In the murk of the ocean depths, catsharks produce a green biofluorescence. Though invisible to the human eye, scientists have captured and studied this phenomena during night dives at Scripps Canyon, located in California’s San Diego County. They found that catsharks can increase the contrast of the lights they emit, leading researchers to theorize the biofluorescence may be used for intraspecies communication.
The research was published in Scientific Reports on Monday by a research team that included scientists from the American Museum of Natural History and the University of California, San Diego’s Scripps Institution of Oceanography, among other institutions.
“Light from the sun is quickly absorbed in the ocean, resulting in a stable narrowband blue spectral setting,” the researchers wrote in their study. “This is an ideal environment for marine organisms to evolve biofluorescent compounds that absorb abundant blue, high-energy, short wavelength photons, which they emit back at longer, lesser energy wavelengths.”
Usually, these reemitted wavelengths are flavored neon, a variety of oranges, greens, and reds.
In order to recreate the catshark’s vision, the scientists used a technique known as microspectrophotometry, which helped them determine how the shark’s eyes absorb light. They used the technique on two catshark species: the chain catshark (Scyliorhinus rotifer) and the swellshark (Cephaloscyllium ventriosum).
According to The Atlantic, the catsharks’ vision only utilizes one pigment molecule, which detects blue-green light. Comparatively, humans use three types of pigment molecules.
Based on that, the research team fabricated a camera that simulates the effect, and used it on night dives at depths around 30 meters.
“During night dives, the team stimulated biofluorescence in the sharks with high-intensity blue light arrays housed in watertight cases,” according to Scripps Institution of Oceanography. “The researchers used custom-built cameras with yellow filters, which block out blue light, as well as the newly developed ‘shark-eye’ camera to get a better idea of how the shark sees the underwater display.”
According to National Geographic, not only did the two species emit different biofluorescent patterns, the sexes within the species emitted different patterns as well.
The contrast in the biofluorescent patterns increased with depth, according to the researchers.
However, biologist Nathan Hart, of Australia’s Macquarie University, told The Atlantic he’s unsure whether blue light would be bright enough in the deep ocean to have a notable effect on the catsharks’ biofluorescence. Study lead author David Gruber told the media outlet the study is an initial attempt to figure out how these sharks see their world.
“Some shark’s eyes are 100 times better than ours in low-light conditions,” Gruber said in a statement. “They swim many meters below the surface in areas that are incredibly difficult for a human to see anything. But that’s where they’ve been living for 400 million years, so their eyes have adapted well to that dim, pure-blue environment. Our work enhances the light to bring it to a human perspective.”