The main classes of cnidaria are:
- Hydrozoa - hydras and caravels;
- Scyphozoa - Jellyfish
- Anthozoa - anemones and corals; and
- Cubozoa Cubezoan, like the Pacific wasp.
The class of hydrozoans has numerous representatives, besides hydra. All other components of this class are marine. Among them, we can cite as an example the Obelia and the caravel (Physalia), this is a very common colonial individual in tropical and temperate seas.
In Obelia, reproduction occurs during a cycle in which polyps (asexual and lasting) and jellyfish (sexual and short lasting) alternate. Two types of polyps exist in a polipeiro (colony): the nourisher and the breeder.
Breeders generate budding jellyfish. These small ones produce gametes found in water (external fertilization). The zygote is formed, embryonic development occurs, and a hairy larva, the seedling, is an important form of dispersal of the species. By attaching itself to an appropriate substrate, the larva becomes a new polyp, which eventually generates a new polyp.
In the class of cyphozoans, the predominant and sexual forms are beautiful jellyfish of varying colors, the true "jellyfish" often seen on our shores. The polyps are small and correspond to the asexual, short lasting phase.
The jellyfish are shaped like an umbrella and are different from the group of hydrozoans. They can reach from 2 to 40 cm in diameter. The giant of the group is a North Atlantic jellyfish that reaches 2 meters in diameter.
In the case of the species Aurelia aurita, fertilization is internal. The seedling swims for a while and gives rise to a fixed polyp, the kyphistoma. This small polyp is the asexual generation and reproduces by a process known as strobilization. In this process, successive fragmentations of the body of the polyp form a stack of discs that remain crammed together. Each disc, an ephira (young jellyfish), stands out and, after a certain period of growth, gives rise to an adult jellyfish, closing the cycle.
Anemones and corals are the best known representatives of this class. Anemones are easily seen on our coast, especially at low tide, on emerged rocks or buried in the sand between the rocks.
The shape of many corals is varied. Some are shaped like small trees, others resemble large colored feathers, and others are sculptural in shape, such as the famous "brain" coral, which resembles the grooves and girths in the human brain.
Antozoa often reproduce by budding or fragmentation. Sexual reproduction involves the formation and fusion of gametes and there is usually a seedling larva preceding the adult phase.
Since in the class of antozoa there is only the polyp form, there is no metagenesis. After sexual reproduction of polyps, seedling larvae differentiate directly into new polyps. The organization of the polyps of this class is more complex than in the other classes.
Anemone protected clown fish
Unlike generally solitary anemones, corals are colonial in the vast majority of species. They are very small polyps, much smaller than anemones.
As they reproduce asexually by budding and the buds do not separate, they constitute large colonial groupings. And since each polyp builds around itself a skeleton usually made of limestone (calcium carbonate), all the skeletons come together, which gives rise to a large limestone formation common to the colony.