Common Names: Mantis Shrimp, Thumb Splitters
Mantis shrimp are the bane of almost every reef aquarist. We all dread hearing that tell-tale snapping sound coming from deep within the rock-work of our aquaria. These shrimp are voracious predators feeding on small fish, shrimp, worms, clams and crabs. They have an elongated body with two well-developed eyes located on short stalks, and a pair of powerful pincers held folded underneath the body; similar to the terrestrial praying mantis. There are numerous species ranging in size from 5 cm to over 30 cm (2-12 in.) in length, many of them brilliantly coloured. The larger species tend to live in burrows in the sand, feeding on passing fish. The smaller species belonging to the genus Gonodactylus, are commonly found in coral rock, and these are the ones we commonly encounter in our aquariums.
A commonly encountered live rock-dwelling Mantis shrimp, Gonodactylus sp. S.W. Michael.
Mantis shrimp are extremely wary and are very difficult to eliminate from the aquarium, but they should be removed at all cost if you value your fish and invertebrate collection. These shrimp are quite intelligent and if you fail to catch them with one method you will have to resort to another because you will never get them the second time. Baited traps, placed close to the mantis* home work sometimes. A trap with a quick release door is a must. These can easily be constructed using a small, rigid plastic box and lid. Weigh the box with a rock to make it sink. Wrap an elastic band over the lid and prop it open with a piece of rigid plastic pipe (rigid airline tubing works well). Tie some transparent line to this post and lead it out of the aquarium. Bait the trap with fresh shrimp, clam or fish and wait. These crustaceans are most active at night so the trap should be set just as the lights go out. If not, your fish will eat most of the food. It would be best to place the trap into the aquarium a week or two before you bait it. This will allow the tank inhabitants time to adapt to its presence. You can also bait the trap each night and train die shrimp to come for the food. Then, when you are ready to trap out the shrimp, wait till it enters the trap and pull sharply on the line. This will pull out the plastic post and cause the trap to shut quickly. Make sure the elastic bands are fresh or else they will not close the trap quickly enough. This trap can also be used very effectively to remove small fish from the aquarium.
If you know exactly which rock the shrimp is living in, it is a simple matter to remove the rock to a separate bucket. Of course this is only simple when the rock is easy to move! Be aware that the rock may have a backdoor where the shrimp can escape, so make sure you have a large enough net to hold underneath the
rock before you pull it out of the aquarium. Be very careful when handling the rock and make sure you are wearing a thick pair of gloves, these shrimp did not earn the name "Thumb Splitters" for nothing! Mantis Shrimp are fascinating animals and once removed from the main tank they can be placed in their own separate little aquarium, where you can safely watch their behaviour.
There are some fish species that wrill eat these shrimp but they are very aggressive species that may not be suitable for all aquariums. Specifically the Australian Dottyback, Labracinus lineatus, is reported to feed on Mantis shrimp but is a very nasty fish (S. Michael, pers. comm.). Roger Bull related to us a rather interesting technique for eradicating mantis shrimp from a newly set up reef aquarium. An octopus will quickly hunt down any crustaceans like Mantis Shrimp or crabs hiding in the rocks. Once they have been eliminated, the octopus can be removed. This technique is meant for new set-ups only, before the fish are introduced, because octopus can feed on fish (see also chapter 7, aquascaping).
Another method that can work to eliminate mantis shrimp is to use a sharp, pointed end scissors to injure or slice the mantis shrimp in two. The shrimp can be encouraged to come out by placing some bait near the entrance of the hole or by prodding the hole with a stick (Mantis shrimps are quite aggressive, and will attack an object inserted down the hole into which they have retreated). If you can hold the open scissor blades just around the opening of the hole, and can manage the beads of sweat mnning down your brow, when the shrimp pops its head out of the hole far enough, you might be able to pop its head off with a quick slice of the scissor. This is not a job for the faint-hearted. It is also possible to skew^er a shrimp with a long sharp needle forcefully inserted into the hole into which it has retreated, or through nearby holes in the same rock. We wish to point out that these rather violent techniques are no more cruel than the methods used by predators of these powerful shrimp.
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