emit it with new colours.

Predominantly blue environments

Museum researchers enlisted scientists from Yale University, the University of Kansas and the University of Haifa in Israel, as well as professional video journalists, to scour tropical waters in the Bahamas and the Solomon Islands.

To our eyes, they blend right into their environment. But to a fish that has a yellow intraocular filter, they must stick out like a sore thumb.- John Sparks, American Museum of Natural History

As the biofluorescence is typically invisible to the human eye, the team had to dive into the waters at night and use recording equipment with special filters to block out blue light. At the same time, they used high-intensity blue light arrays to stimulate biofluorescence.

They found cartilaginous and bony fish species that had biofluorescent characteristics. In all, they were able to identify 180 biofluorescent species.

"The cryptically patterned gobies, flatfishes, eels, and scorpionfishes — these are animals that you'd never normally see during a dive," Sparks said. "To our eyes, they blend right into their environment. But to a fish that has a yellow intraocular filter, they must stick out like a sore thumb."

Scientists believe the biofluorescence could be a way for organisms to communicate with each other and camouflage themselves from predators.