Sea squirtsAscidiacea

A sea squirt starts life able to swim and think, and then gets rid of both.

The larva is a tadpole with a muscular tail, a nerve cord, a light sensor and a tilt sensor, and it has hours rather than days to find somewhere to live. When it does, it lands head first and cements itself down by the front of its head, and then dismantles the equipment it will never need again. The tail is absorbed. The notochord goes. Most of the larval nervous system is broken down and recycled, and what remains of the brain is a single knot of nerve tissue sitting between the two siphons. It is a permanent trade: movement given up for a fixed address in a good current.

What is left is a bag with two holes. Water in through one siphon, through a perforated basket lined with mucus that strains everything edible out of it, and out through the other. Squeeze one, or walk over a colony at low tide, and both siphons fire at once. The name is entirely literal.

The blood is where it gets strange. Cells in many species collect vanadium, a metal present in seawater only in traces, and concentrate it to levels millions of times higher than the water they are pumping, storing it inside the cell at an acidity close to that of stomach acid. No one has settled what it is for. Defense is the usual guess, and it remains a guess.

Not all of them are single animals. Many are colonies: dozens of individuals a few millimeters across, embedded in one shared tunic and arranged in rings and stars around a shared exit hole, so that what looks like a painted crust on a wreck is a settlement with communal plumbing. On this coast they take the shaded, awkward, current-swept surfaces that corals leave alone.