Ten Facts About Jellyfishes

Jellyfish have a bad reputation for the threats they can pose to swimmers and holiday makers visiting beaches around the world. But there's more to jellyfish than a nasty sting. In this article we'll take a closer look at these unique creatures.

1. Jellyfishes are made up of more than 95% water. Their bodies lack a skeletal structure and are delicate and easily damanged. Jellyfishes require water to support their body form and shape.

2. Jellyfishes are invertebrates. They belong to the Phylum Cnidaria, a group of animals that also includes hydra, sea anemones, and corals.

3. A jellyfish is essentially a floating mouth and digestive system. A jellyfish takes food in through its mouth which is located on the underside if its bell. Food is digested in a sac-like structure called a coelenteron.

4. A common analogy used to describe the delicate way jellyfish pounce through the water likens the jellys' movements to 'a simple form of jet propulsion'. To move forward, jellyfishes take water into their muscular bell and then squirt it out behind them, creating a jet of water that propels the jelly forward. In addition to this form of movement, jellies also drift on water currents to move.

5. Jellyfishes have no brain, no blood, and no nervous system. Their senses are primitive and consist of eye spots that can sense light from dark and chemosensory pits that help them identify potential prey.

6. Jellyfishes feed on plankton, fish, and even other jellyfishes. But their fragility requires special adaptations to enable them to hunt and overpower prey without being torn apart in the struggle.

7. The key to jellyfishes' success as predators are specialized structures called nematocytes. Thousands of nematocytes are located on the tentacles, feeding arms, and mouth of a jellyfish.

8. Nenatocysts consist of a capsule that holds a hollow barbed coil, a vemon sac, and chemo-sensitive trigger hairs that detect when something edible brushes against them. When potential prey brushes against the trigger hairs, the nematocytes expel the coiled barb and inject venom into the victim through the hollow thread. The venom immobilizes the prey and the jellyfish uses its oral arms to move the prey into its mouth where it is passed through to the coelenteron for digestion.

9. There are about 200 species of jellyfishes. These species belong to the Class Scyphozoa. The Class Cubozoa, the box jellyfish, includes about 20 species.

10. The species Craspedacusta sowerbii is sometimes referred to as the only species of freshwater jellyfish, although it is not a true jellyfish. Craspedacusta sowerbii belongs to the Class Hydrozoa (the group of animals that includes the hydra).