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The mantis shrimp has to be one of the most dreaded of all uninvited animals to discover in a reef tank. As soon as a clicking noise is heard the reef aquarist's heart jumps into their throat. "Is it a mantis shrimp? What am I going to do? How am I going to get it out of there before it eats all of my fish and shrimps?!" Mantis shrimp are also misunderstood, and not a lot of information is being circulated in the reef keeping hobby about them other than the fact they are undesirable in a mixed invertebrate and fish aquarium.

The common name of "mantis shrimp" comes from their habit of folding their large forelimbs under their heads, rather like a preying mantis. Commercial fisherman also label them, as "thumb splitters", as many have found this ability by getting too close to one or inadvertently picking one up.

Mantis shrimp are long, shrimp like crustaceans with sharp spines on the tail, two well developed eyes on short stalks, and long claws the fold neatly out of sight under the head. They feed on various fish and invertebrates, and are fast, very efficient predators. The long claw is sharp, strong and toothed, with the ability to be rapidly unfolded to impale their prey. They fit into the Crustacea phylum in the class Malacostraca, subclass Hoplocarida (which means 'armed shrimp') and order Stomatopoda. The stomatopods are divided into 15 families, 80 genera and over 400 species. Mantis shrimps are non-decapods and are only distant relatives of shrimps and lobsters, having split off the main crustacean line about 400 million years ago (Mather and Bennet, 1994). Within that time they have evolved into formidable, specialised and intelligent predators. Within the entire group of mantis shrimp there are two types of feeding and fighting appendages which divides the group into smashers and spearers.

The presence of a mantis shrimp is normally given away by a clicking noise, which can be very load and distinct. This is a result of the rock surface being pounded with their claws, in some cases to increase the size of their burrow within the rock. A similar noise can also be generated by pistol shrimp why make their noise by clicking their oversize claw. A distinction between the two can be made by the tendencies of pistol shrimp to only click in ones or twos, and mantis shrimp in multiple, rapid sets.

Some species take up residence in a permanent burrow that can also give them away, using a piece of rock to plug the entrance. They are also very hardy, managing to survive the extreme conditions of transportation, storage, fouling and cycling of live rock. It is lucky that usually only smaller specimens make it in with the rock as they tend to live in the rock and coral heads. The larger ones can be up to 36 centimetres in length, and tend to live in burrows and feed on passing fish.

The grouping of mantis shrimp into spearers and smashers is based on two variations in the predatory claws, which is the second appendage. The raptorial appendages are blade shaped, and either armed with spines which have barbs at the tips (the so called "spearers"), or unarmed but re-enforced with a heavy calcified and inflated heel (the "smashers"). The distributions of these two groupings within five super-families of stomatopods that exist are as follows (Ruppert and Barnes, 1994):

• Gonodactulidea: smasher and some spearers.
• Lysiosquilldea: spearers and some smashers.
• Squilloidea: spearers.
• Bathysquillidae: spearers.
• Erythrosquilloidea: spearer, single species.

Incredible Shrimp

Mantis shrimp are an awe-inspiring predator, being highly skilled and well developed for the role as a high level predator. They are very active hunters, and are lightning quick at lashing out. One finger of their main claw lies folded with a groove in much the same was as the blade of a pocketknife does in its handle. During a strike at prey, this unfolding motion can occur in less than 1/125^th of a second, or 8 milliseconds, with the force of a small caliber bullet. It is one of the fastest animal movements known.

A mantis also has the most complex vision system known; trinocular vision. This super vision is required to give the accuracy that is vital to capture fish at the speed of the motions involved. Add on top of these features their noteworthy intelligence, and it all adds up to a predator that humans are very lucky is so small and lives in the ocean.

Biology

Mantis shrimp have a body that is dorsoventrally flattened with a small, shield like carapace and a large, broad, segmented abdomen. The most distinctive feature is their appendages, with the second pair having developed for raptorial feeding. This appendage has a movable finger that is provided with long spines or shaped like the blade of a knife. It can be rapidly extended to capture prey or to be used in defense.

The eyesight of a mantis shrimp is amazing, their sight is the most highly developed compound eye among the crustaceans. The compound is eye made up from up to 10,000 separate elements, with three "pupils" on each one. This gives them trinocular vision (human vision is binocular, with two eyes) which is one of the most complex range finding systems known currently. This allows them to accurately spear fast moving fish. The eyes are also located on the ends of stalks, allowing them to size up prey, predators and competition from the comfort of the burrow or crevice.

