Urochordata (Tunicates): Sea Squirts Explained Simply

Urochordata, commonly called tunicates or sea squirts, are sessile marine animals that spend their adult life fixed to rocks, shells or boat hulls. The name means "tail-chordates" because only their free-swimming tadpole larva shows chordate features, and the notochord is confined to its tail. Adults look like small leathery bags with two openings.

They matter because they demonstrate retrogressive metamorphosis — the larva is more advanced than the adult. The tadpole larva has a notochord, dorsal nerve cord and gill slits, but during metamorphosis it loses its tail, notochord and much of its nerve cord. This makes tunicates essential for understanding how the chordate body plan can be gained and then simplified.

Key Concepts

• Tunic or test: a tough outer covering made of tunicin, a cellulose-like carbohydrate secreted by the skin — unique among animals.
• Two siphons: an incurrent (oral) siphon that draws water in and an excurrent (atrial) siphon that expels it.
• Pharyngeal basket: a large perforated pharynx with many gill slits (stigmata) used for filter feeding.
• Endostyle: a groove in the pharynx that secretes mucus to trap food; it is homologous to the vertebrate thyroid gland.
• Retrogressive metamorphosis: the advanced larva transforms into a simpler sessile adult, losing the notochord and tail.
• Solitary vs colonial: some species live alone (e.g., Ascidia), while others form colonies of connected zooids (e.g., Botryllus).

How It Works

1. Beating cilia draw seawater in through the incurrent siphon into the large pharynx.
2. The endostyle secretes a sheet of mucus across the pharyngeal wall, trapping plankton and detritus.
3. Cilia roll the food-laden mucus into a cord and move it into the oesophagus and stomach for digestion.
4. Filtered water passes out through the gill slits into the atrium and leaves via the excurrent siphon.
5. Gas exchange happens across the thin pharyngeal wall, so the same current serves feeding and breathing.

Worked Example

A student counts siphons in a small colony of 12 sea squirt zooids. Each zooid has one incurrent and one excurrent siphon.

1. Siphons per zooid = 2
2. Total siphons = 12 zooids x 2 = 24
3. Incurrent siphons = 12, excurrent siphons = 12

So even a tiny colony has 24 openings pumping water — which is why dense tunicate colonies can filter large volumes of seawater.

Common Misconceptions

• "Sea squirts are sponges or plants." They are true chordates — their larvae have a notochord and dorsal nerve cord.
• "Adults have a backbone." The notochord exists only in the larval tail and is completely lost in the adult.
• "The tunic is a shell." It is a living, flexible covering secreted by the animal's own skin, not a mineral shell.

Real-World Applications

• They are a model for chordate evolution, with genomes that reveal the simplest version of chordate genes.
• Marine engineers study tunicate fouling to design anti-fouling coatings for ships.
• Tunicate cellulose is researched as a biomaterial for wound dressings and sustainable plastics.

Urochordates are sessile filter feeders whose tadpole larvae carry all chordate hallmarks but lose most of them as adults. Their tunic, siphons and retrogressive metamorphosis make them one of the most instructive groups in animal diversity.