A sketchbook explainer · 5 min
Explained for ages 8–12
Your turn
Each lab is one step from the video. Tap, drag and see what happens.
Each lab isolates one stage of the life cycle. The bloom model uses made-up numbers.
Tap, drag and slide. See what happens!
Drag through the months. How fast does the sea fill up?
Slide through the season and watch the numbers climb. Why can a bloom appear so suddenly?
Slide and watch the jellies come!
Made-up numbers, just to show the pattern.
Tap the jellyfish to release eggs and sperm cells. Watch a fertilised egg become a fuzzy larva.
Trigger spawning and watch fertilised eggs develop into ciliated planula larvae.
Tap the jelly. Watch the tiny babies!
Drag the larva onto the sand, the wall or a rock. Where can it hold on? Then make clones.
Settle the planula on a hard surface, then bud genetically identical polyps.
Drag the baby to a rock!
Tap the stack to pop off baby jellyfish (ephyrae), one at a time.
Release ephyrae from the strobila one segment at a time.
Tap to pop off baby jellies!
Change the water, the food and the hard surfaces. How many jellies do you get?
Adjust temperature, food and hard substrate in a toy model: polyps × ephyrae × survival.
Made-up numbers, just to show the idea. Real blooms depend on the species and place.
Step around the life cycle, then tap reverse to see the immortal jellyfish’s trick.
Step through the cycle, then reverse it as Turritopsis dohrnii does (medusa → cyst → polyp).
Tap reverse. The jelly starts again!
Some years the sea fills up with jellyfish, all at once. Where do they all come from? The secret is hiding on a rock!
A jellyfish has no brain and no bones. It is almost all water. Grown-up jellies let out tiny eggs and sperm into the sea. An egg and a sperm join and make a tiny fuzzy baby, like a little bean with hairs.
The fuzzy baby swims and looks for something hard. Sand is too wobbly, but a rock is just right. It sticks on and grows into a polyp. A polyp looks like a jellyfish upside down! The polyp makes copies of itself, so soon there are lots of polyps.
When the water gets cold, a polyp grows into a pile of little plates. The top plate wiggles free and swims away. It is a baby jellyfish! Pop, pop, pop: one polyp makes lots of babies, and each one grows into a big jellyfish.
Jellyfish are cousins of corals, which also start as tiny polyps: see how coral reefs are formed.
Most jellyfish reproduce in two different ways and live in two different bodies during one life.1 The bell-shaped jellyfish you see swimming, called a medusa, reproduces sexually: adults release eggs and sperm cells into the water.1 A fertilised egg grows into a tiny swimming larva, which sticks to a rock and turns into a small stalk-shaped polyp. The polyp then reproduces asexually: it makes copies of itself, and later it turns into a stack of discs that pop off one by one as baby jellyfish.14
That second step is the big surprise. Most of the young jellyfish in the sea weren't born straight from an egg. They were "budded off" by a polyp, and one polyp can make many of them. That's why a bay can go from a few jellies to thousands in one season.14
Here is the life cycle of a typical "true" jellyfish, like the moon jelly (Aurelia), which scientists have studied closely.3
| Stage | What it looks like | What it does |
|---|---|---|
| 1. Medusa (adult) | The familiar see-through bell with tentacles | Releases eggs or sperm cells; most adults are male or female1 |
| 2. Egg | A single cell, too small to see easily | Is fertilised by a sperm cell, in the water or in the female's body1 |
| 3. Planula (larva) | A tiny fuzzy oval, a bit like a flattened pear, covered in hair-like cilia | Swims for several days, looking for a hard surface1 |
| 4. Polyp | A small stalk with a ring of tentacles on top, like a tiny sea anemone | Catches food and makes clones of itself by budding13 |
| 5. Strobila | The polyp, now looking like a stack of saucers | Splits into about 10–15 discs (strobilation)1 |
| 6. Ephyra (baby jellyfish) | A tiny star-shaped disc with eight arms | Swims away and grows into a new medusa14 |
Then the new medusa grows up, releases eggs or sperm cells, and the cycle starts again.
They release eggs, but they don't lay them in a nest and they don't give birth to baby jellyfish. Adult jellyfish reproduce by spawning: they release eggs and sperm cells from their mouths into the sea, sometimes with a whole group spawning at the same time.1 In most species the egg and sperm meet in the water. In some species, the female takes in sperm and the eggs are fertilised inside her body.1
Moon jellies do it that way. Males release strands of sperm, females take them in, and the mothers then carry the developing larvae on their frilly oral arms for a while before letting them go.3
Most adult jellyfish are either male or female, and only a few are both (hermaphrodites).1 Some species can even change sex as they age.4 The polyp stage is different: it doesn't need a partner at all, because it reproduces by copying itself.
Yes. This is the part most people have never heard of. A polyp can grow a bud on its side that becomes a new, genetically identical polyp: a clone.13 In a 2022 lab study of moon jelly polyps, budding like this made more than 84% of all the new pieces the polyps produced.8 Over time, one polyp can turn into a whole patch of identical polyps on a rock, a shell or a dock post.
