The Tunicata or „Urochordata“, are commonly known as sea squirts. Sea squirts or tunicate are also called sea grapes, because bunches of sea squirts look like bunches of grapes. Sea squirts have transparent or translucent tunics made of cellulose. They vary in size from a few millimetres to 30 cm in length and are cylindrical, circular, or irregular in shape. The body of an adult tunicate is quite simple, being essentially a sack with two siphons through which water enters and exits. Sea squirts feed by drawing water in through their incurrent siphon. Food particles are filtered through the pharynx and digestive tract. Waste products are ejected out the excurrent siphon.
Tunicate have both male and female reproductive organs. They spawn by releasing eggs and sperm into the water. After about three days, eggs develop into tadpole-like larvae. Sea squirt larvae are fairly sophisticated; they have long tails, a primitive eye and backbone, called a notocord, a slender nerve cord and a hollow, enlarged brain. The free-swimming period lasts for a short time, after which the larvae settle and attach to a hard surface with an adhesive mechanism on the head. In about three to four days, the tail, nerve cord and notocord are absorbed, leaving only a small mass of nerve tissue. The sea squirt's body and siphons, along with the digestive, reproductive and circulatory organs, soon develop.
Sea squirts are very tolerant of polluted water. Their defences against predators include sulphuric acid secretion and the accumulation of vanadium, a toxic heavy metal.