Corals and Jellyfish

Meet the Cnidarians

Corals belong to a large group of colourful and fascinating animals called Cnidarians. As well as such well-known creatures such as jellyfish, sea anemones, hard and soft corals, the group also includes the lesser-known sea fans, sea pens and the fragile fern-like hydroids.

Cnidarians all share in common cnidocytes which are specialised cells in the 'skin'of cnidarians which hold the nematocycts or stinging cells. Cnidarians use these for a variety of functions including defence and food collection.

There are two basic forms of cnidarians: the polyp form, such as corals, and the medusa form of the jellyfish.

About corals

Hard Coral / Acropora horrida / Family Acroporidae

Lion's-mane jellyfish / Cynea capillata / Family - Cyaneidae

Corals are responsible for the formation of the Great Barrier Reef and are extremely beautiful.

Although corals show a wide range of shapes and sizes, they share the same basic body plan - a simple sack-like stomach with a single mouth, surrounded by a ring of tentacles.

However, corals come in a wide range of shapes and sizes. This depends on factors such as genetics, depth of water, current strength and its location on a reef.

About jellyfish

Like most other Cnidarians, jellyfish have a two-part life-cycle. The dominant stage is the familiar free-swimming bell-shaped animal.

Less well-known is the polyp stage that forms early in the life cycle of jellyfish. The polyps are normally very small and are not free-swimming.

Large adult jellyfish are often accompanied by small fish which hide amongst their tentacles for protection.

For humans, jellyfish are among the most dangerous animals in the sea. Box jellyfish and irukandji are capable of inflicting serious and even fatal stings.