Urticant (stinging) Coelenterates must be kept apart from the more delicate species: a safety margin of 10-15 cm is often recommended. The madreporites and anemones are among the most urticant, and they must not be put alongside a leather coral from the Sarcophyton genus, for example. Other alternatives for a Coe-lenterates tank are algae - whether encrusting or not - supported by live rocks, or even caulerpae. Their development must be controlled so that they do not end up smothering the Anthozoa. The introduction of certain fish is possible, even desirable, but there are cases of classic incompatibilities (see table, page 178). Specially designed artificial decor can be colonized quickly, depending on the organisms' rate of growth and reproduction. This can sometimes make it difficult for even a practiced eye to spot the difference between artificial and natural decor in an aquarium which has been in operation for several months.
Some species belonging to the Annelida, a group that includes the sea worms used as bait in fishing, live in a tube which they build themselves: these are known as
This cowry has withdrawn its mantle into its shell.
Shrimpsplayan ecological role by feeding on fishes' leftovers.
sedentary Annelida. The brightly colored branchial plume which sticks out of it traps oxygen, as well as the organisms which make up zooplankton. At the sign of any danger, or if the tank or its support are knocked, this branchial plume retreats into its tube. The feeding of Annelida in captivity is identical to that of Coelenterates. When you buy one, make sure that the branchial plume is functioning correctly and its tube is intact. It is best to buy a worm fixed to a piece of rock.
Few mollusks are found in aquariums, and they are not often available commercially. Among the gastropods, the cowries, well known to collectors of shells, can be kept in captivity. Some fish may nibble the "mantle", the part of the animal's body used outside the shell. Of course, in the event of any danger the animal retreats into its shell. Hobbyists enchanted by the beauty of this shell can put a specimen in a tank containing small Pacific fish (Gobiids, Blenniids, Grammids. Pseudochromis, for example). In the bivalve group there is the famous giant clam, which can sometimes grow up to a length of 1 m in the wild, where it feeds on vegetal plankton. Some smaller species can be acclimatized in an aquarium. However, these animals are only recommended for experienced hobbyists.
It is easy to keep a few species in a marine aquarium, to take advantage of their bright colors. These invertebrates are carnivorous: in an aquarium they should be fed on mussels, fish, or white meat.
• Hermit crabs are highly striking invertebrates which run around the aquarium in all directions to look for their food. They withdraw into their shell when frightened.
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