Polyps are tough, too. While a swimming jellyfish usually lives for a few months, polyps can keep living and reproducing for several years, or even decades.1
When conditions are right, a polyp changes shape. It grows a series of rings until it looks like a pile of little saucers. Scientists call this strobilation, and the stacked polyp is called a strobila.1 A stack usually has about 10 to 15 discs. The top disc wriggles free first and swims off as a baby jellyfish called an ephyra (plural: ephyrae), then the next, and the next.1
The Monterey Bay Aquarium says one moon jelly polyp can produce "tens of ephyrae".3 In the 2022 study, well-fed moon jelly polyps released between 10 and 24 ephyrae each time they strobilated, and hungry ones released 4 to 13.8
What tells a polyp to start? It depends on the species. Changes in water temperature (for many moon jellies, the water cooling) or in daylight can set it off.810
A jellyfish bloom is a sudden, huge rise in the number of jellies in one area.1 The life cycle explains how it's possible: one larva makes one polyp, the polyp makes many clones, and every clone can release a stack of babies. Britannica puts it simply: a single planula larva may end up producing numerous genetically identical medusae.4
Scientists think several things people do may help some jellies bloom more often:1
Some scientists argue that blooms are also part of natural cycles, so this is still being studied.1 The bloom toy in the explainer uses made-up numbers (polyps × babies per polyp × share that survive) just to show the idea. Real numbers depend on the species and the place.
One tiny jellyfish breaks the usual rules. Turritopsis dohrnii is only about 4.5 millimetres wide, smaller than the nail on your little finger.5 When an adult is damaged or stressed, for example by starving, it doesn't just die. It shrinks, pulls in its tentacles, and settles on the seafloor as a small blob. Within about 24 to 36 hours, that blob turns into a new polyp, which later buds off new jellyfish.5
It's as if a butterfly could turn back into a caterpillar. The trick works because its cells can change into different types of cells, a process called transdifferentiation.57 It can repeat this again and again, which is why it's nicknamed the immortal jellyfish.56 A 2022 study of its genes found it's the only known animal that can rejuvenate like this over and over after reproducing.6
Can the immortal jellyfish die? Yes. It may never die of old age in perfect conditions, but fish and turtles eat the jellies, and sea slugs and crustaceans eat the polyps.5 The trick was discovered by accident in the 1980s, when students keeping the jellies in a jar found new polyps where adults had been.5
There are thousands of kinds of jellyfish, and their life cycles vary.1 In box jellyfish, each polyp turns into just one jellyfish instead of a stack.14 In smaller relatives called hydrozoans, polyps bud little jellyfish from their sides.1 A few species, like the mauve stinger (Pelagia noctiluca), skip the polyp stage completely and spend their whole lives drifting in open water.9
Jellyfish have no brain, no heart, no blood and no bones. Instead, a simple nerve net lets them sense light, smells and touch.2 They are about 95% water.12 Even so, this simple body plan has worked for a very long time: jellies have been drifting in the oceans for at least 500 million years, long before the first dinosaurs.1 Jellyfish belong to the same animal group as sea anemones and corals.2 That's why corals start life as tiny larvae and polyps too: see how coral reefs are formed.
A safety tip: most jellyfish stings only cause minor discomfort, but a few kinds are dangerous.21 Even a jellyfish washed up on the beach, or a loose tentacle, can still sting. So look, but don't touch. If someone is stung, get help from a lifeguard or an adult. Rinsing with vinegar or seawater is common first aid, but the right treatment depends on the species, so follow local lifeguard advice.11
Jellyfish aren't the only sea animals with a strange way of growing up or rebuilding themselves. Sea stars can regrow lost arms (how starfish regenerate their limbs). Eels swim thousands of kilometres to spawn in one place that nobody has ever seen (how eels reproduce). And octopuses, which also have no bones, can change colour in a blink (how octopuses change color). Baby jellies go wherever the water takes them, so it helps to know what causes ocean currents and why the ocean is salty. Out in the open sea, jellies drift through water that looks deep blue for a reason (why the ocean is blue), and they share it with fish that pull oxygen straight from the water (how fish breathe underwater).
Adult jellyfish release eggs and sperm cells into the water, where most eggs are fertilised. Some species, like moon jellies, fertilise the eggs inside the female and carry the young larvae on their oral arms for a while. They never give birth to baby jellyfish: the babies are budded off later by a polyp.
Most adult jellyfish are either male or female, and only a few are both. Some species can change sex as they get older. The polyp stage doesn't need a partner, because it reproduces by making copies of itself.
Yes. The polyp stage reproduces asexually. It grows clones from its side (budding), and it turns into a stack of discs that pop off as baby jellyfish (strobilation). Adult jellyfish reproduce sexually with eggs and sperm, so most jellyfish use both methods in one life.
A baby jellyfish that has just popped off a polyp stack is called an ephyra (plural ephyrae). It is a tiny star-shaped disc with eight arms that grows into a bell-shaped medusa. Before the polyp stage there is also a larva, called a planula.
It depends on the species. Most swimming jellyfish live for a few weeks to a few months, and some live a year or more, or 2 to 3 years in aquariums. The polyp stage can live and keep reproducing for several years, or even decades.
Yes. Turritopsis dohrnii can turn back into a polyp when it is hurt or starving, and it can do this again and again, so it may never die of old age in perfect conditions. But it can still be eaten by fish, turtles, sea slugs and crustaceans.
Reproduction doesn't need a brain. Jellyfish have a simple nerve net that senses light, touch and chemicals in the water, and cues such as changes in water temperature or daylight trigger spawning and strobilation. Their bodies simply respond to the conditions around them.