The smashers in particular have sophisticated colour vision that is much more complex than our own. It is not entirely know the purpose of it, but it could enable them to recognise potential mates and even individuals on the basis of the body colours and patterns. The coloured spots inside their forelimbs, which are displayed in threat or courtship, are also likely to be important identification keys.

Since a blow from an opponent during a duel could be fatal due to the force with which they can hit, vision can be important in defense and offense. When they do fight, the blows from the opponent are fended off with the flexible tail or telson.

Mantis shrimp can either mate for life or come together only to mate, depending on the species. And the eggs can be laid and kept in a burrow, or carried around under the females' tale until they hatch. Once the larvae hatch they can spend up to three months as plankton before settling and becoming adult mantis shrimps. (Ruppert and Barnes, 1994)

Habitat

Most live in excavated burrows, either building their own or moving into those built by other organisms, or rock/coral crevices. One particular species, Echinosquilla guerini, have radiating spines over the entire surface of its telson, which is the end of its tail. It then uses this to plug its burrow to the outside world. Gonodactylus bredini on the other hand, closes it burrow entrance at night with debris, and protects it with its claws during the day. (Ruppert and Barnes, 1994)

The burrow can act as a place to surprise prey with the mantis lying in wait at the mouth of the burrow, ready to pounce on any unwary organism that stumbles past. Other species will leave the burrow to feed, crawling over the bottom and rocks or swimming.

Once a suitable burrow is found, it will be stoutly defended from other mantis shrimp. Here is where their telson comes in handy, being thrust forward to defend themselves from the sever impact of their opponents raptorial appendages. (Ruppert and Barnes, 1994)

Spearers

Spearers posses from 2 to 20 spines on the last joint of their claws, and this turns these appendages into deadly and effective spears to capture prey. They feed on soft-bodied prey such as fish and shrimp, spearing them by an extremely rapid extension and retraction of the barbed claws. Typically spearers lie in weighting at the entrance of their burrow for passing prey. When within range, it lashes out in a precise upward spearing motion with a velocity in the region of 10 metres per second. It is one of the fastest animal movements to impale prey ever recorded, taking merely 48 milliseconds. Once the prey is immobilised on the end of the claws, it is pulled in and the meal begins.

Smashers

Smashers on the other hand have heavily calcified heels on the forelimbs, which become very potent clubs. They stalk their prey of molluscs and crustaceans. The hard bodies of the prey are smashed with the heel of the unfolded appendage. A blow from behind, or its claws broken immediately, typically disables crabs. Several more blows are then followed which then smash the carapace and legs. The crab is then dragged back to the burrow to be eaten. Snails or clams are taken back to the burrow first, then smashed and the shells discarded outside after they have been consumed. Smashers are the ones that have the reputation for being able to crack glass.

Removal Techniques

So a mantis shrimp has been discovered in a reef tank, and it will not get along with the other organisms that are going to be kept there. Well, how are they removed? There are a wide variety of different methods available, and the following is a summary of the majority of the more successful ones that have been presented in various sources. A problem that case arise though, is because of their intelligence if the capturing technique is not successful on the first try then they are highly unlikely to fall for the same trick again.

Location Known

• Remove the rock from the aquarium and submerge it into some freshwater or inject it into the burrow with a syringe. The shrimp will come out once the stress of being in freshwater is felt. If the mantis is wanted alive, then quickly remove from the water with something other than a hand, they are not called thumb splitter for nothing. Or perform the freshwater injection over a container so they are contained and captured when they exit the rock. A short period of freshwater dunking will not kill anything on/within the live rock.
• Remove the rock from the aquarium and pour some soda water into the burrow or holes. The acidic water and the fact it is saturated with carbon dioxide will cause anything that can move to quickly leave the rock. Make sure it is performed over something like a sink, so that anything that comes out can either be placed back in or not lost beneath the couch. The effect is not permanent on the live within the rock, just makes them want to move very quickly.
• Put an open pair of scissors over the entrance to the burrow, and place a piece of bait nearby or poke around in nearby holes with a length of wire. It can be possible to close the scissors fast enough to injure or slice the mantis in two when it pops its head out of the hole. (Delbeek and Sprung, 1995)
• Remove the rock from the aquarium and poke around into all the holes with the end of a coat hanger or a piece of wire. This will either drive the mantis out of the rock, or kill it within the rock. Can be a very satisfying technique. (Moe, 1993)
• Remove the rock from the aquarium and place in a separate container with seawater in it. The shrimp may come out eventually. (Fossa and Nilsen, 1996) Good for those that are a bit more scared of doing anything rash to the rock, or don't feel to violent or unforgiving towards the shrimp.
• Either remove the rock from the aquarium or leave it in there if it is easily accessible. Then poke around in the rock that the mantis has made its home with a piece of wire or a sharp needle. It is possible to then impale the shrimp on the poker, although if it has a back entrance then it will very quickly use it. A very satisfying technique if there is some degree of hatred directed towards the shrimp for some reason .
• Remove the rock from the aquarium and place in another where there is a more suitable home for a mantis. They are very fascinating animals to observe, and if the space is available, it is recommended to keep one.

