Jellyfishes

Jellyfishes are gelatinous, free-swimming marine animals in the phylum Cnidaria. The term commonly refers to true jellyfishes of the class Scyphozoa as well as related gelatinous cnidarians such as hydrozoan medusae. They occur in oceans worldwide, from warm surface waters to deep seas, and range in size from a few centimeters to more than a meter in bell diameter.

Anatomy and physiology: Jellyfishes have a bell-shaped body, or umbrella, with trailing tentacles or oral arms. They move by pulsations of the bell. They lack a centralized brain and instead possess a nerve net with sensory structures in the margins called rhopalia, which may include light-sensing organs and balance receptors. Cnidocytes on the tentacles hold nematocysts used for prey capture and defense.

Life cycle: Many jellyfishes alternate between a sessile polyp stage and a free-swimming medusa stage. In typical scyphozoans, the polyp reproduces asexually to form multiple ephyrae, which grow into immature medusae. Fertilized eggs develop into planula larvae that settle to begin new polyps. Some hydrozoans retain different life-cycle patterns.

Ecology and food webs: Jellyfishes primarily feed on small planktonic organisms, crustaceans, and fish larvae, captured with cnidocytes. They serve as prey for larger jellyfishes, sea turtles, and some fish. Blooms can influence local ecosystems and human activities, including fisheries and tourism, particularly where warming waters and overfishing alter plankton dynamics.

Human interactions and conservation: Stings from jellyfishes vary from mild to dangerous, depending on species. Severe envenomation requires medical care. Climate change, coastal development, and shifting plankton communities are linked to jellyfish bloom patterns in some regions. Ongoing research aims to clarify their biology, ecology, and role in marine ecosystems.