Sea squirt larvae swim like tadpoles, then glue themselves down for life
A young sea squirt has a muscular tail, a nerve cord and a notochord, the hallmarks that link it to animals with backbones. Within a day or so it presses its head to a rock, secretes glue, absorbs its tail and becomes a stationary filter bag wrapped in cellulose, a coat more usually found in plants.
Ascidians, or sea squirts, are marine filter feeders found worldwide, usually in shallow, salty water. Around 2,300 species exist, living alone, in clumps joined at their bases, or as colonies of many tiny zooids. Adults range from about half a centimetre to 10 centimetres and stay fixed to rock, coral or shell. Their outer tunic is made of cellulose with proteins and calcium salts, yet unlike a mollusc shell it is living tissue, often with its own blood supply. Pulled out of the sea, many forcibly spray water from their two openings, which is how they got their common name.
Feeding is a matter of plumbing. Beating cilia draw water in through one siphon into a pharynx perforated like a sieve. A groove called the endostyle, possibly related to the vertebrate thyroid, lays down a sheet of mucus that traps plankton and carries it to the gut, while the water leaves via a second siphon. The heart is stranger still: it reverses the direction of flow every three to four minutes. In some species, blood cells carry vanadium, which colours the blood green.
Nearly all are hermaphrodites. Solitary species shed eggs to be fertilised in open water, and each embryo develops over 12 hours to a few days into a free-swimming larva. That larva's notochord always consists of exactly 40 cells. It senses light, gravity and touch, and within 36 hours settles, triggering an irreversible change in which tail and fins vanish and organs shift to adult positions.
Because the larva looks more advanced than the grown animal, the process is called retrogressive metamorphosis, and one hypothesis suggests true chordates arose from larvae that matured without settling. Colonial species, meanwhile, bud asexually and can survive for decades.
Source: Ascidiacea
