Three isn’t enough? A dozen photoreceptor types in Mantis shrimp

By Evelyn Byer (Pitzer College) and Katie Dahl (Claremont McKenna College) [Edited by Lars Schmitz, as part of BIOL 167 “Sensory Evolution”, an upper division class at the W.M. Keck Science Department. Written for educational purposes only].

What animal is as colorful as a clown, but has the punching strength of a .22 calibre bullet? The Mantis shrimp. Besides being a beautiful and terrifying creature, these stomatopods also have a unique way of seeing the world they live in.

The ability of an animal to sense the world around itself is crucial to survival, and eyesight is one of the most important senses that allow animals to do this. While there are many different organisms that have the ability to see, there are multiple different mechanisms and types of eyes that are used. In a recent article, researchers in Australia revealed an incredibly unique mechanism for eyesight in mantis shrimp.. The mantis shrimp eye has twelve types of photoreceptors, which is many many more than most other animals, as nicely explained by Amanda Franklin. This anomaly brought into question what the function of so many photoreceptors may be, and hence the authors sought to determine the possible functional benefits of this type of eye (Thoen et al., 2014).

Figure 1. A colorful mantis shrimp in the shallow waters of Bali (Hannah Tannenbaum, 2012).

Figure 1. A colorful mantis shrimp in the shallow waters of Bali (Hannah Tannenbaum, 2012).

First of all, it’s important to consider what eyesight is mostly used for in the mantis shrimp, and how this might correlate to function. These shrimp are very colorful creatures, which is a characteristic that may be important for communication. If this is the case, then color vision is very important for the mantis shrimp. Also, these animals lead a combative and territorial life, and thus are frequently on the defense and offense. This means that they must be able to quickly sense when a foe is approaching, and respond rapidly in order to escape (Morrison, 2014).

When starting their study, the authors proposed two different hypotheses regarding the function of the unique eye in the mantis shrimp. First, they hypothesized that information from these photoreceptors combine together in a dichromatic system, meaning that there are two channels that the information from color receptors is funneling through, a process that is similar to what happens in our eyes. Second, they thought that information from each photoreceptor type remained separate without interaction between them or integration in the central nervous system.

In order to determine which of these possibilities was occurring, the authors tested the ability of these shrimp to differentiate between similar colors. In order to do this, they trained the shrimp to recognize ten different specific wavelengths. Then, they tested their ability to discriminate between these trained and hues of similar wavelengths. Through doing this, they found that the shrimp were actually quite poor at differentiating colors, despite having many more photoreceptors (Figure 2). This indicated that there was no interaction between different types of receptors.

Figure 2. The mantis shrimp (Stomatopod) has much lower ability to discern similar wavelengths than other organisms with fewer photoreceptors [figure taken from Thoen et al., 2014].

Figure 2. The mantis shrimp (Stomatopod) has much lower ability to discern similar wavelengths than other organisms with fewer photoreceptors [figure taken from Thoen et al., 2014].

A new mechanism was then proposed, which was that these many photoreceptors are used just to distinguish color, rather than to differentiate between similar colors. The shrimp do this through scanning eye movements, which creates a temporal pattern from color. This means that the mental image they process from their eyes might look like a black and white movie. Basically, complexity is being sacrificed for processing efficiency. It’s probably more important for a territorial mantis shrimp to detect a predacious octopus quickly, rather than to determine the different hues of its skin.

The fact that they cannot discriminate similar colors unfortunately means that the mantis shrimp cannot see a “thermonuclear bomb of light beauty” where humans see a rainbow, a belief made popular by The Oatmeal comic “Why the Mantis Shrimp is my New Favorite Animal”. That’s just too bad! Still, Mantis shrimp are awesome.

When thinking about the mantis shrimp’s eyesight in the context of their evolution, it makes sense that quick recognition and responses would give these organisms an upper hand in survival. Evading predators, other territorial mantis shrimps, and catching prey is such a crucial ability to their survival, so it is understandable that speed of recognition would be more important than color determination. Especially when you could get hit by a punch that carries the force of a bullet from a gun if you move too slowly.

Check out this cool video of a mantis shrimp’s eyes surveying its environment!

It was incredibly interesting to find that having more photoreceptors didn’t help color determination, which contrasts many past studies that assumed that the neuronal processing of information from these photoreceptors was similar to our own processing. This goes to show that more is not necessarily better in the ways that you would think, and that less neuronal analysis may actually be beneficial rather than more rudimentary since it can allow for speed of transmission.

This study opens up huge opportunities for future research in this area. Although it was found that the mantis shrimp is likely employing a new method of vision, and that it produces a fast recognition on color, it is unknown how it actually works. Studies on the neuronal connections involved would be incredibly interesting, along with more behavioral implications of this type of eyesight.

For more random facts about the mantis shrimp, and a pretty accurate account of its eyesight, check out this video!

References

Morrison, J. 2014 Mantis shrimp’s super colour vision debunked. Nature Online. (DOI: 10.1038/nature.2014.14578) http://www.nature.com/news/mantis-shrimp-s-super-colour-vision-debunked-1.14578

Thoen, H. H., How, M. J., Chiou, T. H., & Marshall, J. 2014A Different Form of Color Vision in Mantis Shrimp. Science 343(6169), 411-413. (DOI I: 10.1126/science.1245824)

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