Jellyfish

Jellyfish are gelatinous, free-swimming marine animals that belong to the phylum Cnidaria. They are not fish. The typical jellyfish has a bell-shaped body and trailing tentacles. The tentacles bear cnidocytes, cells that contain nematocysts used to capture prey and deter predators. When triggered, nematocysts eject toxins into the target.

Jellyfish have a simple body plan: a nerve net rather than a brain, a gastrovascular cavity for digestion, and no specialized organs for respiration or circulation. They are diploblastic and display radial symmetry.

Life cycle: many jellyfish species alternate between a sessile polyp stage and a free-swimming medusa stage. The polyp reproduces asexually to produce medusae, which release gametes. Fertilized eggs develop into free-swimming planula larvae that settle to form new polyps.

Habitat and ecology: jellyfish occur worldwide in oceans from coastal shallows to the deep sea. They are mostly carnivorous, feeding on plankton, small fish, and crustaceans. They are both predators and prey within marine food webs, and blooms can impact fisheries, tourism, and cooling water intakes at facilities.

Interactions with humans: stings can range from mild to life-threatening depending on species. First aid varies; general guidance includes avoiding contact with tentacles, rinsing with seawater, removing visible tentacles, and seeking medical attention for severe reactions.

Scientific and cultural relevance: jellyfish are studied for their venom and bioluminescence, contribute to marine biodiversity, and serve as indicators of ocean health in some ecosystems.