Life Cycle of Cnidarians

Cnidarians make up the scientific phylum that includes jellyfish, anemones and corals. How the life cycle is continued by these creatures that have tissue but no true organs differs greatly among the different classifications and species within the phylum. Many cnidarians can reproduce both sexually and asexually. While some are hermaphrodites, some individuals are distinctly male or female.

Cnidarians are marine animals, some of which swim and/or move and some of which are fixed, completing their entire life cycle while attached to a rock or other surface.
  1. Coral

    • Coral has one of the simpler life cycles of any cnidaria. It begins life as a tiny larva that swims until it settles on a hard surface, from which it will never move again. It buds, meaning a second polyp sprouts from its side. This reproduction can occur asexually or sexually.

    Anemone

    • The method of reproduction differs among anemone species, with some species capable of reproducing both sexually and asexually.

      Most anemones will spawn at a full moon, low tide or other natural cue. This means the sperm and the egg will be released at the same time, making fertilization more likely. When the egg is fertilized, a larvae --- called a planula -- settles at the bottom of the sea and begins growing into an adult anemone.

      Asexual reproduction in anemones usually occurs when a single anemone splits to create two smaller individuals.

    Jellyfish

    • Jellyfish have one of the more complex life cycles of any cnidaria, or any animal, for that matter. Jellyfish do reproduce sexually. When fertilization occurs, the embryo begins development inside the female jellyfish or in brood pounces along her oral arms.

      Larvae, called planulae, leave the mother jellyfish through her mouth or from the brood pouches. After several days, these larvae attach themselves to a rock, shell or the sea floor and develop into polyps called scyphistoma. Feeding on microscopic organisms, polyps can reproduce by producing buds that separate from the original and develop into individual polyps.

      Eventually, when conditions are best for survival, the polyp enters a larval stage, known as the strobila. A polyp may develop five or more strobila and each strobila will separate from the stack and become a tiny jellyfish in the ephyra stage. The ephyra will grow into an adult, or medusa, jellyfish.

    Sea Cucumbers

    • The sea cucumber spawns from June to August. The male and female individuals shed sperm and eggs into the water and fertilization takes place.

      The fertilized eggs will spend 70 days as larvae, called plankton, suspended in the water. They will then settle on the bottom of the ocean floor and begin the process of changing into sea cucumbers. They will reach an adult size in four to five years.

    Sea Urchins

    • The sea urchin's life cycle is in some ways similar to that of the sea cucumber. Male and female sea urchins spawn from June until September, with July being the peak month.

      Mass fertilization occurs in the water with fertilized eggs developing into larvae, known as nauplii, which feed on plankton. After about eight weeks these larvae settle and develop into juvenile sea urchins. These juveniles will often settle under the spines of an adult urchin and remain there until they are larger and less vulnerable to predators. A sea urchin may live to be seven to 10 years old.