Mantis Shrimp Boasts Most Advanced Eyes

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Shrimp is usually a term for something that goes great with cocktail sauce or the person that is picked last on the basketball team, but is not often thought of as containing two of the most advanced eyes on the planet.

However, the more UC Berkeley biologists study the mantis shrimp, from the order Stomatopoda, the more they are amazed with the animal's eyes.

"Stomatopods have the most complicated visual organ in the animal kingdom" said UC Berkeley integrative biology professor Roy Caldwell. Caldwell has studied about 60 of the 550 known stomatopod species.

A mantis shrimp, or stomatopod, scans an area using two compound eyes that are mounted on two high stalks which enable each eye to move independently.

"It is like having 10,000 telescopes looking out at the world," Caldwell said.

Each eye is broken up into three sections and each of these sections has a pseudo-pupil, something very similar to a human's pupil. Because each one of these sections has the ability to see an object the mantis shrimp is able to triangulate the depth of any object using only one eye.

In addition to their advanced depth perception, stomatopods have an extensive ability to see polarized light.

"While many arthropods can see polarized light, stomatopods appear to have the most developed set of polarized light receptors," Caldwell said.

The only way that humans can see polarized light is through a special lens or certain sunglasses. But the mantis shrimp is able to see three different angles of polarized light. In order for a person to do this they would need to be wearing three different pairs of sunglasses.

Currently Caldwell is trying to figure out how stomatopods take advantage of such advanced eyes. Being able to see polarized light is definitely useful for picking out transparent fish or plankton, which is particularly helpful to the stomatopod because most of them are hunters.

Every type of stomatopod kills using its raptorial appendage. The appendage is used to either spear their prey or to smash them. In either case the stomatopod usually are sit and wait predators, which means that they hide out and wait for prey to get within striking distance.

This type of hunting requires extremely good eyesight and is one reason that the shrimp have such impressive eyes.

Another reason the stomatopod eye is so advanced is so that it can see signals given off by other stomatopods.

"If you want to send out flashy signals without being seen by predator fish then polarized signals are a good way," Caldwell said.

Caldwell has discovered a few signals, and is currently trying to figure out what they mean. He believes that some signals are used to attract the opposite sex during mating, and that others are used to communicate things about the surrounding habitat.

Having such a great ability to see polarized light would make the stomatopod's eye truly remarkable, but that is not its only exceptional feature.

These stomatopods also have an incredible ability to see color. One look at the vibrant shell of a stomatopod will tell you that color is very important to their life. It is essential for identification and other aspects of their life.

"Where our eyes have three different photopigments dedicated to see color, some stomatopods have as many as a dozen different pigments," Caldwell said.

Each type of mantis shrimp emphasizes a different aspect of the photopigments. If it is a type of shrimp that lives 40 meters under the water it has a much more emphasis on the shorter wavelengths. Longer wavelengths are rarely present at these depths so it is not as important to have them.

In addition to using colors to identify members of a species the animals also are able to detect fluorescent light.

"They have three different UV light receptors and can see wavelengths as short as 300 nanometers" Caldwell said.

This ability is very helpful for animals that change depth. Some stomatopods have small yellow color patches that are used in species identification. In shallow waters the patches are vibrant and easy to detect, but if the animal dives deeper yellow wavelengths become rare and therefore species identification could be difficult.

However, these stomatopods are able to increase a fluorescent signal as they dive deeper; this makes it easy for others of the same species to identify them regardless of depth.

Caldwell is trying to find an explanation for the mantis shrimp's evolution of such an elaborate eye. He said it was initially thought that they had great eyes because they had such small brains.

The idea was that animals with more advanced brains did not need to have so much information detected in their eyes. Instead, they would be able to infer a lot of the picture from the limited information that their eye would give them.

Stomatopods have increased the detector's capability rather than increasing the processor's.

Another prevailing idea is that it is closely tied with the lethal appendage that the stomatopods developed. Having such a dangerous weapon makes it imperative that one can distinguish friend from foe.

It also means that the animals must be able to see their surroundings as clearly as possible.

Caldwell is now interested in how speciation and sexual selection played a role in the diversification and evolution of the stomatopod's eye and signals. Understanding the signals of the mantis shrimps will help show what are the best ways of communicating in different habitats.

"Somatopods have pointed out the way to understanding under what conditions signals can be enhanced using polarized light," Caldwell said.

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