Jellyfish have one of the most unusual life cycles in the animal kingdom, alternating between two completely different body forms: a tiny, plant-like polyp anchored to the seafloor and the free-swimming, bell-shaped creature most people recognize. The full cycle passes through five distinct stages, and a single polyp can clone itself into dozens of new jellyfish without ever mating.
The Five Stages at a Glance
The classic jellyfish life cycle, best documented in species like the moon jellyfish, follows this sequence: egg, planula larva, polyp, strobila, ephyra, and finally the adult medusa. What makes it remarkable is that it includes both sexual and asexual reproduction in the same animal’s lifetime. Adult medusae reproduce sexually by releasing eggs and sperm. The offspring that result eventually become polyps, which then reproduce asexually to produce the next generation of swimming jellyfish.
Spawning and Fertilization
Adult jellyfish are either male or female. When conditions are right, increasing light at dawn triggers both sexes to release sperm and eggs into the water simultaneously. In females, light causes the breakdown of the membrane around the egg nucleus, maturing it for fertilization. This synchronized spawning improves the odds that sperm and eggs meet in open water. In some species, fertilization happens externally in the water column. In others, females brood fertilized eggs on their oral arms before releasing larvae.
Planula: The Drifting Larva
The fertilized egg develops into a planula, a tiny, oval larva covered in hair-like cilia that let it drift and tumble through the water. Planulae don’t feed. Their job is to find a suitable hard surface to attach to, whether that’s a rock, an oyster shell, a dock piling, or even a piece of debris on the seafloor.
Choosing where to settle isn’t random. Planulae are influenced by the physical texture of surfaces, the presence of bacterial films already growing on them, and even by other planulae that have already settled nearby. Studies on moon jellyfish planulae show they tend to prefer water-repellent surfaces, though the relationship isn’t perfectly straightforward. The presence of existing bacterial coatings on a surface also encourages settlement. Once a planula lands on a suitable spot, it attaches and transforms into the next stage.
Polyp: The Hidden Phase
The settled planula metamorphoses into a scyphistoma, or polyp. This is a tiny, tube-shaped creature with a mouth ringed by tentacles, rooted to its surface like a miniature sea anemone. It looks nothing like a jellyfish. Most people never see this stage because polyps are only a few millimeters tall and live on undersides of rocks, harbor structures, and other sheltered surfaces.
Polyps are patient. They feed on microscopic plankton, and they can persist for years. In aquarium settings, moon jellyfish polyps have been recorded surviving up to 25 years before producing medusae. When conditions deteriorate, polyps can form protective resting cysts called podocysts, which are extremely hardy and can survive for years until the environment improves. A single polyp can also bud off copies of itself, slowly building a small colony.
Strobilation: Splitting Into New Jellyfish
The most dramatic transformation in the cycle is strobilation, when a polyp begins producing baby jellyfish through asexual reproduction. The trigger varies by species. Moon jellyfish polyps strobilate after a prolonged cold period, essentially responding to the shift from winter to spring. Other species respond to rising temperatures or increased light. The upside-down jellyfish, Cassiopea, takes a completely different approach: it requires colonization by symbiotic algae living inside its tissues before strobilation can begin. Without those algae, the polyps stay in their benthic form indefinitely.
When strobilation starts, the polyp’s body elongates and its tentacles are absorbed. Horizontal grooves begin forming at the top of the polyp, cutting deeper over several days. In moon jellyfish, early strobilation is visible within about 5 days of being triggered, and the polyp is fully segmented into distinct disc-shaped sections by around day 9, with an average of 8 segments stacked like a roll of coins. Starting around day 12, these segments peel off one by one from the top, each becoming a free-swimming ephyra. A single polyp releases about 7 ephyrae on average, and the remaining stump often regenerates its tentacles and returns to life as a feeding polyp, ready to strobilate again in the future.
Ephyra: The Juvenile Jellyfish
Each newly released ephyra is a tiny, star-shaped disc only a few millimeters across, with scalloped edges that pulse to propel it through the water. It looks more like a snowflake than a jellyfish. Ephyrae begin feeding immediately on microscopic zooplankton, including the larvae of barnacles and other small crustaceans. Their nutritional demands relative to body size are enormous. In feeding studies, ephyrae consumed prey equivalent to over 100% of their own body weight per day when small enough prey was abundant.
Over the following weeks, the ephyra’s arms fill in, its bell rounds out, and trailing tentacles develop. It gradually takes on the familiar jellyfish shape.
Adult Medusa: Growth and Death
By early spring, ephyrae have developed into recognizable young medusae. They grow rapidly through summer, feeding on increasingly larger prey. Adult moon jellyfish capture zooplankton, fish larvae, and even small fish up to about 2 to 3 centimeters long using the mucus and stinging cells on their bells and trailing oral arms. Larger prey means dramatically more energy: feeding studies found that when offered large zooplankton instead of small, a moon jellyfish’s daily food intake could jump by 10 to 40 times.
Most adult medusae are seasonal creatures. Moon jellyfish in the wild typically live 1 to 24 months in their medusa form, with a maximum reported wild lifespan of about 2 years. In aquariums, medusae often survive only about 6 months. They reach sexual maturity during summer or fall, spawn, and then die as water temperatures shift. Their offspring, already drifting as planulae or quietly growing as polyps on the seafloor, carry the species forward.
Species That Break the Rules
Not every jellyfish follows the textbook cycle. The mauve stinger, Pelagia noctiluca, skips the polyp stage entirely. Its planulae develop directly into ephyrae in open water, making it fully pelagic throughout its life. Stalked jellyfish in the class Staurozoa go the opposite direction: they combine polyp and medusa characteristics into a single body that remains attached to a surface, never becoming free-swimming.
The most famous exception is Turritopsis dohrnii, often called the “immortal jellyfish.” When injured, starving, or aging, an adult medusa of this species can revert its cells back into a polyp. Every stage of the medusa, from newly released to fully sexually mature, has been observed making this transformation. The process requires specialized cells from the outer bell and digestive system to essentially reprogram themselves into completely different cell types. No other animal is known to return to a juvenile colonial form after reaching sexual maturity. In theory, this cycle of aging and reversal could repeat indefinitely, though in the wild these jellyfish still fall prey to predators, disease, and other hazards.