Classes of the Phylum Cnidaria (currently in Atlas)
What are cnidarians?
The Phylum Cnidaria includes the corals, sea anemones, jellyfish and their relatives. Cnidarians occur throughout the modern oceans and have been a key component of the marine ecosystem since the Proterozoic Eon. They are marveled at by amateur SCUBA divers and scientists alike. Cnidarians are a key component of some of the most dynamic, highly studied and threatened ecosystems in the modern world.
The Phylum Cnidaria is characterized radial symmetry and discrete tissues which, although differentiated into layers, do not form internal organs. A cnidarian body is sac-like, with a mouth surrounded by a series stinging tentacles. Cnidarians are predatory organisms, which capture prey by using stinging cells (or nematocysts) in their tentacles.
Many cnidarians alternate between generations, from a polyp stage to a medusa stage. The polyp stage is attached to a substrate and reproduces asexually. The medusa stage, on the other hand, is free-living in the water column, with tentacles as an adult. Cnidarians in the medusa stage reproduce sexually, by ejecting sperm and eggs into the water column, and the larva grows into a polyp, which repeats the cycle. This alternation is not the same among all cnidarians. Some cnidarians, such as corals, sea anemones, and hydras spend much of their life as a sessile polyp. On the other hand, some cnidarians, such as jellyfish, spend the majority of their lives as free-living medusae.
Cnidarians as reef builders
One of the most interesting capabilities of cnidarians is the ability to create colonies or reefs. These reefs are highly prominent, yet fragile, ecosystems in the modern oceans. These reefs, such as the Great Barrier Reef of the South Pacific ocean, include some of the most ecologically diverse localities on the planet. But this has not always been the case. Throughout geologic time, there have been various phyla which contributed to creating reefs, such as bryozoans or brachiopods. And for a time, after the Devonian biodiversity crisis, all reefs in the world disappeared, leaving a large reef gap in the geologic record. But they recovered eventually, and in the modern world, coral reefs are not only present but imperative.
In the Cincinnatian strata, some tabulate corals built large colonies up to 1m in diameter, which along with some very large bryozoan colonies, may be considered small reefs.