Episode 394: Mantis Shrimp!

Strange Animals Podcast

Thanks to Anbo and Siya for suggesting the mantis shrimp this week! The Kickstarter for some animal-themed enamel pins is still going on! Further reading: Rolling with the punches: How mantis shrimp defend against high-speed strikes The magnificent peacock mantis shrimp [picture by Cédric Péneau, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=117431670]: Show transcript: Welcome to Strange Animals Podcast. I’m your host, Kate Shaw. As invertebrate August continues, this week we have a topic suggested by Anbo and Siya. They both wanted to learn about the mantis shrimp! The mantis shrimp, which is properly called a stomatopod, is a crustacean that looks sort of like a lobster without the bulky front end, or a really big crayfish. Despite its name, it’s not a shrimp although it is related to shrimps, but it’s more closely related to lobsters and crabs. It can grow as much as 18 inches long, or 46 cm, but most are about half that size. Most are brown but there are hundreds of different species and some are various brighter colors like pink, blue, orange, red, or bright green, or a rainbow of colors and patterns. There are two things almost everyone knows about the mantis shrimp. One, it can punch so hard with its claws that it breaks aquarium glass, and two, it has 12 to 16 types of photoreceptor cells compared to 3 that humans have, and therefore it must be able to see colors humans can’t possibly imagine. One of those things is right, but one is wrong, or at least partially wrong. We’ll discuss both in a minute, but first let’s learn the basics about these fascinating animals. The mantis shrimp lives in shallow water and spends most of its time in a burrow that it digs either in the sea floor or in crevices in rocks or coral, which it enlarges if necessary. Some species will dig elaborate tunnel systems while others just wedge themselves into any old crack that will hide them. It molts its exoskeleton periodically as it grows, like other crustaceans, and after that it either has to expand its burrow or move to a larger one. Most species live in tropical or subtropical areas, but some prefer more temperate waters. It has eight pairs of legs, which includes three pairs of walking legs, four pairs with claws that help it grasp items, and its front pair, which are hinged and look a little like the front legs of a praying mantis. That’s where the “mantis” in mantis shrimp comes from, although of course it has lots of other names worldwide. In some places it’s called the thumb splitter. The mantis shrimp has two eyes on stalks that move independently. Its brain extends into the eye stalks, and the section of the brain in the eye stalks, called the reniform body, is what processes vision. This allows it to process a lot of visual information very quickly. Reniform bodies have also been identified in the brains of some other crustaceans, including shrimp, crayfish, and some crabs. Scientists also think that the eyes themselves do a lot of visual processing before that information gets to the reniform body or the brain at all. In other words, part of the reason the mantis shrimp’s eyes are so complicated and so unusual compared to other animals’ eyes is because each eye is sort of a tiny additional brain that mainly processes color. The typical human eye can only sense three wavelengths of light, which correspond to red, green, and blue. The mantis shrimp has twelve different photoreceptors instead of three, meaning it can sense twelve wavelengths of light, and some species have even more photoreceptors. But while our brains are really good at synthesizing the three wavelengths of light we can see, combining them so that we see incredibly fine gradations of color in between red, green, and blue, the mantis shrimp doesn’t process color the same way we do. So while its eyes can sense colors we can’t, its brain doesn’t seem to do anything with the color information.