Deep-sea sharks wield some surprisingly well-adapted eyes that help them see in the dark, according to new research.
Transparent patches of skin above their eyes and a unique arrangement of light-sensitive cells on their retinas, among other things, allow five species of bioluminescent deep-sea shark to collect and focus as much light as possible to hunt prey and find each other in the gloomy depths. (See pictures of other deep-sea creatures that glow in the dark.)A pygmy shark, one of five deep sea shark species with eyes specialized to see in the dark. Photograph by Dr. Jérôme Mallefet, FNRS/UCL
These discoveries are helping to overturn a long-held view that sharks rely heavily on smell to get around.
“Historically, sharks have been considered to have poor eyesight,” said Tom Lisney, a sensory ecologist at Queen’s University in Kingston, Ontario, who wasn’t involved in the research.
Fairly recently, though, studies have shown that that might not be the case.
Take the distribution of cells on deep-sea shark retinas—a light-sensitive layer at the back of the eye, Lisney noted.
The blackbelly lanternshark (Etmopterus lucifer), for instance, has a special pattern of glowing dots running down its flank. That pattern, only found in E. lucifer, allows these lanternsharks to find each other.
The cells sensitive to the light given off by those markings are concentrated in a streak across the shark’s retina, boosting the ability of blackbelly lanternsharks to detect that pattern.
“They have an eye that’s designed to see both ends of the marking at the same time,” said Julien Claes, a shark biologist at the Université Catholique de Louvain in Belgium. He also examined closely related lanternshark species that lacked E. lucifer‘s particular pattern across their retina.
“This supports the use of flank markings as a visual communication tool,” said Claes, lead author of the study, published August 6 in the journal PLOS One. (See “Pictures: Fish Light Up in Neon Colors.”)
One of the more interesting findings, said Christine Bedore, a marine biologist at Duke University in Durham, North Carolina, are the transparent patches above the eyes of all five shark species studied.
This could allow these sharks to expand their field of view in an upward direction, she speculated. That would help them find other individuals in their species swimming above them, said Bedore, who was not involved in the study.
The study authors also speculated that these clear patches—found only in these five species so far—could help the sharks gauge the type and amount of light filtering down into the gloom, allowing them to tune the light they produce to match.
This kind of camouflage, called counterillumination, prevents animals from standing out as darker silhouettes when viewed from below. (See “Glowing Pygmy Shark Lights Up to Fade Away.”)
Keeping an Eye on Deep-Sea Sharks
Claes was surprised no one had seen these clear patches before, since they’re obvious to the naked eye. One of the species he studied, the velvet belly lanternshark, is quite common in the Atlantic, and other researchers had studied its anatomy previously. “It’s quite strange that no one has mentioned [it] before.”
This just goes to show much work still needs to be done on these deep-sea animals, Claes added.
He plans to continue his work on deep-sea shark vision, looking further into whether these shark species are better at detecting movement than are non-bioluminescent deep-sea sharks.
Follow Jane J. Lee on Twitter.