- Many deep-sea creatures that emit light to help find prey or avoid predators do so using small organs called photophores.
- A recent study of deep-sea shrimp shows that photophores can also detect light, acting like rudimentary eyes all over the body.
- The finding adds to a growing body of research documenting photosensitive organs outside the eyes in a variety of animals, and is the first demonstration in deep-sea creatures.
Surrounded by darkness, many deep-sea creatures emit light to help find prey or avoid predators. Scientists have long known that small organs called photophores are responsible for this bioluminescence. Now they’ve learned that photophores can also detect light, acting like rudimentary eyes all over the body.
A team of marine biologists made the discovery by studying deep-sea shrimp in the Florida Straits. They published their findings in the journal Scientific Reports earlier this month. Their work adds to a growing body of research documenting photosensitive organs outside the eyes in a variety of animals, and is the first demonstration in deep-sea creatures.
“When we think about sensing light we think of the eyes,” Heather Bracken-Grissom, a marine biologist at Florida International University who led the study, told Mongabay. “These shrimp have the ability to see beyond their eyes, in a sense.”
Janicella spinicauda shrimp take part in one of the planet’s largest daily migrations — one that is not so much geographical as vertical. Every night, they rise from depths as great as 1,500 meters (almost 5,000 feet) to shallow waters to feed and mate. On the move, they camouflage themselves by glowing so that predators below can’t see their silhouettes against light from above. But it’s been unclear how they and other deep-sea animals that perform this trick can adjust the intensity of their glow to match that of the light emanating from above.
Bracken-Grissom’s discovery provides a clue: they have more than their eyes to “see” with. The shrimp have dozens of photophores at various places on their bodies, especially along the undersides and sides. Her team found that the photophores contain some of the same genes as the shrimps’ eyes, including opsins that help with light detection and phototransduction genes that help convert light into electrical signals.
“I was surprised to find identical genes to the ones in the eyes,” Bracken-Grissom said. “It was exactly what we were going for, but I was surprised to find the diversity of visual opsins,” which indicates an ability to sense light at different wavelengths.
The researchers complemented this genomic research with a shipboard experiment. In a dark room, they exposed live shrimp to bright light. Under the microscope, the impact on photophores was clear: cell structures moved to protect tissues from the light. This further established the sensitive nature of the photophores — indeed, the eye structures of lobsters, crabs, and other crustaceans responded similarly to bright lights in other studies.
“That’s a nice hint that these cells are photoreceptive,” Victor Benno Meyer-Rochow, a visiting science professor at Andong National University in South Korea and the author of the cited lobster study, told Mongabay. “They are getting damaged by the light.” He said that proving the photophores are light sensitive and understanding the feedback loops with the brain would require electrophysical recordings of the cell activity of live shrimp, something the shrimp study did not do.
Bracken-Grissom’s team also complemented its genomic research using a staining method to see where the relevant genes were expressed. They found them in photophore cells but not in surrounding tissue.
The conservation status of J. spinicauda, like many marine invertebrates and deep-sea species, is not known due to lack of research. The species is common in the Gulf of Mexico, the Atlantic and the Pacific.
Beyond the eye
This study is the latest in a series of work that looks at light sensitivity in organs other than the eye. The most direct precursor to Bracken-Grissom’s work was a 2009 study on the Hawaiian bobtail squid (Euprymna scolopes) that found that its photophores could detect light. The squid differs from J. spinicauda in creating light by hosting bioluminescent bacteria. Todd Oakley, a biologist at the University of California, Santa Barbara who co-authored the bobtail squid study, later discovered that octopus skin has opsins and detects light.
“Extraocular” perception is not unique to marine animals, Oakley told Mongabay. A recent study of peppered moth (Biston betularia) caterpillars found that they change colors to mimic their surroundings even when blindfolded. And even humans have opsins in our skin that sense light and send signals to increase melanin production as part of the process we call tanning.
Scientists say that such perception, whether it comes from photophores or not, can’t be called “vision” or “sight” because it’s not image forming. That is, in sensing light, the photophores of a deep-sea shrimp, for example, carry out only part of the function of an eye.
Bracken-Grissom hopes that her work will lead to research on bioluminescent animals in other taxa. “The more we look for photosensitivity in structures outside the eyes, the more we will find it,” she said.
Banner image: Researchers studied Janicella spinicauda, a deep-sea shrimp species covered with bioluminescent organs called photophores, which appear as red dots. Image by Danté Fenolio.
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