The form of an adult squirt is based on a U-shaped tube with filtering equipment in its middle, all contained in a leathery tunic. The two ends of the tube are usually obvious. Sea water is pumped through the tube and suspended food particles removed. When taken out of water, or otherwise disturbed, the squirt contracts to produce an impressive jet of water, hence the name.

Sea-squirts look to be very simple creatures but, amazingly, belong to the same animal group (called chordates) as truly back-boned creatures such as birds, fish - and us. The squirtÕs key characteristic is the chord that runs down the back of its larvae. Akin to a rudimentary spine, it sets the squirts apart from animals such as sponges, sea anemones, worms and shellfish.

Depending on the species, sea-squirts live as individuals or in colonies with various degrees of togetherness. In colonies of the lightbulb sea-squirt (pictured), members are joined only at the base, and retain their individuality. In star sea-squirt colonies, the individuals virtually lose their identity and are embedded in clusters within a common tunic.
The chord present at the larval, or tadpole, stage helps the young squirt to swim efficiently. When the larva settles and develops into an adult, the chord is resorbed, so a spineless, totally sedentary way of life is adopted. Unkind students have compared this process to an ambitious academic obtaining a professorship!

Even the stationary tube-like adult has some intriguing features. A blood-circulation system unique to squirts pumps blood one way round the body for several seconds before reversing the flow for an equivalent length of time. The body wall of some species is highly acidic, to deter predators.

With their actively swimming young, sea-squirts can take over an area very quickly. In a disused dock basin, for example, they often dominate space on ropes put out to cultivate mussels. With an individual squirt capable of filtering 3 litres of sea water an hour, they will also compete fiercely with shellfish for planktonic food.