Marine invertebrates are just as beautiful and just as interesting as the fishes, if not more so. Sea fans, corals, crabs, snails, sea anemones and starfishes are only a few of the dfferent kinds one is apt to keep in the home aquarium. Locomotion among the sea anemones is limited to a slow creeping motion on a pedal disk. However, these animals can really move great distances over periods of weeks and months. The size of sea anemones varies from 1 inch to the 3-foot-wide giant Stoichactis, found on the Great Barrier Reef off Australia.
The phylum Cnidaria contains an odd assemblage of seemingly very dissimilar creatures. Numbering at least 10,000 species, and likely many more, the jellyfish, sea anemones, sea fans, and corals are, despite their very different appearances and lifestyles, actually quite closely related.
The majority of sea anemones are filter feeders, and many rely on commensally living single-celled algae known as zooxanthella for a large portion of their food supply. The host animals apparently utilize carbohydrates and oxygen produced by the algae, and the algae may derive some benefit from the invertebrate's waste products. Proper lighting is essential if the algae, and therefore their host animals, are to thrive in the aquarium. Larger species of sea anemones will consume small animals and, in the aquarium, thrive on bits of shrimp and fish.
Sea anemones are among the heartiest of the phylum Cnidaria, although even they will perish quickly if kept in poor water quality. Most also require a fairly steady current of water flowing over them at all times. A very commonly available species is the Carribean anemone (Condylactis gigantea). Varying in color from white to brown or pink, this species is quite hearty, but unfortunately it is rarely adopted as a home by clownfish, which often find shelter in the appendages of anemone. More attractive to these popular fish is the purple-based anemone (Heteractis magnifica). This is a very active species that quite frequently travels about the aquarium. All anemones will thrive on infrequent (once or twice a week) meals of shrimp, clam, fish and similar foods.
The body of this anemone ranges from purplish-pink to white and even avocado green, although much of it is hidden by the tentacles, which are typically 3 in long. If the tentacels are largely retracted, the water quality is likely to have deteriorated. This anemone sometimes occurs in groups on he reef, often close to the surface.
All species of anemone, coral, and jellyfish possess stinging cells that are capable of causing painful rashes, and none should be handled with bare hands.
Some anemones, such as Yellow Indonesian Polyps (Parazoanthus spp.) live in colonies. They are vulnerable to predators, such as marine angelfish, when their tentacles are exposed. If danger approaches, an anemone will pull its tentacles into tis body. One advantage of communal living is that when one anemone withdraws its tentacles, all its neighbors are instantly alerted to the threat. The length of the extended tentacles gives an insight into the health of the colony. Shortened tentacles suggest poor water quality, ill health, or individuals that have recently inflicted a sting. All the anemones in a colony are likely to be clones of one anemone.
The Cnidaria phylum includes organisms that are among the most venomous animals. The composition of cnidarian venoms is not known in detail, but they appear to contain a variety of compounds. Currently around 250 of those compounds have been identified. Toxins are used for prey acquisition, but also to deter potential predators. Cnidaria toxins have been identified on the nematocysts located on the tentacles, acrorhagi and acontia, and in the mucous coat that covers the animal body. At least four toxic living classes of cnidarians are currently recognized by most systematists: Anthozoa, Hydrozoa, Scyphozoa and Cubozoa.4