The Things You Could See With 12 Retinas

One of probably quite a few paradoxes involved with living in the twilight zone of Earth's oceans has to do with seeing stuff. It's black down there, but that's a relative thing. Eyes can be boosted. Lots of animals come with reflector-equipped retinas for seeing in the dark, a system of light magnification. This has a downside, however.

If you're living life as a deep-sea shrimp, darkness is also your protection. And in the absence of physical barriers or landmarks, it's really your only protection. As my dog might give himself away in a (relatively) darkened bedroom by the glow of his eyes, shrimps outfitted with supersized or reflective optics might give themselves away in the inky abyss. ​A clever solution (evolutionarily speaking) devised by the Paraphronima gracilis—and ​recently described in the journal Current Biology—lies instead in the addition of an entire extra array of light-sensitive tissues.

There are 12 in all: a single row of giant red retinas. It's an optical system that occupies nearly half of the shrimp's body. The configuration has never been seen before.

Paraphronima gracilis live within what's known as the ​mesopelagic zone, a region that extends from about 200 meters below the ocean's surface to 1,000 meters below. It's know alternatively as the "twilight zone." Light here isn't completely absent.

"Solar illumination is visible in the upper 1,000 meters of the ocean, becoming dimmer and spectrally filtered with depth—generating a nearly monochromatic blue light field," the paper explains. "The struggle to perceive dim downwelling light and bioluminescent sources and the need to remain unseen generate contrasting selective pressures on the eyes of mesopelagic inhabitants." This is the problem.

There's an added twist. The wavelength of light the Paraphronima gracilis eyes see is different from the light that's usually surrounding them. The retinas appear to be specially calibrated to look upwards. ​In a separate blog post, Jamie Baldwin Fergus, one of the authors behind the new study, explains: "Animals looking upward into the down-welling light may have an easier time spotting targets using an offset spectral sensitivity. Light models indicate that mesopelagic animals searching overhead would most benefit from sensitivity near 515 nm. This corresponds closely to that found in P. gracilis, indicating that their eyes are well suited for searching for overhead targets."

How the retinas are processed and integrated neurologically remains to be explored, but the model demonstrates that the arrangement results in a better "spatial summation" than other configurations so-far observed by biologists. Just within the same suborder of tiny amphipods, the ​hyperiidea, 10 different variations have been cataloged, ranging from no eyes at all in relative deep-dwellers to four giant eyes in fellow mesopelagic swimmers (floaters?). The better to see you with.