Mantis shrimp keep turning up lately. These charismatic little crustaceans have been featured recently in both National Geographic and Wired, and are the subject of newly published research in several academic journals, including The Royal Society’s Philosophical Transactions B and Science. So, what exactly are mantis shrimp, and why are we making such a fuss over them?
Well, mantis shrimp are marine crustaceans composing a group known as stomatopods. They inhabit the shallow, sunlit waters of tropical seas, where they make a living as voracious ambush predators. Imagine living in this environment, occupying a burrow in the sandy seabed, smack in the middle of a coral reef. Your world would pulse with all the color and noise of an undersea metropolis, and immersed in this hubbub, you would somehow have to find your way.
Without strong eyesight, mantis shrimp would be poorly equipped for survival here. Good eyes allow them to discern between potential predators, prey, and mates from the sea of color all around them. And thanks to this particular lifestyle, the shrimp have developed extremely complex eyes- some of the most complex known to science in fact.
What makes stomatopod eyes so complicated is their unusually diverse array of photoreceptors. These cells detect light, transmitting visual information from the eyes to the brain. Humans have a grand total of three photoreceptor types, while mantis shrimp boast twelve just to discern color.
This week in Science, researchers from The Queensland Brain Institute, and from Taiwan’s National Cheng Kung University dug into this impressive visual system further, asking why stomatopods use so many different types of photoreceptors to process color. What they found is surprising: a visual system previously unknown in the animal kingdom.
To you and I, colors are highly nuanced and blend along a continuum. There are many different kinds of blue for example: cerulean, azure, turquoise, etc. Why are some blues more yellow, while others are tinged with purple or red? The answer lies in our photoreceptors- cells sensitive to different, but specific, wavelengths of light. Some receptors are stimulated by red or green wavelengths, while others pick up the blue end of the spectrum.
As photoreceptors are engaged, they transmit a pattern of signals to the brain. Using this pattern, the brain compares the relative proportion of each color observed by the eye, blending those hues into a complex palette. And because the brain is able to discriminate between wavelengths that are very similar (differing by up to single nanometer) we see a rainbow of color that blurs and bleeds together.
Mantis shrimp also see this rainbow (same ol’ ROYGBIV, and then some), but the mechanism of their color vision is very different from ours. For all of their visual complexity, they are actually much worse at distinguishing between colors than humans are. In laboratory experiments, researchers found that while the shrimp could discern between drastically different wavelengths of light (50-100 nanometers apart), they often couldn’t tell the difference between light varying by 25 nanometers or less. So, while stomatopods could tell blue from orange, they might not pick up on subtler variations in a single color.
So, how is it that an organism with an unprecedented twelve types of photoreceptors has relatively limited vision? The short answer is: we don’t really know. However, researchers hypothesize that for mantis shrimp colors are distinguished in the eye rather than in the brain. Each of those twelve photoreceptors is sensitive to a specific and unique range of wavelengths, and each range is interpreted as a single color. When a pattern of visual signals is sent to the brain, the shrimp doesn’t process the image through comparative analysis. Instead of synthesizing the pattern into a cohesive, blended whole, the shrimp identifies the individual, recognizable colors within the larger pattern.
To humans, tiny variations in wavelength are processed as slightly different hues of a color, but for mantis shrimp this fine-scale variation doesn’t seem to exist. This means that the shrimp can still see the rainbow, but with more abrupt transitions between each color in the spectrum. In a coral reef environment, writhing with colorful and fast-moving life, it may be advantageous to see crisp delineations between colors. And as ambush predators, stomatopods may not have time for complex neural processing. Rather, certain colors may be attack or retreat cues, particularly when recognized in specific temporal and spatial patterns.
Although we do not yet know the specific evolutionary drivers of the stomatopod visual system, it is possible that landmarks and other organisms are more easily recognized when their boundaries are both quickly and sharply defined. Further research will shed light on the highly complex (yet surprisingly limited) visual system of these fascinating reef-dwellers.