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During my recent foray into the world of saltwater aquariums, I learned that most hobbyists are horrified if they find one of my favorite marine animals in their tanks.

This didn't make sense to me. "If you don't want one in there," I said to an experienced keeper, "well then, don't put one in."

He laughed. "We don't. They hide inside newly harvested live rock. You don't know you've got one until your fish and invertebrates start disappearing in the night. It can make for some real bad mornings."

Have you guessed who these little scalawags are? They're mantis shrimp.

These nocturnal shrimp get their name from the praying mantis insect, which folds its front claws under its body in an attitude of prayer. Mantis shrimp do this, too.

There are about 300 species of mantis shrimp in the world. They grow from 1 inch to 12 inches long.

Most of us don't think of shrimp as being efficient predators, but mantis shrimp are exceptions. Some strike and kill passing fish or soft-bodied invertebrates, such as barber pole shrimp or nudibranchs. Others stalk snails, crabs and clams.

If you leave a marauding mantis in a home tank, you will soon have nothing in it but a well-fed mantis shrimp. If you still have a tank, that is. Some mantis shrimp pack such powerful punches they can break glass heater tubes and crack aquarium glass.

These so-called smashers have a pair of bulky front claws folded beneath the body. A few punches with the heavy elbow of one of these unfolded forelegs can crack an aquarium, smash open a snail or knock out a crab.

Sometimes, a smasher disables a crab by breaking its claws. The shrimp then drags its injured prey to its burrow where it peels open the shell and eats the meat. Broken, empty shells lying on the ocean floor (or the bottom of your aquarium) signal a mantis shrimp hideout.

Other species of shrimp, called cutters, deserve their nickname of thumb-splitters because they occasionally slash a human hand that's moving in the tank. The front claws of these shrimp are pointed and razor-sharp, sometimes with barbs on the edges. When a fish or nudibranch passes by, the cutter strikes out, spearing or slashing its prey.

Mantis shrimp also use their powerful legs to defend themselves. You'll know immediately if your hand or foot gets too close for a mantis shrimp's comfort.

In Hawaii, mantis shrimp live in burrows in sand, shoreline mud or rock and coral cracks. If you are wading or playing in the ocean and suddenly have an unexplained cut or bruise, you've probably met a mantis.

This is not something to lose sleep over. Mantis shrimp don't carry venom, and the occasional wounds they cause are usually minor. Injuries can be avoided to net fishermen by wearing gloves when retrieving nets pulled from a reef, and to reef explorers by turning stones away from you. (Please remember to turn these animal dwellings back the way you found them.)

Mantis shrimp aren't all bad. The big ones, I am told, taste as good as lobster. In some parts of the world, fishermen trawl for mantis shrimp and sell them in fish markets. Hawaii anglers fish for a tasty species called banded mantis shrimp, about 12 inches long, in Kaneohe Bay.

Mantis shrimp have stalked, moveable eyes, and some bear exquisite colors, like the well-named peacock mantis.

These fascinating shrimp are worthy aquarium pets when invited. Just be sure to give them a tank of their own -- and watch your hands.