Cnidarians are apparently simple animals, radially symmetrical and exclusively aquatic. There are more than 10,000 species described, the great majority of which are marine, although there are species that live in fresh water such as Craspedacusta sowerbyi, which is a species of cnidaria that inhabits the great African lakes. Cnidarians vary in size from 1-2mm to 3m in diameter, and are characterised by specialised stinging cells called cnidocytes, which, at the slightest contact, trigger a spike impregnated with a stinging substance. This mechanism is used by Cnidarians both defensively against predators and actively to capture 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.

They have 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 species, as well as colonial ones.
  • Jellyfish - Flared body, these are free-living, swimming and pelagic individuals.

The diet of the Cnidarians, being such a large group, is very varied. There are species that capture their prey (zooplacton, fish,...) with their tentacles, other species that possess photosynthetic zooxanthellae that in a symbiotic relationship provide the Cnidarians with their food source.