True jellyfish are ancient, cup-shaped predators that have thrived since the Cambrian period.

Nature · Published 3 October 2026 Swipe more like this →

In short: Known as Scyphozoa, these marine animals are not fish but complex invertebrates. They lack skeletons, brains, and organs, yet they dominate the world's oceans. By utilizing a gelatinous structure and a unique two-phase life cycle, they have survived for millions of years, from the surface to the deepest waters.

The name Scyphozoa derives from the Greek word skyphos, meaning a drinking cup, which perfectly describes the organism's iconic umbrella shape. These creatures have existed since the earliest Cambrian period. While they are often perceived as simple, they possess a sophisticated internal structure. Their bodies are composed of up to 98% water, supported by a gelatinous material called mesoglea that acts as a skeleton. Because they lack durable hard parts, they are rarely preserved in the fossil record.

Their life cycle is a marvel of biological transformation. Most species alternate between a long-lived, bottom-dwelling polyp called a scyphistoma and a free-swimming medusa. The scyphistoma reproduces asexually by budding or through strobilation, where it releases microscopic medusae. These medusae eventually grow into the large, colorful jellyfish often seen in coastal waters. Unlike other jellyfish, Scyphozoa lack a vellum—a membrane used for propulsion—relying instead on a ring of muscle fibers within their mesoglea to contract and swim.

Scyphozoans are formidable predators, using stinging cells called nematocysts located on their tentacles and oral arms to capture fish and crustaceans. Their nervous system is typically a distributed net, though some species coordinate movement using specialized lobes called rhopalia. While about 200 species are currently recognized, scientists estimate the true diversity could reach at least 400. From the massive Cyanea capillata, which can span two meters, to the commercially fished Rhizostomeae, these animals remain a vital, if often misunderstood, component of marine ecosystems.

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