Cnidarian Life Cycle

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Cnidarians are a group of aquatic organisms that undergo a two-part life cycle. This lesson describes the Cnidarians, their characteristics, and the detailed (yet fascinating) reproductive process they employ.

What Are Cnidaria?

The phylum Cnidaria is comprised entirely of aquatic organisms, including the corals, sea anemones, jellyfish, hydra, and sea pens. All Cnidarians are invertebrates and have bodies with radial symmetry, meaning the body is symmetrical around a centralized point. All Cnidarians possess some form of stinging cells, called nematocysts. These nematocysts have tiny sharp points tipped with poison, so you don't want to accidentally cross one. This is easy to remember when you know that the word 'Cnidaria' comes from the Greek word for 'sea nettle'; if you've ever been stung by a nettle, you know the pain they can cause! These stinging cells allow them to paralyze their prey so they can eat them.

Cnidaria Reproduction

The different groups of Cnidarians have slightly different details when it comes to reproduction. However, if we take a broad look at the reproductive cycle we can create a generalized description.

Cnidarians as a whole have two body forms: the polyp and the medusa. An individual with the polyp form is usually anchored to the substrate with the mouth facing upwards (the term 'substrate' refers to the materials at the bottom of a water body, like sand, rocks, and organic debris). In contrast, individuals with the medusa form are free-swimming with the mouth facing downwards.

The medusa shape is on the left and the polyp shape is on the right.
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This is important, because if we look at the general reproductive cycle of Cnidarians, both body types are present during some point of the life cycle (not all Cnidarians adhere to this cycle, but we'll discuss exceptions later in the lesson). Free-swimming medusa Cnidarians release their gametes (eggs or sperm) into the surrounding water when they are ready to mate. Eggs are fertilized by sperm (sexual reproduction) and develop into the larval stage, during which they are still free-swimming and are called planulae. Once the larva reaches a certain stage of development it will attach to the substrate and become a polyp. Over time the polyp begins to segment itself, and each segment eventually becomes a separate organism. When the segments break apart, the now free-swimming medusae become independent individuals in a process called strobilation. These released individuals are called ephyrae and they are essentially clones of the parent polyp (making this asexual reproduction). The part of the parent polyp still attached to the substrate remains behind and may grow more segments that eventually split off. The released ephyra juveniles eventually mature into adults and repeat the reproductive cycle. This type of alternating sexual and asexual reproduction is called metagenesis.

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