Sunday 8 January 2012

When Aliens Fall In Love - Jellyfish Lifecycle

From the pulsating bell to the shimmying tentacles, jellyfish have got to be some of the weirdest lifeforms our planet has to offer. Oral arms wave mindlessly around a central mouth while stinging cells puncture and poison without a thought. Not only does the jellyfish not even have to be alive for it to sting, but the stinging tentacles needn't be attached to the rest of the body at all. The futility of such endeavours is totally lost on them, as if they're more a force of nature than an animal. The sheer depth of their apparent mindlessness is quite amazing. It's like watching someone watching TV. Or, with all these reality shows, watching someone watching someone watching TV on the TV.

But no matter how strange a jellyfish may seem as it drifts in the ocean's currents, its life history is stranger still. Jellyfish really are weird from start to finish.

It all begins with an adult jellyfish, known as a medusa. Actually there are two, a male and female. How very traditional! Many will just release their gametes into the sea and leave them to it. Some females hold the eggs within her body or oral arms, protecting them as they develop to the larval stage.

Jellyfish eggs grow into jellyfish larvae, known as planulae. These are tiny, oval shaped creatures covered in hair-like cilia. Beating the cilia allows the planula to swim around to some extent, although they're largely at the mercy of ocean currents just like their parents. This changes when the planula finds a suitable surface to rest on. At this point it ceases its wandering and transforms into a polyp to better take on life at the ocean floor. A most rebellious attitude.

At the polyp stage the jellyfish is much like a Sea Anemone. They are stuck to a surface with tentacles waving around for capturing tiny planktonic prey. Sometimes they remain at this stage for years! They might even reproduce! This is done asexually by budding, with a clone breaking off from the original. Can you believe that? One wonders if this jellyfish even needs to become a jellyfish at all.

The next step is strobilation, when the the polyp strobilates and becomes a strobila. I love sentences like that. Even if you know what it means it's STILL almost entirely useless!

Strobilation is when the polyp segments into a sort of tower of plates. Each layer becomes increasingly separated from the rest of the body until it's completely independent and swims away. These are the next stage, the ephyra. The number of ephyra released from each strobila varies, but it can be several dozen.

These, finally, are what we could call "baby jellyfish". They're just a couple millimetres across and have a strange, leggy, star shape to them. They eat tiny plankton in the water and live like a proper jellyfish, sort of swimming, mostly drifting. As they grow over the next weeks or months, the bell develops and they begin to look more like an adult.

There's only one place to go from here (aside from getting eaten).
It's not called a cycle for nothing!

Remarkable! You hear about relatives like the Portuguese Man o' War and it's difficult to imagine that they are a whole colony of individuals all working together to make one, single organism. With jellyfish we see another peculiarity: two individuals could have both come from a single polyp. In fact, 30 could be from one tiny polyp and another 30 from a clone of that original polyp! And I thought sextuplets was quite surprising! It's all gloriously mad.


Crunchy said...

Fact: jellyfish are actually a form of weather.

Joseph JG said...

Hahaha! Yeh! A form of weather with stinging tentacles!