Scientists including Dr. Vincent Pieribone have shown that catsharks – spotty, reticent fishes of deep ocean waters – have extraordinary vision that helps them distinguish their mates from other species. The research performed in collaboration with the City University of New York and the American Museum of Natural History, among others, has just been published in Nature’s Scientific Reports.
At depths of 300 to 500 meters in the ocean, where catsharks thrive, only blue light from the sun makes its way through. All other colors of the spectrum are dispersed away by the immense volume of the ocean. When blue light strikes their bodies, the molecules on the shark skin absorb it, and in return, emit light of lower energy, lending a green hue to their facade. This biophysical phenomenon is what we call bio-fluorescence.
The team of scientists, who are in fact expert divers, unveiled in 2014 for the first time that marine fishes inhabiting the ocean waters of the Caribbean and the Western Pacific exhibit fluorescence. The finding came in as a surprise since, until then, fluorescence had only been observed in the aquatic inmates of shallower reefs. The recent study is a follow-up of the 2014 report. Here, the scientists probe deeper into whether and how catsharks perceive the fluorescence on their peers.
To do this, they isolated the pigment in the shark’s eyes and examined its light-absorbing properties. Just as the human eye is equipped with pigments that can detect the red, green and blue colors of the spectrum, the shark eye is also fitted but with pigments that can only perceive blue and green. For the sharks, this comes in handy because their skin is composed of complementary pigments that emit the exact same colors when blue light from the sun bounces off of their bodies. This means that, in the secret world of the deep, the sharks cloak themselves in an outfit only perceivable to their own kin. Even the human eye is insensitive to the fluorescence. Divers need blue illumination from an external light source and yellow barrier filters to be able to see the shark’s greenish hue.
What’s more, the team also established that catshark fluorescence elevates the contrast at the patches on their skin so much that they “stand out like a sore thumb” among themselves. The dark patches are areas of intense blue fluorescence while the paler tone on the rest of the body glows green in front of their eyes.
The other authors in the study are David Gruber, Ellis Loew, Dimitri Deheyn, Derya Akkaynak, Jean Gaffney, Leo Smith, Matthew DAVIS, Jennifer Stern and John Sparks.
Contact information: Dr. Vincent Pieribone, The John B. Pierce Laboratory, New Haven, CT. E-mail: vpieribo@jbpierce.org.
