The photo above, as most of you know, is of an insect called a praying mantis. It is a fierce predator on other insects (and even hummingbirds) and uses those two spine-covered forelimbs to trap its prey. As intimidating as those raptorial appendages look, you can gently pick one up without fear of injury. They simply aren’t powerful enough to inflict any damage to a person’s thick skin.
The common name of this animal comes from its supplicant-like stance, not from it predatory behavior (note that it is spelled "praying", not "preying", mantis).
Ok, so what does this have to do with marine life? In the oceans around the world live a group of crustaceans known as mantis shrimp. Although not remotely related to the terrestrial insects they are named after (except that both groups belong to the enormous phylum Arthropoda), they have a remarkably similar appearance.
Both have elongated bodies protected by an exoskeleton, large compound eyes and forelimbs that make them appear to be deep in prayer. Both groups also use these raptorial appendages for capturing prey. Mantis shrimp, however, are strictly marine animals. And if you try to gently pick one of these guys up, you’re liable to get hurt.
Despite the name, sea mantids are not actually shrimp. They belong to their own order of animals called Stomatopoda. Although there are hundreds of different species of mantis shrimp around the world, they are divided into two general groups based on those two front feeding arms. Some prey on soft-bodied animals such as true shrimp and fish. Others prey on hard-shelled creatures such as clams and crabs. The former are known as "slicers" and latter are called "smashers".
On crabs and shrimp the first pair of legs are usually tipped with claws, used to pick up food and transfer it into the mouth. Mantis shrimp that are slicers have a front pair of legs, again positioned like those of the praying mantis insect, tipped with sharp, serrated blades. These two blades are used to slice, impale or trap prey as it swims by. And the striking action is ruthlessly effective.
Slicers are generally very secretive and found on muddy bottoms where they form complex burrows that would make a mole rat proud. These burrows have multiple entrances, dead ends and interweaving passageways. When feeding they sit at the opening of one of the entrances and wait patiently for prey to pass by. If a fish swims past the burrow, the two front "praying" legs will extend at lightning speed and pierce the fish’s body, sometimes slicing it clean in two.
The speed of this movement is so fast it actually creates a mini-sonic boom. As the legs strike out to capture the food they create a small vacuum behind them. This vacuum creates a bubble which bursts just after the strike. Although this may be just a side effect of the movement, if the mantid misses its prey on the first strike, the collapsing of this bubble is powerful enough to stun the prey anyway.
In my neck of the woods, "Swamp Yankees" call these animals "thumb splitters". Even though the mantids are shy during the day, it is not uncommon for a clam digger to pick one up in his bullrake. Reach into the rake to pull it out and those front limbs will slice your hand open, hence the name.
Like the slicers, smashers also have a pair of forlimbs used for capturing food. Only instead of a blade-like tip, these animals have a club structure on the end that looks sort of like a pair of boxing gloves.
Most smashers are tropical and live in crevices on coral reefs rather than underground. They are also nocturnal, and at night they go out to hunt their favorite foods: crabs, clams and oysters. Like their cousins, the front feeding legs can move with unbelievable speed and power. When a smasher finds a crab or oyster it will slowly approach the prey, raise the front part of its body up slightly and then slam down on the prey’s shell, smashing it to bits. Stomatopods rarely grow larger than six inches in length but they can destroy an oyster’s shell that you or I would need a hammer to open. To give you an idea of the power involved here; the force of the strike is equivalent to that of a bullet shot out of a pistol.
In the past twenty years or so, a fad in the aquarium hobby called "live rock" has become popular. Basically this involves taking a chunk of dead coral reef, usually the size of your fist or a bit larger, and keeping the entire mini-community alive in captivity. Each live rock is unique and contains various anemones, algae, tube worms, mollusks and crustaceans. The bane of the live rock hobbyist is the mantis shrimp. If a tiny one is lurking in the crevices it may go unnoticed for months, all along coming out at night to feed on the other tank inhabitants. Eventually it can grow large enough that it feels it needs to escape the aquarium and will search for a way out. Of course it won’t find one so it may rear up in attack position and hit the glass. The aquarist will come home to find a broken aquarium and lots of water and dead animals on the floor.
I’ll put together another diary on this animal at some point. Although extremely solitary and territorial, some species form long-term monogamous relationships, their eyes are perhaps the most complex of any animal alive and because their legs don’t work on land, they are actually known to curl into a donut shape and roll back to the water if stranded on the beach by a wave. Stay tuned.
Fun Fact: Mantis shrimp are edible, and in Chinese cookery they are known as "pissing shrimp" because of their habit of urinating when placed in boiling water.
Other diaries in this series can be found here.