Cnidarians are apparently simple, radially symmetrical and exclusively aquatic animals. More than 10,000 species have been described, most of them marine, although there are species that live in fresh water, such as Craspedacusta sowerbyi, which is a species of Cnidarian that inhabits the large African lakes. Cnidarians vary in size, ranging from 1-2mm to 3m in diameter, and are characterized by specialized stinging cells called cnidocytes. The cnidocytes, at the slightest contact, shoot a spike impregnated with a stinging substance. This mechanism is used by Cnidarians both defensively against predators and actively to capture and immobilize their prey.
There are Cnidarians that live individually, and also species that form more or less numerous colonies. As for their habitat, there are free-living or swimming species, and others are fixed to the substrate.
There are two basic types of individuals...
- Polyp - Tubular individual, whose mouth is surrounded by tentacles, while the aboral end is fixed to the substrate. There are solitary as well as colonial species.
- Medusa - Bell-shaped body, free-living, swimming and pelagic individuals.
During the life cycle of Cnidarians it is common for the polyp and medusa phases to alternate. The cycle begins with a larva, called a planula, which is able to swim through the water until it finds a suitable substrate to grow. Once found, it attaches itself to the substrate, giving rise to the polyp. The polyp will grow until it eventually gives rise to jellyfish through a process known as strobilation. Jellyfish, free-living and carried by currents, reproduce sexually through the generation of gametes, which, when fertilized, generate the egg that will give rise to the larva that starts the cycle again.
The feeding of Cnidarians, being such an extensive group, is very varied. There are species that capture their prey (zooplankton, fish, ...) with their tentacles, other species that have photosynthetic zooxanthellae that in a symbiotic relationship provide the Cnidarians with their food source.
