Cnidarian Facts: Corals, Jellyfish, Sea Anemones, and Hydrozoans

Scientific Name: Cinadaria

Purple-Striped Jelly Fish Dancing Underwater

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The Cnidaria (Cnidaria spp.) is the phylum of animals that contains corals, jellyfish (sea jellies), sea anemones, sea pens, and hydrozoans. Cnidarian species are found throughout the world and are quite diverse, but they share many similar characteristics. When damaged, some cnidarians can regenerate their body parts, making them effectively immortal.

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
  • Weight: Up to 440 pounds
  • 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

Description

There are two types of cnidarians, 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.

Despite their diversity, cnidarians share several basic characteristics:

  • Radially Symmetrical: Cnidarian body parts are arranged around a central point.
  • Two Layers of Cells: Cnidarians have an epidermis, or outer layer, and a gastrodermis (also called the endodermis), which lines the gut. Separating the two layers is a jelly-like substance called the mesoglea, which is most visually apparent in jellyfish.
  • Digestive Cavity (The Coelenteron): The coelenteron contains their stomach, gullet, and intestines; it has one opening, which serves as both the mouth and anus, so cnidarians eat and expel waste from the same location.
  • 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.

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; including its tentacles. it can exceed 250 feet long.  

Close up of Jewel Anemone
Dania Chesham/Getty Images 

Species

The Cnidaria phylum is made up of several classes of invertebrates:

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. They fish using their stinging cells: when a 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.

Some cnidarians, such as corals, are inhabited by algae (e.g., zooxanthellae), which undergo photosynthesis, a process that provides carbon to the host cnidarian.

As a group, the Cnidarians have the ability to reorganize and regenerate their bodies, which somewhat controversially suggests they may be essentially immortal. The oldest cnidaria is arguably corals in a reef, which have been known to live as a single sheet for more than 4,000 years. In contrast, some polyp types only live 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.

Cnidarian life cycles are complex and vary within the classes. The archetypal life cycle of a cnidarian begins as a holoplankton (free-swimming larvae), then develops into a sessile polyp stage, a hollow, cylinder-shaped tube with a mouth at the top surrounded by tentacles. Polyps are attached to the seabed, and, at some point, the polyps bud off into a free-swimming, open-water medusa stage. However, some of the species in the different classes are always polyps as adults such as coral reefs, some are always medusas such as jellyfish. Some (the Ctenophores) always remain holoplanktonic.

Lunar controlled coral spawning (Acropora sp.), underwater view
Pete Atkinson/Getty Images

Conservation Status

Cnidarians such as jellyfish are likely to be tolerant of climate change—in fact, some are even thriving and ominously taking over the habitats of other lifeforms—but corals (such Acropora spp) are listed as threatened by ocean acidification and environmental damage, according to the International Union for Conservation of Nature (IUCN).

Cnidarians and Humans

There are many ways cnidarians may interact with humans: They 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. Not all cnidarians have stings that are painful to humans, but some do, and some may even be fatal. Some cnidarians, such as jellyfish, are even eaten. Different cnidarian species may also be collected for trade for aquariums and jewelry.

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