Jellyfish: basics

In short: Jellyfish are among the oldest animal groups on Earth, existing almost unchanged for over 500 million years. Despite their simple body plan without a brain or skeleton, they’re highly specialised predators.

In more detail: A jellyfish’s body is about 95 percent water, which explains its transparent, gelatinous appearance.

In detail

• Body structure: Jellyfish have no central nerve node but instead a diffuse nerve net that allows simple reactions to environmental stimuli.
• Stinging cells: Special cells on the tentacles contain toxins used upon contact for catching prey and for defence.
• Reproduction: Many jellyfish species go through a complex life cycle with a stationary polyp phase and a free-swimming medusa phase.
• Locomotion: By rhythmically contracting their umbrella-shaped body, jellyfish expel water and propel themselves forward.
• Jellyfish blooms: Under favourable conditions, jellyfish populations can multiply explosively and dominate entire coastal regions.
• Danger to humans: The sting of many species ranges from mild burning to serious health complications in highly toxic species.
• Ecological role: Jellyfish are an important part of marine food webs, both as predators of plankton and as prey for other marine animals (fische-und-meerestiere, nahrungskette-und-oekosystem).