Eyes of the peacock mantis shrimp. The black bands show where it’s looking. Credit: Mike Bok
by Ed Yong
Eyes are testaments to evolution’s creativity. They all do the same basic things—detect light, and convert it into electrical signals—but in such a wondrous variety of ways.
There are single and compound eyes, bifocal lenses and rocky ones, mirrors and optic fibres. And there are eyes that are so alien, so constantly surprising, that after decades of research, scientists have only just about figured out how they work, let alone why they evolved that way.
To find them, you need to go for a swim.
This is the eye of a mantis shrimp—an marine animal that’s neither a mantis nor a shrimp, but a close relative of crabs and lobsters. It’s a compound eye, made of thousands of small units that each detects light independently.
Those in the midband—the central stripe you can see in the photo—are special. They’re the ones that let the animal see colour.
The rock mantis shrimp. Credit: Mike Bok.
Most people have three types of light-detecting cells, or photoreceptors, which are sensitive to red, green and blue light. But the mantis shrimp has anywhere from 12 to 16 different photoreceptors in its midband.
Most people assume that they must therefore be really good at seeing a wide range of colours—a “thermonuclear bomb of light and beauty”, as the Oatmeal put it. But last year,
Hanna Thoen from the University of Queensland found that they’re much worse at discriminating between colours than most other animals! They seem to use their dozen-plus receptors to recognise colours in a unique way that’s very different to other animals but oddly similar to some satellites.
Thoen focused on the receptors that detect colours from red to violet—the same rainbow we can see.
But these ultra-violent animals can also see ultraviolet (UV).
The rock mantis shrimp, for example, has six photoreceptors dedicated to this part of the spectrum, each one tuned to a different wavelength. That’s the most complex UV-detecting system found in nature.
Divers exploring warm waters around the world often encounter Chilean devil rays, gentle marine creatures that can grow up to ten feet long.
Scientists have just discovered that the rays harbor an impressive secret, however: they regularly undertake epic dives more than a mile deep.
These remarkable dives came as a surprise to researchers who reported the finding today in Nature Communications. In retrospect, they note, the rays’ physiology did hint at this ability.
Chilean devil rays possess a special organ called the retia mirabilia, which is also found in deep-diving species such as great white sharks. In those animals, the veined structure fills with warm blood that exchanges heat between vessel walls.
This helps to keep the marine creatures’ brain warm when they descend to freezing depths. But Chilean devil rays, researchers assumed, spent all of their time at the surface. Why would they need such a structure?
To solve the puzzle, an international team of marine biologists attached satellite tags to 15 Chilean devil rays captured off the northwest coast of Africa, near the Azores archipelago. The team monitored the rays’ movements for nine months and found that the animals were tremendously active.
They sometimes traversed up to 30 miles of ocean per day, with each covering a distance of up to 2,300 miles over the nine-month period. Even more impressive, however, was the rays’ diving abilities.
They regularly dove below 1,000 feet, with a maximum-recorded depth of 6,062 feet. This means that Chilean devil rays undertake some of the deepest dives ever recorded for marine animals, the team reports.
The journeys into the deep seem to be no sweat for the animals. One individual, for example, dove nearly 4,600 feet six days in a row, and overall, the rays spent more than five percent of their time in deep water.
The deep dives explain the presence of the previously enigmatic retia mirabilia, the team writes. At the depths recorded by the trackers, rays would encounter temperatures as chilly as 37˚F, so the extra flush of warm blood provided by that organ likely makes those dives possible. Additionally, the researchers found that the rays spend more time basking near the water’s warm surface both one hour before and one hour after a deep dive, implying that the animals are preparing for and recovering from encounters with the cold.
The rays aren’t undertaking these dives just for fun, of course. Based on the animals’ movement patterns—oftentimes a quick bee-line descent followed by a slower step-wise ascent—the researchers think they are probably foraging on fish or squid that live well below the surface.
The unexpected findings, the authors write, demonstrate “how little we know” about Chilean devil rays and the role they play in ocean ecosystems.
Given that these animals were recently listed as endangered (largely due to a growing demand for their gills by practitioners of traditional Chinese medicine), “this ignorance has significant conservation implications,” the team continues. As with any species, the more we know about them, the better equipped we will be for protecting them—and for knowing what we stand to lose should they disappear.