Tunicate

Lore & Background

Some are solitary and sessile, attached to rocks or other hard surfaces, while others are colonial, replicating by budding into units called zooids. They are marine filter feeders with a sac-like body and two siphons: an incurrent siphon for drawing in water and an excurrent siphon for expelling filtered water. Adult ascidians are permanently attached, but thaliaceans (pyrosomes, doliolids, salps) and larvaceans swim in the pelagic zone as adults.

Reader's Guide

Tunicates are significant as the only chordates that have lost myomeric segmentation, yet they retain key chordate features in their larval stage, confirming their close evolutionary relationship to vertebrates. Their fossil record is rare, but the well-preserved Middle Cambrian fossil Megasiphon thylakos shows that the basic tunicate body design existed 500 million years ago. Tunicates also produce microscopic spicules that can be preserved as microfossils, though they are often mistaken for sponge spicules. Their unique tunic, made of proteins and carbohydrates, acts as a growing exoskeleton, and their classification remains under study, with evidence suggesting that Ascidiacea may be paraphyletic.

Did You Know?

• Tunicates are the only chordates that have lost myomeric segmentation, except possibly for the seriation of gill slits.
• The tunic is composed of proteins and carbohydrates, including tunicin, a variety of cellulose, and can grow without being shed.
• Some tunicates, like sea squirts, squirt water when disturbed, giving them their common name.