Pauline Walsh Jacobson (CC BY 4.0)
Mantis shrimp (hitchhiker)
Order Stomatopoda (common hitchhikers include Neogonodactylus and Gonodactylus spp.)
A burrow-dwelling predator that smashes or spears snails, hermits, shrimp and small fish; most keepers remove it from a community reef and many rehome it to a species tank where it is prized.
Identification
A shrimp-like crustacean, commonly 3-10 cm for hitchhiker species, green, brown or mottled, with stalked, independently moving eyes, an elongated segmented body, and folded raptorial arms under the head tipped either with a club (smasher) or barbed spears (spearer). Often heard before it is seen: repeated clicking or popping from the rockwork, plus a burrow entrance under rock with rubble and empty smashed snail shells nearby.
Often confused with
Pistol shrimp, which make single loud snaps with one oversized claw, stay smaller, often pair with a watchman goby, and rarely harm livestock beyond scaring it. Forum keepers separate them by sound pattern and by losses: smashed hermit and snail shells and vanishing small fish point to a mantis, while a pistol alone usually explains noise without a body count.
What it targets
Smashers specialise in hard prey: snails, hermits, crabs and clams. Spearers take soft prey: small fish and shrimp. Neither type eats coral, although burrowing can undermine rockwork, and fingers poked into a burrow can be struck or sliced.
How it spreads
Hitchhikes in live rock, including rock attached to corals — one widely shared WetWebMedia account traced two mantis shrimp to a worm rock and a moon coral rather than the inspected live rock. They do not breed their way through a display in normal circumstances; one hidden animal is the usual case.
Control (safest first)
Baited bottle trap: cut the top off a drinks bottle, invert it into the body, bait with a piece of shrimp and lay it by the burrow at night. Cheap and safe; the shrimp are intelligent, so expect several nights and do not give up after the first empty morning. Rock-out method: identify the home rock, lift it out and hold it over a bucket with a little water, or squirt fresh water or soda water into the burrow hole until the shrimp bails out. Wear heavy gloves and keep fingers away from the burrow mouth. Most decisive method in keeper reports. Commercial mantis traps baited with krill, shrimp or scallop: work for some animals and endlessly catch snails and hermits for others, so check them each morning. Once caught, rehome rather than kill where possible: mantis shrimp have an active following of species-tank keepers, and stores or local hobbyists often take them. Never reach into the container barehanded.
Natural predators
No natural predators are listed.
Field notes
Split verdicts run through the Reef2Reef threads: one keeper whose fish were vanishing and hermits were turning up smashed wanted the animal out at any cost, while another answered that a similar mantis had lived in the tank for several years eating snails by the batch — replaced 50 to 100 at a time — without ever taking a fish. A keeper with a small Florida-type mantis reported constant clicking, two dead crabs found in the rockwork and total indifference to starfish, and planned to keep fish stocking flexible around the shrimp; other members warned the planned gobies, blennies and leopard wrasses would be snacks and advised eviction before stocking. A WetWebMedia warning letter described the long version of ignoring one: two mantis shrimp that arrived on coral rock took two months to catch and in that time killed a long list of small gobies, cleaner and anemone shrimp, a dwarf angel and a firefish — the reason experienced keepers quarantine and inspect coral rock as carefully as live rock.