If a mantis shrimp roll its eyes at you, don't assume it thinks you are pathetic for not having a punch a boxing champion would envy or for only seeing a tiny part of the electromagnetic spectrum. It's probably just trying to see you better (although thinking you are pathetic is quite possible too).
Mantis shrimp vision is among the wonders of zoology. Although there are differences from species to species, some mantis shrimps can see far into both the ultraviolet and infrared, as well as being able to differentiate between all six types of polarized light. Others make do with distinguishing two or four, which is still pretty good seeing as humans can't do any.
A paper in Nature Communications reveals that one of the ways stomatopods achieve this is by rolling their eyes.
The mantis shrimp's eyes can rotate on two axes. Daly et al./Nature Communications.
“They can use 12 different color channels (we use only three), and can see the polarization of light. But the eye movements of mantis shrimp have always been something of a puzzle,” said author Dr Nicholas Roberts in a statement.
“Many animals go to great lengths to stabilize their gaze, using eye, head and body movements to avoid motion blur,” the paper notes. Having your eyes stay stable, irrespective of what the rest of you is doing, helps to detect objects, to “estimate relative motion within a visual scene,” and to “maintain orientation relative to a local horizon.”
Yet mantis shrimps do the opposite. They move their eyes frequently, including rotating them up to 90º, which previous studies have shown can be either coordinated or completely independent, as required.
Mantis shrimp eyes contain photoreceptors on two hemispheres that are bisected by a narrow midband. When shown highly polarized light, two species of mantis shrimp rotated their eyes so that they maximized the polarization distance, a measure of how strongly an object's polarization contrasts with its background.
Video still frames demonstrating the rotation range of Gonodactylus smithii (above) and Odontodactylus scyllarus, the two species studied. Daly et al/Nature Communications.
The paper reveals that the eye rolling occurs when the mantis shrimp encounters linearly polarized light, and appears designed to maximize the polarization contrast for the receptors in one of the hemispheres, without having any benefit for the midband.
Mantis shrimps have likely developed this ability because they dwell close to the sea floor, where most of the light reflected from nearby objects is only weakly polarized, but light from overhead can be strongly polarized, so it is important for them to be able to make out objects against both a polarized and depolarized background.
The authors hope that their work will inspire visual systems for robots operating underwater that face similar challenges, or for instruments designed for detailed study of the structure of materials, where polarized light is often useful.
Gonodactylus smithii. If your potential mates looked like this, you'd probably want amazing vision too. Daly et al/Nature Communications
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