Keeping a reef aquarium can be tricky and sensitive. Adding one extra creature could upset the balance of the ecosystem you've created, and you can do so unwittingly. When you add live rock to your aquarium, some common hitchhikers may try to make your tank their home.
All sorts of worms can hitchhike into your aquarium. Some of them are harmless, whereas others require removal for the sake of your other creatures. Feather dusters and spionid worms are two forms of tubeworms that commonly hitchhike. Both of these species are harmless to your tank. Bristleworms, which look like centipedes, are another common hitchhiker; these too are harmless -- or even beneficial -- to a reef tank. Clear flatworms are generally no problem, but red flatworms reproduce rapidly and can take over a tank.
Shrimp are another of the main contenders that come crawling out of new live rock and into your tank. The two most commonly found hitchhiking shrimp are pistol shrimp and mantis shrimp. Pistol shrimp -- and most other species of shrimp -- are peaceful and pose no threat to your tank. Mantis shrimp are another story. They're highly predatory and won't hesitate to kill and eat fish and invertebrates in your tank. They're highly sought for species tanks, so you should be able to easily find your hitchhikers new homes.
If you have any hitchhiking snails, you should beware. The most common species that are likely to make their way into your tank, such as sundial or pyramid snails, are predatory and unsafe for reef life. However, some snail species are perfectly benign, such as Stomatella varia and collumbellid snails. If you're unsure what type of snail you have, consult some reliable reference material or ask advice from an aquarium expert.
As a rule, only commensal crabs -- those that have a mutually beneficial relationship with another organism in your tank -- are safe for keeping in a reef aquarium. Other species can be too predatory or destructive. You should also watch out for any crabs that will eventually grow too large. Porcelain crabs and acro crabs are two common reef-friendly species.
Other Common Hitchhikers
Juvenile echinoderms, such as starfish and sea cucumbers, often hitchhike in rock. Some of these species are suitable, whereas others are too aggressive, too large or potentially poisonous. Do your homework before you decide to keep one. Sponges are another common occurrence, but are no threat at all to a reef tank. Watch out for sea anemones, as hitchhiking species are usually unsuitable for reef aquarium living. Unfortunately, there is no way of ensuring that a live rock contains no hitchhikers.