Tunicate
Marine chordates with a tunic exoskeleton and tadpole-like larvae.
Tunicates are marine invertebrates that form the subphylum Tunicata within the phylum Chordata, a group that also includes all vertebrates. The name Tunicata, coined in 1760 by nurseryman James Lee, refers to their distinctive outer covering, a "tunic" made of proteins and carbohydrates that functions as an exoskeleton; this tunic can be thin and gelatinous or thick and tough depending on the species. At one time, the group was called Urochordata, and that term is still occasionally used, but Tunicata has precedence and is now the standard scientific name, established by Jean-Baptiste Lamarck in 1816. Despite their simple adult form, tunicates are clearly close relatives of vertebrates because, during their mobile larval stage, they possess key chordate features: a notochord, a hollow dorsal nerve cord, pharyngeal slits, a post-anal tail, and an endostyle, giving them a tadpole-like appearance. They are unique among chordates in having lost myomeric segmentation, though doliolids retain segmented muscle bands. Tunicates are filter feeders with a sac-like body and two siphons: an incurrent siphon draws in water for respiration and feeding, and an excurrent siphon expels it. Many adult ascidians are sessile, permanently attached to hard surfaces like rocks or ship hulls, while thaliaceans (including pyrosomes, doliolids, and salps) and larvaceans swim in the open sea. Some tunicates live as solitary individuals, while others form colonies through budding, with each unit called a zooid. About 3,000 species exist worldwide, mostly in shallow waters, with ascidians being the most numerous. They display a wide range of colors and forms, sometimes resembling seeds, grapes, or bottles; one of the largest, the sea tulip *Pyura pachydermatina*, can grow over a meter tall. Common names include sea squirts, named for their habit of squirting water when disturbed, and sea liver or sea pork, based on the appearance of their dead colonies.
- field
- Marine biology, chordate taxonomy
- known_for
- Being chordates with a tunic-like exoskeleton; filter-feeding siphons; larval stage resembling a tadpole
- oldest_fossil
- Megasiphon thylakos from the Middle Cambrian, about 500 million years ago
Quick Facts
- Taxon
- Tunicata
- Subdivision
- "Ascidiacea" / Thaliacea / Appendicularia / Yarnemia? / Megasiphon · ia / Shankouclava?
Facts from the source article.
Lore & Background
Tunicates exhibit a wide range of appearances, from solitary individuals to colonial aggregations of units called zooids. Their sac-like, water-filled bodies are enclosed in a unique outer covering known as a tunic, which acts as an exoskeleton and is composed of proteins and carbohydrates. This tunic varies greatly among species, sometimes being thin, translucent, and gelatinous, while in other cases it is thick, tough, and stiff. The animals are found in a variety of solid or translucent colors and can resemble seeds, grapes, peaches, barrels, or bottles. Some species, like the stalked sea tulip, are supported by a stalk, but most attach directly to a substrate such as rock, shell, coral, seaweed, mangrove root, dock, piling, or ship’s hull. About 3,000 species inhabit the world’s oceans, primarily in shallow waters, though fewer than 100 ascidian species occur at greater depths. The most numerous group is the ascidians, which are sessile and permanently attached as adults. In contrast, thaliaceans (including pyrosomes, doliolids, and salps) and larvaceans swim in the pelagic zone throughout their lives. Defining characteristics include two tubular openings, or siphons: an incurrent siphon for drawing in water and an excurrent siphon for expelling filtered water. During their mobile larval stage, tunicates possess a notochord, hollow dorsal nerve cord, pharyngeal slits, post-anal tail, and an endostyle, resembling a tadpole and confirming their chordate affinity. They are the only chordates to have lost myomeric segmentation, though doliolids retain segmented muscle bands.
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.
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