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Nothing sees color like the mantis shrimp

Gonodactylus smithii, a species of mantis shrimp.
[Image courtesy of Roy L. Caldwell]

Most mammals have two types of photoreceptors—cells that convert light into electrical signals—in their eyes. Humans and many other primates have three. Some birds and reptiles have four. Certain butterflies can even have six. But a crustacean, known as the mantis shrimp, which lives among colorful coral reefs, has 12 different types of photoreceptors in their eyes—and researchers haven't understood why until now.

Previous studies have shown that four to seven photoreceptors are enough for animals to see every color under the sun. But, a new study by Hanne Thoen and colleagues reveals that the mantis shrimp uses a unique and previously undocumented color vision system.

The researchers associated food rewards with various colors and discovered that the mantis shrimp, despite their baffling number of photoreceptors, were not easily able to discriminate between some similar colors. For example, the mantis shrimp might not be able to tell the difference between light orange and dark yellow. But, the mantis shrimp are still able to quickly recognize basic colors while using less brain power than humans or other creatures.

This might be because the 12 photoreceptors of mantis shrimp—each one set to a different sensitivity—scan over objects to recognize basic colors almost immediately. Unlike human eyes, which send signals to the brain for comparison, the eyes of mantis shrimp are able to skip this step. And this neat trick might help them out in the extremely colorful—but dangerous—world of coral reefs, full of predators and prey, the researchers suggest.