One of my favorite aspects of keeping a marine aquarium is the opportunity to observe symbiotic relationships up close. The reef is a highly competitive environment, and many reef animals develop relationships with other animals to increase their chances of survival. Some of these relationships benefit both animals, while others only benefit one of the two. Setting up the proper conditions to observe some of these relationships in the aquarium is, in my opinion, well worth the effort, and here are a few symbiotic relationships most aquarists with a stable, mature system can attempt to replicate in his or her system.
The symbiotic relationship shared between a clownfish and an anemone is one of the most well-known and oft replicated symbiotic relationships within reach of most aquarist. To insure success, the aquarium must be able to support the anemone, which is, in general terms, a more difficult-to-keep animal than the fish. Most anemones require a stable tank that is at least six months old, and some anemones have even more specific requirements—so be sure to do your research. The bubble tip anemone (or BTA, Entacmaea quadricolor) is a good anemone with which to start, and many clownfishes will readily form a relationship with it. Having said this, not all clownfishes will host in an anemone, even if one is present in the aquarium.
Commensal Crab-Coral or Anemone
There are many crabs that will readily form a relationship with various invertebrates including both anemones and many coral species, especially from the genus Euphyllia (e.g., torch coral). The anemone or porcelain crab, for example, (Neopetrolisthes spp.) is a small, non-aggressive crab that will readily live within the tentacles of an anemone or coral species. Again, the conditions in the aquarium must be able to support the anemone or coral, and this usually means the tank will be mature (at least six months old) and stable. In addition, the salinity should be kept at near ocean levels, as most inverts will not do well in hypo-saline conditions.
Commensal Shrimp-Coral or Anemone
Like the commensal crab that lives in association with an anemone or coral, commensal shrimp will readily form a relationship with both coral species (especially from the genus Euphyllia) and anemones. One of the best commensal shrimps to keep is the so-called sexy shrimp (Thor amboinensis). This is a small shrimp with gaudy colors and a unique movement that resembles a somewhat lurid dance (hence the name). While they will host in anemones, the sexy shrimp does particularly well when living in close association with Euphyllia spp.
The relationship formed between a goby and a shrimp is wonderful to watch. While it is possible to observe this relationship in a large reef tank, the pairing of a goby with an appropriate shrimp can easily be the centerpiece of a nano tank. In general, the goby will dig a burrow, which it will then share with the shrimp. When the shrimp ventures out of the burrow, it relies on the goby to make sure the coast is clear. Consider gobies from the genera Amblyeleotris and Stonogobiops and pair them with shrimp from the genera Alpheus and Synalpheus.
There are many other symbiotic relationships which the aquarists can host in his or her aquarium, but these are some of my favorite. Remember to check compatibility charts and to do your research with any species BEFORE you acquire it. In the case of commensal shrimp and crabs, it is often wise to purchase several of these animals if you are planning to add them to a community reef tank, as you may lose one or more from natural predation. To increase your chances of success, add the animals at night directly adjacent to the intended host.