Location Unknown

• Place a glass jar with a string tied around the neck, and baited with a piece of seafood on the bottom of the, tank or against the rocks after the lights go out. The bait will attract the mantis, and when the shrimp enters the jar rapidly remove it from the tank. This can be a bit time consuming, waiting for the shrimp to enter the jar. They can also swim, so if it is not removed quickly enough they can swim out of the jar.
• Commercial traps are available. These are baited with some seafood and placed on the bottom of the tank after the lights go out. When the shrimp enters the trap to get the food, the trap is triggered and a door drops down and hopefully captures them. Can be a problem when have other types of crustaceans in there, such as cleaner shrimp and hermit crabs. Would be a good idea to remove all of these first, so that they cannot get caught instead.
• A trap can be easily constructed out of a small rigid plastic box and a lid. Wrap an elastic band around the box and the lid, and then prop over the lid with a small piece of plastic. Tie a length of transparent fishing line to this piece of plastic and lead it out of the aquarium to a nearby chair. Place some bait of seafood within the box, and then lie in wait for the shrimp to come out. When the shrimp enters the trap for the food, sharply pull on the fishing line and trap the shrimp. May have to weight down the box so that is does not move when the fishing line is pulled. (Delbeek and Sprung, 1995)
• During the curing process of the new live rock in the aquarium add an octopus. Octopuses are predators of crustaceans, and will soon capture any shrimps or crabs that are present in the live rock. When the supply of crustaceans runs out the octopus will be visible hungry and can be captured by a baited jar very easily. This will only work well when the reef is being established, as octopuses will also eat fish and any added crustaceans. Some local fish stores will be able to work out a sort of hiring deal. (Delbeek and Sprung, 1995)
• After the curing process, the Bird Wrasse Gomphosus varius can be added to the tank for a couple of weeks. Their prey included mantis shrimp and bristle worms, but also will eat desirables such as fan worms and feather dusters. Bird Wrasses are not a good fish to leave in a reef tank, as they also like to feed on Tridacna clams.
• Another mantis eating fish is the Australian Dottyback, Labracinus lineatus. It can be added in much the same manner as the Bird Wrasse. These fish are rather aggressive, so have to be removed once the shrimp has been if other fish are to be added. (Delbeek and Sprung, 1995)
• Place a powerhead on the bottom of the tank with a piece of seafood bait at the unshielded intake. When the mantis comes to eat the bait, and is within range, turn on the pump. A very gratifying technique, and supplies the reef with some food.
• For the DIY type people, take a cigar tube and drill a hole in the closed end. In the input cap cut several radiating slits with a razor blade or fine, sharp knife. Remove roughly half of the triangles that are formed in the cap, and bend the remaining ones inwards. The thinner the strips are the better it will work, so they are nice and soft. Place a piece of bait of seafood within the trap, and place on the bottom of the tank after the lights go out. The mantis can then go into the trap to get the food, but because the strips point inwards, cannot get out again.

References:

Delbeek J.C., and Sprung J., The reef aquarium: a comprehensive guide to the identification and care of tropical marine invertebrates, vol. 1, Richordea:Coconut Grove, 1995.
Fossa S.A., and Nilsen A.J., The modern coral reef aquarium, vol. 1, Birgit Schmettkamp Verlag:Bornheim, 1996.
Marther P., Bennet I., editors, A coral reef handbook: a guide to the geology, flora and fauna of the Great Barrier Reef, 3^rd ed., Surrey Beatty: Chipping Norton, 1994.
Moe M.A., The marine aquarium reference: system and invertebrates, Green Turtle Publications:Plantation, 1993.
Ruppert E.E., and Barnes R.D., Inverebrate zoology, 6^th ed., Saunders College:Sydney, 1994.