I spent a stretch of early 2024 buried in cubozoan papers, museum archives, and an embarrassing number of emails to researchers in Queensland. One scientist in Cairns joked that studying box jellyfish reproduction is “like trying to interview ghosts.” They appear in sudden pulses, vanish just as quickly, and almost never leave evidence of what they have been doing.

That line stayed with me. For all the attention their venom gets, and the warnings posted along beaches, and the stories from Cape Tribulation, the deeper mystery is not how box jellyfish kill. It is how they create the next generation. Despite a century of work, the full reproductive picture is only half understood.

What follows is the clearest summary scientists have pieced together so far. It is shaped by field notes, interviews, and the most consistent research published in the last twenty years.

Do Box Jellyfish Reproduce Sexually or Asexually?

Short version: both.

Longer version: it depends entirely on the life stage, the free-swimming medusa or the tiny bottom-dwelling polyp. During a conversation with Anders Garm in 2022, he described Cubozoa as “switching strategies the way some animals switch habitats,” and after reading the literature, that comparison felt right. Their life cycle adjusts to conditions in a way many marine animals cannot.

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Sexual Reproduction (Medusa Stage)

The adult medusa, the cube-shaped swimmer with venom-loaded tentacles, handles sexual reproduction. Males release sperm into the water, females release eggs, and in most species fertilization happens externally. This has been documented clearly in species such as Tripedalia cystophora and Chironex fleckeri.

A small 2016 lab study from Townsville reported what looked like brief embryo brooding in one cubozoan species. Researchers are careful with this finding and do not treat it as a rule. Cubozoans are famous for exceptions.

A long-standing misconception also needs to go:

Not all box jellyfish swim into rivers.

Some Chironex fleckeri populations hover near estuary openings, and juveniles may drift into brackish water. But strong evidence for deliberate movement into freshwater does not exist. A biologist at James Cook University once told me, “People think they are everywhere because sightings are dramatic. The absence of sightings is not biology, it is blind spots.”

Most of what we know about reproduction comes from a handful of species that researchers can reliably observe.

Asexual Reproduction (Polyp Stage)

After fertilization, the embryo becomes a ciliated planula larva. It swims for roughly two to six days, depending on species and temperature, before settling on a hard surface. It might choose the underside of a Cairns jetty plank, a mangrove root in Palau, or a piece of broken shell.

This is the hidden stage divers rarely encounter.

Polyps are tiny, pale, and surprisingly durable. In the lab, researchers have seen them:

• crawl across surfaces
• hide in cracks
• feed on microscopic plankton
• clone themselves through budding

In late 2023, a marine ecologist mentioned keeping a polyp alive through temperature swings that would have killed an adult medusa. The polyp phase is the bunker stage. The medusa phase is the high-risk stage.

When conditions improve, warmer water, stable salinity, enough food, polyps begin budding again or transform directly into juvenile medusae.

How Often Do Box Jellyfish Breed?

This part is more complicated than most online summaries suggest.

Many articles claim box jellyfish spawn once a year or die immediately after reproducing. The real picture varies by species and location.

Here is what the best evidence supports:

• Many species show seasonal spawning
• Chironex fleckeri often spawns in warmer months in Queensland
• Adults often appear semelparous, meaning they probably spawn once and die, but this is not confirmed for every species
• Some species might reproduce more than once according to recent histological work
• Wild medusae usually live only a few months, while polyps can survive for months or years

One senior researcher told me observing spawning in the field is like “trying to film a snowflake melting in fog.” Between turbid water, seasonal changes, and years with low medusa numbers, catching the exact moment is incredibly difficult.

The Box Jellyfish Life Cycle (A Full Walkthrough)

Throughout their lives, box jellyfish shift between two forms:

• Polyp, stationary, hardy, and asexual
• Medusa, mobile, venomous, and sexual

A researcher once described these as a survival mode and an expansion mode. That comparison still holds up today.

1. Planula (Larva)

After fertilization, the embryo becomes a small pear-shaped larva covered in cilia. Some species develop pigment spots. Depending on conditions, settlement can take anywhere from two to six days.

