Revert to the Ascidian

I've spent most of the last week running full-tilt at work. I had a higher-than-usual number of talks and meetings to attend, got a stack of papers to grade, and have spent this weekend with an experiment that has me run to the ground. /end grad student whining.

One of the talks I attended this week was about ascidian development, which reminded me that I have been meaning to write about these cool invertebrates (some of our closest relatives!). 

This is a group of Ciona intestinalis, the solitary vase tunicate.

Ascidians are also called tunicates or sea squirts. Their body is covered by a protective covering called a tunic, hence "tunicate," and as you can see in the photo above, they have two siphons to pump water through their bodies. If you squeeze them, water shoots out the excurrent siphon, hence "sea squirt." The ones pictured above are solitary animals, but they also live in colonies like this orange species below.

The colonial ascidian Botrylloides violaceus. Each of the holes is a zooid, and the whole gloopy thing is a colony.

 Ascidians are part of a group of organisms known collectively as "fouling organisms" because they live on pretty much any hard substrate they can find. Sometimes that is rocks, and sometimes it is the underside of docks or the bottoms of boats. They generally grow really quickly and can outcompete other species like barnacles. Many species are nasty invaders all over the world.

I am, of course, particularly interested in tunicate larvae. They look like this:

Tadpole larvae of B. violaceus. Photo from UNH.

 These larvae are called tadpole larvae for obvious reasons. They do not feed, and they only live in the water for a few hours to a day before metamorphosing and turning into a little colony of ascidians.

And these larvae bring me to the part where they are your closest invertebrate relatives. They have all of the traits that make chordates unique as a phylum. So, just like you did when you were an embryo, they have a notochord, a muscular postanal tail, a hollow dorsal nerve chord, and pharyngeal gill slits. Many people are interested in studying the larvae because of these similarities with the rest of the chordates (like us!). I, on the other hand, am interested in the larvae because they can only disperse over short distances before they metamorphose -- and that can affect their ecology and evolution in turn.

Walter Garstang, author of The Ballad of the Veliger, did not write about ascidian larvae. But I have found this gem, by Andrew Lang, and excerpted the part of it describing metamorphosis here. Go read the whole thing -- it's a lovely metaphor of life.

Th' Ascidian tadpole, young and gay,
Doth Life with one bright eye survey,
His consciousness has easy play.
He's sensitive to grief and pain,
Has tail, and spine, and bears a brain,
And everything that fits the state
Of creatures we call vertebrate.
But age comes on; with sudden shock
He sticks his head against a rock!
His tail drops off, his eye drops in,
His brain's absorbed into his skin;
He does not move, nor feel, nor know
The tidal water's ebb and flow,
But still abides, unstirred, alone,
A sucker sticking to a stone.

-Andrew Lang, from "Man and the Ascidian"