Sea Squirt

Image: Nick Hobgood

Oh dear. It's that time again. That time when we, the proud and noble vertebrates, are forced to visit the other side of the family. Bah! Those spineless cousins of ours are so embarrassing! OK. Take a deep breath and hold your nose. We're going to say... "hello". Yuck!

Sea Squirts are around 2,300 species in the class Ascidiacea. This in turn is within the phylum Chordata, like we are, but they are not in the subphylum Vertebrata. That's where all us good, honest, hard-working vertebrates reside.

We've seen a couple non-vertebrate chordates before. There was the Lancelet, who was like a fillet of fish with uppity aspirations of being an actual fish. And there was the Salp, a tube of jelly who did nothing but take water in one end and let it out the other. This was how they ate, breathed and moved.

Now we're going one down, because Sea Squirts don't even move! They're sessile, spending all their time attached to the sea floor. And what do they do there, you ask? They have 2 siphons, one for taking water in, the other for letting water out. Hmph! I hope my taxes aren't paying for that!

Image: ozfoodie via Flickr

I can almost forgive them when they look pretty, though. Sea Squirts come in an amazing range of shapes and sizes, and reach anywhere between 0.5 and 10 cm tall (0.20 to 3.9 in). Some of them are even colonial, the more simple of which are individuals attached by a kind of root system while the more advanced may be physically fused together and form colonies several metres across.

Image: Nick Hobgood
I think it looks great because it looks disgusting

The best of them are the ones that look like a funky heart. Or, I suppose, HALF a funky heart. Which is, of course, even better. I will gladly pay taxes to have half-hearts all around the coral reefs! That's art. It's pretty good that they're not actual hearts though; Sea Squirts get their name from their habit of squirting a jet of water when they're bothered.

One big difference between a Sea Squirt and a heart is that Sea Squirts don't pump like a heart does. They just don't have the flexibility due to a tough, outer layer called the "tunic". It would be nice to think that they've dressed up for our visit, but the tunic is actually living tissue made of stuff not unlike the cellulose you find in the cell walls of plants.

Image: Nick Hobgood

The innards of a Sea Squirt is dominated by the pharyngeal basket, which makes the whole creature look like a sieve in a bucket and a literal basket case. Water enters the top end, drawn in by the beating of tiny, hair-like cilia. Sticky mucus on the inner wall catches plankton while everything else passes through the holes and goes out through the other siphon.

The mucus is constantly oozing downward, like a cold, biological lava flow which may or may not be worse than an actual lava flow depending on how much dignity you have. Once it reaches the bottom of the basket, it goes down into the stomach taking all that plankton with it.

Image: richard ling via Flickr
A colonial Sea Squirt.
Each white dot is an individual inhalant siphon,
all sharing one, large exhalent siphon at the top.

Digestive waste products leave through the "out" siphon, so... don't let Auntie Sea Squirt blow you a kiss. Even worse are some of the more advanced, colonial Sea Squirts. A whole bunch of different ones can all share the same "exit", and all their waste products are cast out into the sea together. It's a sewage system, basically. But disgusting because you can see it. Just imagine if all the aqueducts were full of crap! Not in my back yard.

Tiny ones! On stalks!

Right next to the stomach is the heart. Remember how some Sea Squirts look like half a heart? Well... they kinda have half a heart! It pumps for a few minutes and blood goes one way, then it pumps for another few minutes and blood goes the other way!

Even their actual blood is kinda weird as it contains huge amounts of a metal called vanadium. It's an element, found between lutetium and lanthanum in The Element Song.

Image: prilfish via Flickr
More colonies, even bigger and with gigantic chimneys of stink.

Sea Squirts have been found to contain concentrations of vanadium 10 million times higher than the surrounding sea water. They're hoarding the stuff like a dragon hoards gold. No one is quite sure why, but it may be some sort of defence against predators. Either that or it's like a drug to them and they're high like every day. It would make sense.

And then we have the gonads. One of each, since Sea Squirts are hermaphrodite. Indecisive, I guess. Colonial Sea Squirts can multiply via a whole range of different budding methods. Sometimes a parent just splits in two, other times lots of youngsters grow from the tissue of the parent.

More common is the good old fashioned eggs and sperm way. Sperm is always chucked out. Eggs are either also cast out or kept inside to develop. Either way, they soon develop into a tadpole.

Image: Arjan Gittenberger
Larval Sea Squirt. So much potential!

A tadpole? Yeh! Sort of... It has a mouth and a tail and an embryonic brain, and it has a hollow nerve tube running down its back, supported by a flexible rod called the notochord. THE notochord, from which the phylum Chordata gets its name.

Good little tadpoles have their notochord become a backbone so as to earn a living as a frog. Bad little tadpoles - Sea Squirt tadpoles - don't do that. They use all their wondrous gifts; the ability to tell light from dark, up from down, the sensuous delights of touch and the ability to MOVE, and immediately set about trying to lose them all.

Within a day or two, our larval Sea Squirt settles on the sea floor, secretes a load of adhesive stuff and goes through a metamorphosis. The tail and notochord are absorbed as spritely youth is overtaken by hollowed-out couch potato. What could have been a brain become a cerebral ganglion, a kind of simple, not-quite-brain that Winnie the Pooh would find embarrassing.

Sea Squirts. Doing everything they can. Everything.

Adult Sea Squirts still have senses for touch, chemicals and telling light from dark, it's just that... well, they can't do much about it can they? Except squirt. They don't even have boot straps with which to pull themselves up! No finger to pull out! And yet there they are without a want in the world... the cousins.

Almost makes a fine, upstanding vertebrate want to sit down and squirt for a living. I already have a heart... maybe the rest is overkill?