Pyrosome

Image: Nick Hobgood

Why'd you do it, Jimmy? Why'd you set fire to the orphanage and the puppy rescue centre and the soft-and-lovely-things factory? What are you? Some kind of pyromaniac?

No! I'm not a madman! I just... pyrosome.

Image: SERPENT Media Archive Project

Yet again we take to the ocean to learn about tunicates, our chordate cousins who have that tough, flexible notochord running down their back during embryonic development but never acquire a backbone. In fact, they don't even keep the notochord, they turn into a kind of jelly creature instead.

This time it's the handful of species that make up the genus Pyrosoma. They're colonial animals; each Pyrosome is formed of hundreds or thousands of individuals called zooids, themselves all clones of the first, founding zooid.

These zooids are like tiny Salps, just a few millimetres long. They spend their time taking water in one end, filtering out bacteria using slits, mucus and hair-like cilia, then passing the water out the other end. So if you've become bored of bacteria and want to spice up your food poisoning, there's one option.

Image: SERPENT Media Archive Project

Of course, you never see just one zooid! Instead you see what looks like one of those really annoying, long balloons people use to make animals. Just get one of those balloons and you have already created an impressively lifelike sculpture without even having to blow the blasted thing up!

The balloon here is gelatinous, closed on one end, open on the other and with the zooids embedded within. Their mouths are all along the outer surface, while all the water they take in is passed on into the empty space in the middle to flow out into the sea.

So it's actually like a balloon that's perpetually and very slowly losing air. The colony can swim a little, but they are mainly at the mercy of the ocean currents.

That, however, doesn't stop one Pyrosome called P. atlanticum from making extremely impressive vertical migrations. Really tiny colonies can migrate about 90 metres (300 ft), rising during the day and descending at night. Big colonies can travel some 760 metres (2,500 ft)! That's fantastic! And a wonderful example of what can be achieved with a massive army of clones.

Second half shows the gaping, muscular mouths of Salps.
First half shows the absurdity of Pyrosomes.

But how big is "big"? Well, for that particular species, 60 cm (2 ft) or so. Not too shabby. For other species... 20 metres (65 ft)! That's longer than most whales! The sight of a gigantic tube of jelly several times bigger than yourself is truly surreal. It just doesn't look like a living thing! And it doesn't look like much else either.

You can add to the madness when you realise why they're called 'Pyrosomes' in the first place. The word means "fire body", and comes from their spectacular bioluminescence. Before becoming "Darwin's bulldog" and inventing the term "agnostic", T. H. Huxley said:

“I have just watched the moon set in all her glory, and looked at those lesser moons, the beautiful Pyrosoma, shining like white-hot cylinders in the water”

Each individual zooid is capable of emitting light, and when one does it, the neighbours do too. Sometimes light from one colony will cause a whole other colony to start lighting up. It's as if they're communicating... talking... discussing...

I've seen The Day of the Triffids. Those tubes of jelly are up to something, they're just waiting for an opportunity... I know it.