I was living in Africa the first time I saw a pyrosome, and I nearly cried. I was doing research on plankton, which meant long days staring down a microscope plucking through tiny dead things. And then there it was. I actually gasped in recognition. My first real life pyrosome. Among many marine-inclined folks such as moi, pyrosomes are like unicorns. Completely improbable, utterly mysterious. And why?
For starters: if the Borg and the Clone Wars had a baby it would be a pyrosome. One long pyrosomes is actually a collection of thousands of clones, with each individual capable of copying itself and adding to the colony. And like members of the Borg, which are mentally connected, pyrosome members are physically connected– actually sharing tissues. And while the Borg live in a big scary ship, pyrosomes are the big scary ship. The whole colony is shaped like a giant thimble with a point on one end and an opening on the other, and in some species this opening can be up to 6 feed (2 meters) wide– large enough to fit a full grown human inside . Under normal circumstances, this sort of thing would scare the crap out of me.
But the pyrosome I found was cute. No larger than a jellybean. As I looked closer, I realized I could see right through it. Right into the guts. Each little “wire basket” is the stomach of one member of the colony. They take water in through a mouth on the outside of their space-ship body, pass it through the little basket to filter out the nom bits, and squirt water out the other end, into the big hollow space in the middle. Pyrosomes look terrifying, but like many giants of the sea, they’re actually filter feeders.
And it’s this filter feeding that gives pyrosomes their rocket power. Well, almost. My friend, aerospace engineer turned biologist, Henry Astley, put it like this: “jets have to do four things: Suck, squeeze, bang and blow.” Suck in air, compress it, explode some fuel, and blow the resulting force in the direction opposite where you want to go. The squeeze and bang part need compressible gas and fuel. Two things pyrosomes don’t have. But the other two, suck and blow, are expert territory for these guys.
Many animals use sucking and blowing movement to get around. Squid and octopuses use it, as well as certain types of jellyfish, which move like squid by sucking water under their bell and then shooting it out. This is a pulsing jet-like movement. Only one opening doing both the sucking and the blowing, and therefore the animal moves in fits and starts. Not at all like the smooth, constant stream of a true jets engine. The only animal to move with such fluid jet-like propulsion is the pyrosome. Because each member noshes on tiny plankton, they must constantly suck water in and over their baskets, and constantly blow waste out the hollow center. Thus they are moving at a steady, albeit painfully slow, speed.
So they’re giant, terrifying looking, and trolling through the depths of the ocean, waiting for you to swim in one end and get stuck. Maybe. But fortunately, in addition to being slow moving filter feeders, they’re also delicate and fluffy. One diver described a pyrosome saying “it felt like an exquisitely soft feather boa” . And this is why I almost cried when I saw my first wee pyrosome. Despite their improbable nature, these horrifying giants, the spawn of the worst movie villains, are actually delicate and fragile. The bizarre unicorns of the sea.
Update (03August 2013): Do not swim inside a pyrosome. While they are said to be quite soft, K Gowlett-Holmes reports finding a 2 meter (~ 6.5 ft long) pyrosome with a dead penguin trapped inside. K Gowlett-Holmes writes in a comment below: “The penguin had obviously swum in the open end of the tube then couldn’t turn – it was jammed in the apex of the pyrosome and its beak was just poking through the colony matrix. Even fairy penguins are quite strong – the fact it could not break free shows just how tough some pyrosomes are”. O_O
 Edward E. Ruppert, Richard S. Fox, Robert D. Barnes (2003). Invertebrate Zoology: A Functional Evolutionary Approach Seventh Edition. Brooks/Cole Thompson Learning, Belmont, California.