Sunlight is only visible in the top one thousand meters of the ocean, and the pressures of living in the dim, blue deep has led to some bizarre adaptations. A tiny shrimp-like critter called Paraphronima gracilis lives in intermediate, or mesopelagic, habitats—depths that are totally dark to our eyes. And according to findings published in Current Biology this week, each of their giant eyes has 12 distinct retinas, the light-sensitive layer of the eyeball.
Creatures living in mesopelagic environments face a constant struggle: Be able to perceive dim light and bioluminescent sources, but also remain inconspicuous to predators. These struggles generate contrasting selection pressures on the eyes of these inhabitants. Hyperiid amphipods are a family of small crustaceans with at least 10 different eye configurations, ranging from deep-sea species with no eyes to mesopelagic ones with four enlarged eyes.
A team led by Jamie Baldwin Fergus of the Smithsonian Institution collected the hyperiid amphipod P. gracilis on night trawls off the R/V Western Flyer in the Monterey Submarine Canyon between depths of 200 and 500 meters. These transparent crustaceans have a pair of large compound eyes—about 45 percent of their whole body—and they swim with their eyes positioned up to hunt for prey swimming above.
Each of their compound eyes, the researchers found, have a large upward-looking portion and a smaller lateral-looking portion. The strangest part is that the upward-looking portions have a row of ommatidia (the facets that make up a compound eye) that ends in 12 discontinuous retinas. In the image above, the reddish orange coloration is from screening pigments located within and around the “retinula cells” contributing to each of the ommatidia.
“We have never seen the retina split up this way in any other arthropod eye, not in insects, not in crustaceans or other animals with a compound eye,” Baldwin Fergus says in Smithsonian Science. In most compound eyes, the retina is a single continuous sheet. “This eye design has not been described previously and its function is unknown,” she adds. They think that each retina captures an image that’s then passed onto the brain, Science reports, which integrates the 12 images to increase brightness and contrast sensitivity. It likely confers an advantage under the changing light conditions of those dim, monochromatic depths.