Sound familiar? These and similar questions are not odd, but what is odd is how many aquarists don't realize that when it comes to keeping a saltwater system, it's not unusual for strange things to show up. We don't understand why someone is so surprised when it happens, because it's all a part a marine environment!
Although organisms are usually introduced into an aquarium by way of newly added live rock and live sand, there are other reasons why things appear out of what "seems" like nowhere.
- Many organisms are microscopic or plankton sized when they start out, so until they grow large enough to be seen with the naked eye, you don't know they are there.
- Some organisms that can be seen with the naked eye, no matter how large or small, are experts at hiding. They hitchhike in on live rock and sand, and it is only after you have placed it into your aquarium that these organisms then crawl out and make themselves at home.
- Often corals, anemones, urchins, cucumbers, starfishes, nudibranchs and other forms of marine life play host to other, much smaller hitchhikers on the reef. Just a few examples of the many types of symbiotic relationships that occur in nature are the Pacific Clown Anemone Shrimp (Periclimenes brevicarpalis) and Anemone Crab (Neopetrolisthes ohshimai) that live with various types of sea anemones. The Sea Cucumber Crab (Lissocarcinus orbicularis) lives commensally on the body, but most often among the tentacles or in the mouth, as well as the anus of several species of sea cucumbers.
- Marine animals have babies. As examples, crabs and shrimps can hatch out live fry when mated pairs are present, snails reproduce by laying weird looking egg sacs, and corals spawn or split.
For these reasons, organisms that suddenly show up in an aquarium were most likely there all along or were spawned by other tank inhabitants. You just couldn't or didn't see them yet.