How Do Jellyfish Have Sex?

How Do Jellyfish Have Sex?

Jellyfish sex is really weird. If they could speak, jellies would argue they're an ancient group of animals who mastered sexual reproduction a long time before us. Frankly, we're the ones who are odd-balling it with our penises and vaginas and miserable, painful childbirth.

Let's look at the jellyfish way of making babies.

Step 1: The Male Sprays His Sperm Everywhere

We begin with the male jelly releasing a joyless mist of sperm into the water. It drifts around in the ocean until it's sucked up by a female jellyfish.

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

The sperm is gobbled up by a single opening in the female - an opening which functions as a mouth, an anus, a vagina, and any other specialised orifice you care to think about.

Male Jellyfish Releases Sperm into The Water

Jellyfish are fairly simple animals in terms of their physiology and evolution has not bestowed them with complex internal systems.

No blood, no brain, no organs, no respiratory system, no circulatory system and no digestive system.

The body of a jellyfish is basically a glob of glue (mesoglea) surrounded by a layer of skin that's one-cell-thick (epidermis) which takes a slightly different form upon lining the inner cavity (gastrodermis).

And that's pretty much it for jellyfish anatomy.

Step 2: Fertilisation Takes Place Inside The Female

The sperm make their way inside the female jellyfish and fertilise her eggs-in-waiting.

She hosts the zygotes for a brief time until they hatch into free-swimming planula larva.

These are flat, immature jellies that look absolutely nothing like jellies. The hairy little grains are then released into the ocean to swim about and get a feel for the place.

Planula Larvae of Jellyfish

You'd think this would be a sensible time for the larvae to develop into adult jellies and complete the cycle. But that would be too easy.

The story of jellyfish sex gets a lot more convoluted.

Step 3: The Juveniles Hunker Down to Become Polyps

After a few days of furiously beating their cilia only to be tossed around by ocean currents, the larvae settle down on the sea bed.

Jellyfish have two official body forms. The medusa form is the pulsating bell-shaped jelly with tentacles. The polyp form takes a little more explanation.

On landing on a substrate, jellyfish larvae develop into polyps: stalk-like structures that look more like plants than jellyfish at this stage.

They do, however, have the characteristic tentacles by now, which ring the upward-facing mouth.

Fun fact for evolutionary biologists: at this point in its lifecycle, the immature jellyfish polyp resembles its sister class: the humble sea anemone.

Polyp vs Sea Anemone

Step 4: They Clone Themselves to Form a Colony

As the polyps grow, they go through a process of strobilation.

This is where they reproduce themselves asexually by budding off bits here and there, which grow into genetically identical polyps for the colony, all the while sharing the same "stomach". These are clones.

Growing lots of different polyp-limbs is the equivalent of growing a third arm out of your chest. Then a fourth arm. Then a fifth arm. Then a leg. Then another head. And then calling yourself a colony.

Colonies are great for survival, though. In the case of jellyfish, a colony can undergo polymorphism, where different polyps can develop different structures to fulfil specialised roles.

Some polyps specialise for feeding (gastrozooids), some capture prey (dactylozooids), some reproduce (gonozooids), and some protect the colony with their stingers (nematophores).

In this way, the colony covers every base and functions extremely well despite being a relatively small, primitive and, for now, sessile animal with multiple personality disorder.

Crucially, this asexual budding also results in star-like blobs called ephyra, which go on to become adult medusa jellyfish we can more easily recognise.

Strobilation of Jellyfish Polyp

Step 5: The Colony Breaks into Bits and Swims Away

In the final stage, the ephyrae break off from the main polyp body and swim away.

They will soon develop into mature medusa jellyfish forms and the whole insane cycle starts anew.

Ephyra Develops into Medusa Form

The processes surrounding jellyfish sex may sound complicated, and it has evolved entirely at the hands of nature.

When the ocean environment creates new pressures on these marine creatures, the mutants among them provide novel solutions and go on to parent a population of offspring mutants.

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

How Do Jellyfish Have Sex?
Rebecca Turner Bio

ABOUT THE AUTHOR: Rebecca Turner is the founder of Science Me, a biology student and a nurturer of tiny people. Read more about her story here.