The box jellyfish life cycle alternates between a tiny attached polyp and a free-swimming medusa. Details differ among species, so the sequence below is a general cubozoan pattern with notes on the best-studied Australian species.
1. Eggs and sperm
Adult medusae reproduce sexually. Fertilization may occur in the water or through species-specific courtship and transfer. The fertilized egg develops into a ciliated larva called a planula.
2. Planula and settlement
The planula swims briefly, settles on a suitable surface and develops into a small polyp. Because the polyp is minute and hidden on the seafloor or in estuarine habitat, this stage is much harder to observe in nature than an adult medusa.
3. Polyp growth and budding
Polyps feed and can reproduce asexually by budding. That lets one sexually produced larva establish multiple polyps when conditions are suitable. The exact form of budding and dormancy varies by species.
4. Metamorphosis into a medusa
Unlike the stacked-disc strobilation familiar from many true jellyfish, a cubozoan polyp generally transforms into a single juvenile medusa. Field research on Chironex fleckeri suggests juvenile production can continue over an extended period, helping explain why several sizes may be present in the same season.
5. Growth and sexual maturity
The medusa swims, hunts crustaceans and fish, grows, and develops gonads. Mature animals complete the sexual phase and begin the next generation.
What is still uncertain?
Researchers have not documented every stage in the wild for every cubozoan. Temperature, light, salinity and rainfall may influence development, but evidence from one species should not be treated as universal.
Life-cycle stages compared
| Stage | Form and habitat | Main biological job |
|---|---|---|
| Egg and embryo | Developing after fertilization | Early cell division and body-axis formation |
| Planula | Small ciliated swimming larva | Dispersal and finding a settlement surface |
| Polyp | Tiny attached animal on submerged substrate | Feeding, persistence and asexual budding |
| Transforming polyp | Polyp reorganizing into one medusa | Transition to the swimming stage |
| Juvenile medusa | Free-swimming cubozoan | Growth and active feeding |
| Adult medusa | Sexually mature swimmer | Reproduction and dispersal |
How cubozoan metamorphosis differs
Many scyphozoan polyps produce a stack of disc-like young through strobilation. A cubozoan polyp generally transforms into a single juvenile medusa. That distinction is important when comparing “jellyfish life cycles,” because the everyday word groups several classes.
Asexual reproduction extends the polyp phase
Polyps can bud and create additional polyps before medusa production. The visible seasonal population can therefore arise from an attached population that persisted when adult medusae were absent. “The jellyfish disappeared” may mean only that the swimming stage is not being observed.
Environmental triggers
Temperature, salinity, food and light have been associated with development in laboratory or species-specific work. Evidence from one cubozoan should not become a universal recipe. Wild polyps experience interacting conditions that are difficult to reproduce.
Reproductive variation
Some species release gametes into the water, while Copula species are known for direct sperm-package transfer and courtship. Lunar-associated spawning aggregations in Alatina show another route to bringing adults together.
Research gaps
- Locations of wild polyp populations
- Survival and dormancy across seasons
- Triggers for transformation
- How juvenile medusae move from nursery habitat
- Differences among poorly studied species
Sources
- Growth and season onset in Chironex fleckeri
- Animal Diversity Web: development and reproduction
- Natural History Museum: jellyfish life cycles
- Species-specific cubozoan development research
Editorially reviewed: 22 August 2026.
Interactive egg-to-polyp-to-medusa timeline
Open each stage to follow the change in body plan. Timing is deliberately not presented as universal: temperature, food, species and laboratory conditions alter development.
1 · Fertilized egg — microscopic beginning
Cell division produces an embryo. Fertilization can be external or linked to species-specific courtship and sperm transfer.
2 · Planula — ciliated larva
The planula is a small swimming or crawling dispersal stage. In Carukia barnesi, laboratory work documented an unusually long encapsulated planula phase, showing why one “jellyfish egg timeline” is misleading.
3 · Settled polyp — attached and feeding
The polyp attaches to submerged material, catches tiny prey and may bud asexually. Wild cubopolyps are notoriously difficult to find.
4 · Transforming polyp — body-plan reset
In the common cubozoan pattern, the whole polyp reorganizes into one medusa rather than producing a stack of discs like many scyphozoans.
5 · Baby box jellyfish — juvenile medusa
The newly detached medusa is free-swimming and already has functional stinging cells. It grows its bell, tentacles, sensory structures and prey range; “baby” is an informal term, not a taxonomic stage.
6 · Mature medusa — sexual phase
Adults reproduce and close the cycle. Some populations aggregate predictably; others remain poorly observed.
Stage-by-stage scale illustration
egg
planula
polyp
change
juvenile
adult
Conceptual relative scale; not a universal measurement chart.
Primary evidence: Carukia barnesi early life history and cubozoan polyp seasonality and metamorphosis.
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