Mantis shrimps are not the shrimps normally chosen by a marine aquarist to live in their display aquarium, and for good reason. They are very efficient predators. Aquarists are much happier with cleaner shrimps etc.
Sometimes a mantis shrimp gets into the display aquarium, perhaps with live rock, or the rock attached to a coral. The shrimp is noticed as it scurries into its hide hole and the best way of removing it is to lift out the entire rock if that is practical.
If not practical the normal way is to attempt to catch it using a narrow necked bottle of suitable size – clear glass ones are the best. The bottle should be of the type where the neck is more narrow than the bottle itself, the glass opening out from the neck to the body.
Ensure that the bottle is absolutely clean inside and out. Then, place some food in the bottle, but not too much. Flake isn’t of use, but a small piece of fish such as sold for aquarium use or the like will do. Then lower the bottle into the aquarium allowing it to slowly fill with seawater. Leave it standing more or less upright if possible, with the top rim close to the rock where the shrimp lives, again if possible. Don’t leave the food in the bottle in the aquarium long enough for it to rot – remove the bottle and renew the food.
The idea is that the shrimp will detect the food and track it down. Having found it, the shrimp cannot get out of the bottle again because it is too smooth.
Alternatively the aquarist could try using a commercial device designed to catch livestock, though the aquarist could end up with a caught fish.
The capture method will need patience and a number of attempts may be required before there is success.
As said, the easiest and quickest way is to remove the shrimp complete with its rock.
Once caught what can be done? Put it in the sump is one possibility, where it can become interesting in its own right. If there isn’t a sump available, perhaps pass it to an aquarist who has one, or take it to the local retailer.
If they can be interesting in a sump, why are they not welcome in a reef? As said, they are efficient predators and can take fish and crustaceans as prey. This isn’t going to please the aquarist.
Where does the name mantis shrimp come from? It is derived from the land based praying mantis as it is thought there is some similarity in the way that insect sits and waits for and then captures prey. There are two attack methods used by mantis shrimps, spearing and clubbing.
The spearing mantis will usually attack fish and other shrimp. If these come within reach the forward legs flash forward and the victim is speared on the spines.
The smashing mantis will normally hunt hermit crabs, ‘normal’ crabs, snails and other shrimp. Once in range it will club the victim so hard that it is stunned. Three or so hits with the club will normally cause the victim’s armour to crack. It is interesting to note that scientists have measured the power of the club in action, and have found it to be only a little less powerful than the impact of a .22 bullet!*
There are around 300 species of mantis shrimp of various sizes, and all of them are predators.* If prey is not available (for example they have been moved to a sump) they will take mysis shrimp, krill, and small de-frozen fish and the like.
The shrimps can be just 5cm (circa 2″) in length up to a whopping 40cm (circa 15¾”).* I wouldn’t like my finger near one of the latter! Most seen in aquaria are of course small types.
So if a mantis is spotted in the aquarium it needs to come out.
If a sump is available, or another small tank, keeping a mantis is interesting and different. They are efficient predators, and can be fed quite easily. All they need is a rock or two to call home.
(* Reference: Marine Atlas. Hans A Baensch & Helmut Debelius)