Scientists discover why two shark species emit green glow
The secret behind the eerie glow of two shark species has been revealed in a study which sheds light on the origin and possible advantages of their fluorescent green bodies.
Chain catsharks and swell sharks are deep-dwelling and live in the western Atlantic and eastern Pacific respectively, where they hide among rocks and rubble. While at first glance they appear to be in various shades of brown, recent studies have shown that under blue light they glow green. Crucially, only blue light penetrates the depths of the ocean.
Now scientists have pinpointed the chemicals behind the phenomenon, which not only cause the green glow, but possibly help defend the sharks against harmful microbes.
“We have so much more to learn about life in the ocean and sharks,” said Prof David Gruber, of City University of New York, a co-author of the study.
The green glow emitted by the sharks is a type of biofluorescence, which arises when blue light in the oceans is absorbed and then re-emitted, in this case as green light.
A number of difference chemical substances can produce such effects. The best known is the green fluorescent protein (GFP). Discovered in the jellyfish Aequorea victoria, the glowing green molecule has become a workhorse in biological research: scientists often use the gene for GFP as a tag to explore how cells work and follow changes. Work on GFP won a trio of scientists the Nobel prize for chemistry in 2008.
Gruber and colleagues say a group of very different molecules is behind the green glow of the sharks, many of which have not been found before in natural settings.
Writing in the journal iScience, the team said the green glow of the sharks was limited to certain parts of their bodies – typically where the skin was paler or where there were white spots.
The team took skin tissue samples and analysed them, finding between the two species a range of eight fluorescent molecules that glowed when blue light was shone on them. Among those predominantly found in the paler areas of the skin was one in particular, present in both species, that gave off a bright green glow.
The team’s previous research has suggested the visual system of these sharks seems tuned in to such hues. “They just have one visual pigment in their eye,” said Prof Gruber. “It sees right at the blue-green interface.”
Gruber said the research suggested the glow might aid communication between the sharks, noting that males and females showed different light patterns. “If these sharks are absorbing blue and then creating green [light], they are creating contrasts for each other, because there is no green [light] down there except the green that they are producing on their skin,” he added.
The latest study suggests there might be additional benefits. The team tested the fluorescent substances for their action against certain bacteria: one found in marine environments and the other MRSA – the hospital superbug. Growth of both bacteria were found to be hampered by some of the substances within the sharks’ skin – a finding the team says might indicate that molecules involved in biofluorescence could also be part of microbial defence.
“These sharks lay on the bottom and the bottom has a high density of bacteria,” said Gruber, adding that substances in the skin might protect the animals. “We are just hypothesising it could be a way to stop things from growing on the shark.”