Invertebrates can add a whole new dimension of interaction and interest to a marine aquarium. The term invertebrate simply means without a backbone so many different kinds of animals meet the criteria. They include the sponges, corals and anemones but also snails, shrimp, crabs and clams. Even the creatures as different as octopus, starfish and sea squirts are all invertebrates.
Many of the immobile or slower moving species are prone to predation by fish and other more mobile invertebrates like some crabs and shrimp. Many invertebrates are filter feeders that can be challenging to keep adequately fed in a captive environment for the long term.
A lot of others are scavengers whose willingness to feed on a wide variety of materials makes them very useful in keeping the tank clean but also potentially puts just about everything in the tank on the menu when hungry.
Still others like many of the corals and anemones have formed a symbiotic relationship with marine algae called zooxanthellae that provides them with oxygen and sugars but leaves them dependant on sufficient lighting.
It is not uncommon for an anemone to pick up and move if lighting conditions are not to its liking. However this is not an option for some like the corals and sea fans so careful consideration and monitoring when first placed in the tank is critical.
Lacking a backbone many if the inverts rely on shells or external skeletons composed of minerals they draw from the water. This makes additional testing not necessary in a fish only tank an important factor in their care.
As a group the inverts are all very sensitive to copper making treating some fish parasites difficult. The large variety of niches that they fill in nature means that correctly identifying the species and researching their needs is important