How Do Jellyfish Have Sex? | Science Me

How Do Jellyfish Have Sex?

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Jellyfish sex is really weird. Although if jellies could talk, they'd argue they're ancient animals who mastered sexual reproduction long before us. Frankly, we're the ones odd-balling it with our penises and vaginas and miserable, painful childbirth.

So how do jellyfish have sex?

1. The Male Releases a Joyless Mist of Sperm

The male's role in procreation begins and ends with a release of sperm into the ocean. Ocean currents disperse the sperm until it's sucked up by a female jellyfish.

Illustration of male jellyfish releasing sperm into the ocean

It's a little presumptuous, but it's effective.

"Sucked up? But how?" you ask, wide-eyed and captivated.

The sperm wafts into an opening in the female jelly—an opening which functions as a vagina, a mouth, an anus, and any other specialised orifice you care to imagine.

Jellyfish are simple animals and evolution didn't bestow them with complex internal systems. (Indeed, if it did, they would no longer be jellyfish.) They have no blood, no brain, and no organs. In fact, the entire body of a jellyfish is basically a glob of internal glue, surrounded by a one-cell-thick layer of skin.

That's pretty much it for jellyfish anatomy.

2. Fertilisation Produces Jellyfish Larvae

Inside the female, the sperm do their thing and fertilise her eggs-in-waiting. This is the crux of sexual reproduction—where the DNA of two parents mingle to produce a genetically original offspring.

The genital dance we associate with sex on land is completely unnecessary in the ocean. The water provides a top notch medium for the eggs and sperm to meet.

In fact, many aquatic species see fertilisation take place outside the body all together, with females doing an unfertilised egg dump in the somewhat blind hope they'll be fertilised by passing sperm.

Even when internal fertilisation takes place, the female's role is brief. She hosts the zygotes for a few days until they hatch into free-swimming larvae. Ready to meet the world, they swim off into the ocean and the mother buggers off.

Illustration of planula jellyfish larvae

Larval jellies look nothing like jellyfish.

This seems like a sensible time for the larvae to develop into adults and complete the reproductive cycle of jellyfish. But that would be far too easy. The whole story of jellyfish sex gets a lot more convoluted...

3. The Larvae Hunker Down and Become Polyps

After a few days of furiously beating their cilia against the ocean currents, the larvae settle down on the sea bed or any other convenient substrate. Rocks, shells, shipwrecks, car tyres, pool tables—you name it.

The larvae develop into polyps: stalk-like structures that look more like plants than jellyfish.

In fact, jelly polyps most closely resemble their sister class, the humble sea anemone, in a another wink to the theory of evolution by natural selection.

Illustration of jellyfish larvae developing into young polyps

Jelly polyps look like tiny sea anemones

They do, however, have the characteristic tentacles of jellies which ring the upward-facing mouth.

You might think that because we can sort-of see a jellyfish emerging here, the cycle is almost complete. Don't think that! There's an even weirder thing about to happen. Not only will the polyps drop their tentacles, they'll also go through a round of asexual reproduction.

4. The Polyps Clone Themselves to Form a Colony

As the polyps grow, they go through a process called strobilation where they divide in half to produce a genetically identical twin. Then each twin divides again. And again. Soon, an entire colony of clones emerges, all from a single original larva.

Illustration of jellyfish polyps dividing into colonies

Each larva is now a colony of immature jellyfish

This budding process is a type of asexual reproduction. Only a single parent is involved, so there's no genetic variation produced, except in rare circumstances where a mistake is made in DNA replication.

Why do this? As with everything in life, the jellyfish lifecycle occurred randomly and stuck around because it conveyed a survival advantage.

The main advantage of strobilation is that it produces many more jellies, multiplying the chances that the parent DNA will be passed on to the next generation.

What's really bizarre is the colony now undergoes polymorphism. Different clones develop different structures to fulfil specialised roles. Some capture prey, some feed, some reproduce, and some provide defence with stingers.

In this way, the colony covers every base, functioning extremely well despite being a small, primitive, sessile animal with multiple personality disorder.

5. The Polyp Colony Disbands

The colony, now called a strobila, finally matures and begins to separate. The uppermost clone peels away from the strobila and floats free, now a jeuvenile jellyfish, also known as ephyra.

Illustration of ephyra peeling away from strobila

Ephyrae more closely resemble the classic jellyfish form

The little squirts have just a few months to mature and reproduce before they die. Will they make it? Five hundred million years of evolution says yes.

6. Ephyrae Mature into Adult Jellyfish

When they leave the colony, the ephyrae are tiny—about the size of that freckle on your bottom that you don't know about.

But they get bigger, developing into the classic jellyfish form known as the medusa. While Irukandji jellyfish max out at 1-2cm in size, the largest species, Lion's mane jellyfish, can reach two metres across.

Illustration of ephyra developing into medusa form

The body grows larger, with longer tentacles, to become a medusa

Jellyfish Sex Round-Up

The lifecycle of jellyfish is a wild ride and, strictly speaking, this is only one version of events. Different jelly species undergo equally strange variations of the process above.

Depending on how much you enjoyed this article, you'll be overjoyed or disappointed to learn that comb jellies have a much simpler lifecycle altogether. They're hermaphrodites—possessing both the sperm and eggs to self-fertilise if needed—and the larvae grow directly into adult comb jellies.

What's truly amazing to me is the jellyfish lifecycle was produced by random mutation and pruned by natural selection. When the ocean environment created new pressures on these physiologically basic animals, the mutants among them provided novel solutions to parent a new generation of ocean-savvy mutants.

And this is what we end up with. A bizarre, twisty rollercoaster ride of submarine jellyfish sex.

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Rebecca Casale, Creator of Science Me

Rebecca Casale is a science writer in Auckland, New Zealand. If you like her content, please share it with your friends. If you don't like it, why not punish your enemies by sharing it with them?

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