How Do Jellyfish Have Sex?
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.
Not like that. Here's how jellyfish really have sex.
The Sperm and Egg Dump
The male's role in procreation begins and ends with a sperm dump.
Ocean currents disperse the sperm until it's sucked up by a female jellyfish.
"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. The body is essentially three layers: an inside (the gastrodermis), a middle (the mesoglea, functioning as a hydrostatic skeleton), and an outside (the epidermis, one-cell-thick).
Meanwhile, some jellyfish species opt for external fertilisation, allowing the females to make an egg dump and be off with it.
The sperm do their thing and fertilise the eggs. This is the crux of sexual reproduction, where the DNA of two parent individuals mingles to produce genetically original offspring.
The genital dance we associate with sex on land is completely unnecessary in the ocean, where the water provides a top notch medium for eggs and sperm to meet.
Even when fertilisation takes place inside the female, her mothering role is brief. She hosts the zygotes for a few days until they hatch into free-swimming larvae and swim off into the ocean.
This seems like a sensible time for the larvae to develop into adults and complete the lifecycle of the jellyfish. But that would be far too easy. Jellyfish sex gets a lot more convoluted.
Clones and Polymorphisms
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 young polyps: stalk-like structures that look more like plants than jellyfish.
This is where jellies more closely resemble their sister class, the humble sea anemone, in another wink to the theory of evolution by natural selection. In time, they become scyphistomae with what look like tentacles around an upward-facing mouth.
You might think that because we can sort-of see a jellyfish emerging here, the lifecycle is almost complete. Don't think that! There's an even weirder thing about to happen. Not only will they drop their apparent tentacles, but they'll also go through a round of asexual reproduction.
The scyphistoma matures into a strobila, which means it's ready to strobilate. Sounds saucy? It isn't! Remember, jellyfish sex is not sexy. Not at all.
The polyp divides 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.
This budding process is a type of asexual reproduction. Since only one parent is involved, there's no genetic variation produced (except in rare circumstances where meaningful mistakes are made in DNA replication).
Why do this? As with everything in life, this part of the jellyfish lifecycle occurred randomly at least once and then 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.
Now the colony now undergoes polymorphism. Different clones develop different structures to fulfil specialised roles. Some parts of the colony specialise in capturing prey, some specialise in digestion, some do more cloning, and some defend the colony using stingers.
The tiny polyp covers every base, functioning extremely well despite being a small, primitive, sessile animal with multiple personality disorder.
Mobilisation and Medusae
Now, the uppermost clone peels away from the strobila and floats free. This jeuvenile jellyfish is called an ephyra.
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.
When they leave the colony, the ephyrae are tiny—about the size of that freckle on your bottom that you don't even know about. But they do get bigger, developing into the classic jellyfish form known as the medusa.
How big do jellyfish get? While Irukandji max out at just 1-2cm wide, the largest known jellyfish species, called the Lion's mane jellyfish, can reach 200cm across.
Jellyfish Sex Round-Up
The jellyfish lifecycle 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 emerged through random genetic mutation and pruned by natural selection. Each time the ocean created new pressures on these otherwise simple animals, the mutants among them provided novel and more complex solutions to parent a new generation of ocean-savvy oddballs. And this is what we end up with. A bizarre, twisty rollercoaster ride of submarine jellyfish sex.