Cnidarians Facts

Corals, Jellyfish, Sea Anemones, Sea Pens, and Hydrozoans

Purple-Striped Jelly Fish Dancing Underwater

Elfi Kluck / Photographer's Choice / Getty Images

The Cnidaria is the phylum of animals that contains corals, jellyfish (sea jellies), sea anemones, sea pens, and hydrozoans. Cnidarian species are diverse, but these animals share many similar characteristics. Cnidarians are also known as coelenterates, a reference to the name for their digestive cavity.

Fast Facts: Cnidarians

Scientific Name: Cnidaria

Common Name(s): Coelenterates, corals, jellyfish, sea anemones, sea pens, hydrozoans

Basic Animal Group: Invertebrate

Size: 3/4 of an inch to 6.5 feet in diameter; up to 250 feet long

Lifespan: A few days to more than 4,000 years

Diet: Carnivore

Habitat: Found in all the world's oceans

Conservation Status: Some species are listed as threatened


First, a bit about the body plan of cnidarians. There are two types, called polypoid and medusoid. Polypoid cnidarians have tentacles and a mouth that face up (think of an anemone or coral). These animals are attached to a substrate or colony of other animals. Medusoid types are those like jellyfish—the "body" or bell is on top and tentacles and mouth hang down.

  • Radially Symmetrical: Their body parts are arranged around a central point.
  • Composed of Two Layers of Cells: The epidermis, or outer layer, and the gastrodermis (also called the endodermis), which lines the gut are the two types. In between, there is the mesoglea, which is a jelly-like substance. This mesoglea is most obvious in jellyfish.
  • Have a Digestive Cavity (The Coelenteron): This consists of their stomach, gullet, and intestine. It one opening, which serves and both the mouth and anus. So, a cnidarian eats and expels wastes from the same spot.
  • Stinging Cells: Cnidarians have stinging cells, called cnidocytes, which are used for feeding and defense. The cnidocyte contains a nematocyst, which is a stinging structure made up of a hollow thread that has barbs inside. If the trigger at the end of the cnidocyte is activated, the thread unfurls outward, turning inside out, and then the thread wraps around or stabs into the tissue of the prey, injecting a toxin. Not all cnidarians have stings that are painful to humans, but some do, and some may even be fatal. The name 'cnidaria' comes from the Greek word for nettle, which is a type of plant with stingers.

    The smallest Cnidaria is the Hydra, which measures under 3/4 of an inch; the largest is the lion's mane jellyfish which has a bell that can measure more than 6.5 feet in diameter, and including its tentacles can exceed 250 feet long.  

    Close up of Jewel Anemone


    • Kingdom: Animal
    • Phylum: Cnidaria
    • Classes: Anthozoa (sea anemones, corals); Cubozoa (box jellyfish); Hydrozoa (hydrozoans, also known as hydromedusae or hydroids); Scyphozoa or Scyphomedusae (jellyfish); and the Staurozoa (stalked jellyfish).

    Habitat and Distribution

    With thousands of species, cnidarians are diverse in their habitat and are distributed in all the world's oceans, in polar, temperate and tropical waters. They are found in a variety of water depths and closeness to shore depending on the species, and they may live anywhere from shallow, coastal habitats to the deep sea.

    Diet and Behavior

    Cnidarians are carnivores and use their tentacles to feed on plankton and other small organisms in the water. Some cnidarians, such as corals, are inhabited by algae (e.g., zooxanthellae), which do photosynthesis, a process that provides carbon to the host cnidarian.

    Cnidaria has the ability to reorganize and regenerate their bodies, which somewhat controversially suggests they are essentially immortal. The oldest cnidaria is arguably corals in a reef, which can live as a single sheet by more than 4,000 years. Some polyp types only life between 4–8 days. 

    Reproduction and Offspring

    Different cnidarians reproduce in different ways. Cnidarians can reproduce asexually by budding (another organism grows off the main organism, such as in anemones), or sexually, in which spawning occurs: male and female organisms release sperm and eggs into the water column, and free-swimming larvae are produced.

    Diver swimming in coral reef

    Cnidarians and Humans

    There are many ways cnidarians may interact with humans: cnidarians may be sought-after in recreational activities, such as scuba divers going to reefs to look at corals. Swimmers and divers may also need to beware of certain cnidarians because of their powerful stings. Some cnidarians, such as jellyfish, are even eaten. Different cnidarian species may also be collected for trade for aquariums and jewelry.

    Cnidarians such as jellyfish are likely to be tolerant of climate change, but corals (such Acropora spp) are listed as threatened by the ICNU wherever they are found, by ocean acidification and environmental damage.


    • Coulombe, Deborah A. 1984. The Seaside Naturalist. Simon & Schuster.
    • Fautin, Daphne G. and Sandra L. Romano. 1997. Cnidaria. Sea anemones, corals, jellyfish, sea pens, hydra. Version 24 April 1997. The Tree of Life Web Project,
    • "Listed Animals." Environmental Conservation Online System, U.S. Fish and Wildlife Service.
    • Petralia, Ronald S., Mark P. Mattson, and Pamela J. Yao. "Aging and Longevity in the Simplest Animals and the Quest for Immortality." Ageing Research Reviews 16 (2014): 66-82. Print.
    • Tillman, Patricia, and Dan Siemann. Climate Change Effects and Adaptation Approaches in Marine and Coastal Ecosystems of the North Pacific Landscape Cooperative Region: National Wildlife Association, 2011. Print.
    • University of California Museum of Paleontology. Cnidaria.