Choosing Marine Invertebrates
It's possible to keep many invertebrates, including anemones, corals and shrimp, in a saltwater aquarium, but there are a few things to keep in mind before you get some for your own tank. For example, some species are not compatible and putting them together often results in one becoming the meal. Here are a few pointers on some common invertebrates that may help you select what will work best for your set-up.
Anemones and Corals
Cnidarians are a group of species that includes the anemones and corals. Anemones and corals are generally sessile, in that they attach to the bottom or some hard surface and usually stay there. Both of these invertebrates are popular among hobby aquarists but require careful monitoring of water quality.
Anemones are much easier to keep than corals and are even appropriate for a beginner marine aquarist, but they still need outstanding water quality, meaning no nitrates, good oxygen content, good aeration and circulation. They can be pink, green, purple, orange, yellow or white, depending on the color of the photosynthetic algae that lives in their tissue. Anemones are mobile and can creep along surfaces with their "basal disk." It will find a spot that it likes and settle down. Common types are the carpet anemone, known for being the favorite companion of the clownfish and the Florida anemone.
Tubeworms are found on coral reefs. These sedentary species are known for their tendency to build tubes around their soft bodies using various materials such as sand grains or shells; some secrete calcium carbonate. They can range in shape from straight to spiral and make an eye-catching addition to any reef tank. A feathery and colorful crown of tentacles sprouts from the top and serves as both a respiratory organ and a food-net. When tubeworms are startled, they retract their tentacles.
Tubeworms are divided into two general groups, the Sabellid and Serpulid worms. Sabillid worms secrete a soft substance to which fine sediments and shells will adhere. The most common species found in stores are the Pacific Sabellastarte indica and the Caribbean Sabellastarte magnifica. Serpulid worms secrete a limestone tube that forms small spirals. Their crown of tentacles can be blue, red, yellow or white and these are usually arranged in a whorl. Christmas tree worms (Spirobranchus giganteus) are among the most common on coral reefs. Tubeworms are not compatible with banded coral shrimp, arrow crabs or most other crustaceans. Fish that feed on tubeworms include mandarinfish, blennies, filefish, pufferfish, triggerfish and wrasses.
Crabs and Shrimp
Crabs and shrimp are also common to keep as pets. These creatures are collectively called crustaceans and are a part of the phylum Arthropoda. They are distinguished by their exoskeleton, which they will molt as they grow (so you need lots of nooks and crannies in which they can hide when they are vulnerable). Hermit crabs, arrow crabs, boxer crabs, sponge crabs, banded coral shrimp, harlequin shrimp and candy-striped shrimp are among the most available for hobby aquarists. Be sure to know if they have a tendency to munch on any other invertebrates you may have – or, in the case of hermits, that they are not so large that they may attack small fish.
Mollusks like clams, scallops and snails are also worth mentioning. This is a huge and diverse group of invertebrates, and scientists estimate that there are more than 100,000 species worldwide. Clams are very delicate, but most hobby aquarists won't keep them anyway since they are burrowers and quickly out of sight once they get into the tank. Scallops are popular to keep since they are not usually burrowers and "clap" their shells to move about. Clams and scallops are called bivalves (the term "valve" means shell). They are filter feeders and require a constant supply of food particles in order to flourish.
Snails, also called gastropods (meaning "stomach-foot"), are popular as well since many of them provide a natural way to control algal growth. They move about with a strong muscular foot, and those available for the aquarium usually have shells. When danger approaches, they pull into their shell until they feel safe enough to emerge again. Cowries are regularly imported for the hobby marine aquarist and are unusual in that they will wrap their soft mantle around their shell, which gives it the brilliant patterns and colors. Crabs, however, will commonly pick at the mantle of cowries.
Snails can be herbivorous, mostly feeding on algae, or carnivorous, feeding on fish, anemones, soft coral or other invertebrates. Some are scavengers and feed on whatever organic material they encounter, but most commonly available snails are usually all-out vegetarians.
Starfish, Brittlefish and Sea Urchins
Echinoderms (meaning "spiny skin") include starfish, brittlefish and sea urchins. With the exception of sea cucumbers, which aren't all that popular or available, most of these are bottom dwellers. Many of them feed on other invertebrates such as corals and even each other, so careful selection is crucial.
Starfish are compatible with most fish but will eat bivalves and sea urchins, their natural foods. Sea urchins are difficult to keep because they require excellent water quality and they should be placed in your aquarium only if it is very well established. Since they are voracious eaters, they demand a constant supply of algae on which to graze. Readily available in many good stores are the long-spined urchin, pencil urchin and blue-spotted urchin. With a few exceptions such as triggerfish, sea urchins can be kept with just about any fish. Their spines will fall out or droop when they are sick or dying.
As always before you decide on adding any creature to your tank, check into their habits and lifestyle to be sure that you are able to provide the right conditions and handle the extra work it may entail.