Jellyfish, Sea Anemones, Sea Fans, and Corals (Phylum Cnidaria)

This phylum 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 basic body plan of all the cnidarians is a saclike structure with a single opening that serves as a mouth and exit port for waste products. All creatures within this phylum possess stinging cells, known as the nematocysts, whose needlelike barbs readily pierce the skin of both predators and prey. The various species within this group exist in two forms, as polyps attached to the substrate or as swimming animals known as medusas. In some groups, such as the jellyfish, the animal passes through both stages at different points in its life.

Jellyfish (Class Scyphozoa)

Comprised of at least 95 percent water, the jellyfish are quite appropriately named. Rarely exhibited until fairly recently, public aquariums are currently making great strides in their captive husbandry. Most species are, however, very delicate, and not suitable for home aquariums. All are carnivorous, feeding on a range of tiny creatures, and, in some cases, small fish that are overcome by the stinging tentacles.

Many species of jellyfish, especially those from tropical waters, are quite toxic to humans, and their venoms have been little studied. Even temperate species should be avoided or handled with extreme care, as their stings may cause a severe allergic reaction in people sensitive to their particular venom.

Sea Anemones and Corals (Class Anthozoa)

It is among the sea anemones and corals that we find many of the cnidarians most sought after by aquarists. Approximately 6,500 species inhabit the world's temperate and tropical seas, with many species of sea anemones being found in quite cold waters as well. The majority of these animals 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 invertebrates' 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.

Soft corals are not often kept by aquarists but are actually fairly hardy, as corals go. Usually sold as pulse corals or leather corals, they require good water quality and an ample supply of tiny food items. Hard corals possess a calcareous exoskeleton and are responsible for the formation of coral reefs. The animal itself is somewhat similar to a sea anemone in its body plan and sits in a calcium-based cup. Individual coral animals range in size from 0.2inches to 20 inches across. Most hard corals offered for sale today require the presence of thriving colonies of zooxanthella and water with high pH and calcium levels.

Coral reefs build up very gradually in tropical seas as living animals colonize the exoskeletons of deceased corals. The world's largest coral reef is the Great Barrier Reef of Australia, which is 1,250 miles in length.

Corals reproduce by releasing sperm and eggs into the sea. A number of species have evolved the reproductive strategy of simultaneous spawning, with all of the animals in a given area releasing gametes at the same time. Even in ecosystems as large as the Great Barrier Reef, the individual coral animals are somehow synchronized to reproduce at a specific time.

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