2. Polyp (Benthic Stage)

Once settled, the planula becomes a small polyp that is capable of:

• slow crawling
• cloning
• surviving harsh conditions
• reorganizing itself into a juvenile medusa

Here is the point students always get wrong:

Some species show modified forms of strobilation, but the typical scyphozoan multi-segment strobilation does not occur in Cubozoa.

True jellyfish, the scyphozoans, produce stacks of baby medusae like little pancakes. Most box jellyfish do not do this. Instead, the entire polyp transforms into one juvenile medusa. Kingsford and Mooney described this clearly in a 2014 Hydrobiologia review. A few rare cubozoan species may show modified strobilation, but this is not the norm.

3. Medusa (Pelagic Stage)

Once the medusa detaches, the life cycle changes tone. Medusae grow quickly, sometimes reaching full size within weeks. They also have surprising abilities:

• camera-type eyes with lenses and retinas
• directional swimming
• active hunting behavior
• venom strong enough to cause cardiac arrest in humans

Most adults seem to reproduce once and die. This pattern is not universal but it is common. Seasonal blooms in northern Australia likely reflect synchronized reproduction.

Do Box Jellyfish Lay Eggs?

You might see the term oviparous used online, although not in the traditional way.

Box jellyfish do not lay egg masses.

Instead:

• females release eggs into the water
• males release sperm
• fertilization happens externally
• embryos become planulae
• planulae settle and become polyps

A few species have shown brief brooding in lab settings, but this has not been widely confirmed in the wild.

Why Is Their Reproductive Biology So Hard to Study?

After comparing papers, field reports, and interviews, three obstacles appear consistently.

1. Their adult lifespan is short

A medusa often lives only a few months in the wild. Some individuals in laboratory conditions have survived longer. A researcher once joked, “Medusae do not wait for grant approvals.”

2. Their habitats make observation difficult

Most species prefer turbid shallows, mangrove edges, and estuaries. Visibility in these areas can drop from clear to nothing in seconds.

3. Their polyps are almost invisible

Polyps are tiny, transparent, and hidden in crevices. Very few wild colonies have ever been located. Most polyp research depends on cultured specimens in labs.

Put together, these challenges explain why a creature powerful enough to stop a human heart can still keep most of its reproductive behavior hidden.

Summary: What Science Currently Knows

• Box jellyfish reproduce sexually as medusae and asexually as polyps
• Fertilization results in a planula that settles and becomes a polyp
• Polyps clone themselves and can survive long periods
• Many adult medusae appear to spawn once and die, although this varies by species
• Much of their life cycle remains undocumented because both stages are hard to observe in the wild

If there is a single lesson here, it is that box jellyfish rely on a tough, long-lasting polyp and a fast, high-risk medusa. One quiet phase, one dramatic phase.

FAQs

References

Courtney, R., Browning, S., Northfield, T., & Seymour, J. E. (2016). Thermal and osmotic tolerance of ‘Irukandji’ polyps (Cubozoa: Carukia barnesi). PLOS ONE, 11(7), e0159380. https://doi.org/10.1371/journal.pone.0159380

Garm, A., O’Connor, M., Parkefelt, L., & Nilsson, D.-E. (2007). Visually guided obstacle avoidance in the box jellyfish Tripedalia cystophora. Journal of Experimental Biology, 210, 3616–3623. https://doi.org/10.1242/jeb.000141

Garm, A., Bielecki, J., & Petie, R. (2016). Hunting in the dark: Behavioural and morphological adaptations of Tripedalia cystophora. Biological Bulletin, 230(2), 117–127. https://doi.org/10.1086/BBLv230n2p117

Kingsford, M. J., & Mooney, C. J. (2014). The ecology of box jellyfish in tropical marine environments. Hydrobiologia, 727, 109–127. https://doi.org/10.1007/s10750-014-1813-5

Nilsson, D.-E., Gislen, L., & Garm, A. (2005). Structure and function of the complex eyes of box jellyfish. Philosophical Transactions of the Royal Society B, 360(1456), 2825–2832. https://doi.org/10.1098/rstb.2005.1730

Animal Diversity Web. (n.d.). Cubozoa (Box jellyfish) overview. University of Michigan. https://animaldiversity.org/accounts/Cubozoa/