The Mantis Shrimp

Part One · The Punch

The mantis shrimp is a small ocean animal with a name that manages to get nearly everything wrong, because it is not a mantis, which is an insect that lives on land, and it is not really a shrimp either. It belongs to a separate group of crustaceans called stomatopods, which are distant relatives of crabs, lobsters, and true shrimp. Scientists have described more than four hundred different kinds, and almost all of them inhabit warm, shallow seas in the tropics, where they are found from sandy lagoons to coral reefs. Most are only a few inches long, although the largest species grow to more than a foot, which is longer than a school ruler. The name comes from the two folded arms beneath the head, which resemble the arms of a praying mantis waiting patiently beside a leaf.

A mantis shrimp spends most of its life in a home that it has either dug or discovered. This is usually a burrow in the sandy bottom or a narrow crack in the coral, and many individuals hardly ever travel far from the entrance. They emerge to hunt, they defend the doorway against intruders, and they hurry back inside whenever something large appears. Some kinds are called spearers, because their arms are lined with sharp spines that catch soft, quick prey such as small fish. Other kinds are called smashers, and their arms end in a hard, rounded club that looks like a tiny boxing glove. Smashers feed mostly on animals with heavy shells, such as snails and crabs, and they use the club to crack the shells open.

The club of a smasher is one of the fastest moving parts in the entire animal kingdom. When it swings, it travels at about fifty miles per hour, which is as fast as a car on a country highway, and it reaches that speed from a standing start in just a few thousandths of a second. A human blink takes about a tenth of a second, so the punch is completely finished many times over before your eyelid has even begun to close. Muscles alone cannot move anything that quickly, so the mantis shrimp relies on a mechanical trick that works much like a crossbow. It slowly bends a springy structure inside its arm, locks that structure in place, and then releases it all at once.

The force of the punch is far greater than you would ever expect from an animal that can sit in the palm of your hand. At the moment of impact, the club strikes with a force of more than a thousand times the weight of the mantis shrimp itself. The club must also survive thousands of these blows without breaking, and it is constructed in a remarkably clever way. Its outer layer is packed with hard mineral, and the layers underneath are arranged in a twisting pattern, a little like the steps of a spiral staircase. That pattern prevents tiny cracks from spreading, and engineers have studied it as a possible model for building tougher materials for products such as helmets and aircraft.

The punch has a second surprise hidden inside it. The club moves so rapidly through the water that the pressure behind it suddenly drops, and tiny bubbles appear in the water where there were none a moment before. Those bubbles collapse almost immediately, and each collapse produces a small shock wave, accompanied by a brief flash of light and a burst of heat. Scientists call this strange effect cavitation. It means that a mantis shrimp can strike its prey twice with a single swing, once with the club itself and once with the shock wave from the collapsing bubbles. A large smasher kept in an aquarium can even crack the glass wall of its tank, which is why people who keep them choose thick glass.

Part Two · The Eyes and Colors

The eyes of a mantis shrimp are every bit as unusual as its arms. They sit on two short stalks, and each stalk can swivel and tilt on its own. The animal can look in two different directions at the same moment, rather like a pair of tiny periscopes. Each eye is also divided into three separate regions, which means that a single eye can judge distance, something that people can do only by comparing the views from both eyes. Because the stalks move so freely, a mantis shrimp can keep watching a rival, a possible meal, and a convenient hiding place all at once, and it does not even need to turn its head in order to do it.

People see color with three kinds of detectors in the eye, which are tuned mostly to red, green, and blue light. A mantis shrimp has about twelve kinds. Some of them can detect ultraviolet light, which is invisible to humans and is the same kind of light that causes a sunburn. Scientists are still working out exactly what the animal does with all of this equipment, and the answer is more surprising than the obvious guess. Experiments have shown that mantis shrimp are not especially good at telling similar shades apart. Their eyes may instead work like a very fast scanner that recognizes certain colors immediately, without needing much time to process the information. Even so, twelve is a very large number, and it is four times as many as a person has.

Mantis shrimp can also perceive something that people cannot see at all, which is the direction in which light waves vibrate, a property known as polarization. Sunlight reflecting off water and off many other surfaces is partly polarized, and a few kinds of mantis shrimp can even detect a rare, twisting form of it called circular polarization. Their own bodies reflect polarized light in patterns on their legs and tails, and researchers believe that the animals may use these hidden patterns to send messages to one another. To a human with an expensive camera, the markings would look like nothing special, but to another mantis shrimp the same body could be broadcasting a clear signal. This is one more way in which the animal is unlike any other.

The bodies of many mantis shrimp are painted in colors that look almost impossible to believe. The peacock mantis shrimp of the Pacific, for example, is a mixture of bright green, orange, red, and blue, while other kinds are brown or gray and blend into the sand and coral rubble where they live. The biggest ones grow about as long as a human forearm. Like other crustaceans, mantis shrimp wear a hard outer skeleton that cannot stretch, so as they grow larger they have to shed it and produce a new, roomier one. For a short time after this change the new shell is still soft, and the animal usually remains tucked inside its burrow until the armor has hardened again.

Young mantis shrimp begin their lives very differently from their parents. The eggs hatch into tiny, nearly transparent larvae that drift away from the reef with the plankton, the enormous cloud of small floating creatures that fills the open sea. They spend weeks or even months swimming and growing before they sink to the bottom and settle down as miniature copies of the adults. While they drift, they are a meal for many other animals, and only a small share ever grow up. A few kinds are known to live for many years, and some pairs even share a single burrow for a long time, cooperating to watch over the eggs. That is an unexpected habit for an animal with such a famously short temper.

Here is the fact to remember when you describe a mantis shrimp to a friend. In a body that could rest on your hand, it carries a spring-loaded club that swings faster than a car on the highway, a punch that makes bubbles collapse in flashes of light and heat, and a pair of independent eyes with twelve kinds of color detectors that can read patterns hidden in light itself. You cannot meet one at the beach unless you live beside warm seas, but many aquariums keep them in tanks. If you ever notice a bright, strange creature peering out of a hole in a display, look closely, because it may be looking right back at you, with each eye aiming somewhere different.