- Also called:
- urochordate
- Related Topics:
- sea squirt
- larvacean
- Thaliacea
- Pleurogona
- Doliolida
- On the Web:
- National Center for Biotechnology Information - Early Development in Tunicates (Nov. 02, 2026)
tunicate
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What are tunicates?
Tunicates are marine animals belonging to the subphylum Tunicata (Urochordata) within the phylum Chordata. They are small, typically 0.4 to 2.0 inches long, but colonies can reach 59 feet. They inhabit oceans from the polar regions to the tropics.
Adult tunicates are usually encased in a robust tunic that contains cellulose. They possess two siphons: the branchial aperture for water entry and the atrial aperture for the exit of water, wastes, and gametes. Water circulation occurs through ciliary activity on the pharynx of the animal.
Most tunicates are hermaphrodites and can reproduce sexually or asexually through budding. They have a tadpole larva in their life cycle, which, in some groups, resembles the adult form.
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Where do tunicates live?
Tunicates are marine animals inhabiting ocean waters from the polar regions to the tropics. They are small, typically 1 to 5 centimeters long, but colonies can reach 18 meters. Tunicates are found as free-swimming plankton throughout the oceans, while sessile forms attach to solid surfaces like wharf piles, ship hulls, rocks, and seashells. Some tunicates, especially in the tropics, have symbiotic relationships with unicellular plants and blue-green algae, which may provide them with food.
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How do tunicates get their food?
Tunicates are filter feeders that eat small food particles suspended in the water. Cilia, or hairlike extensions, on the gill slits draw a current of water into the mouth and through the pharynx. The endostyle, a glandular organ, secretes a mucous sheet that filters food particles from the water. Cilia lining the pharynx then move the food-rich sheet of mucus to the posterior gut.
Some tunicates, mainly ascidians and thaliaceans, use an internal mucous sheet to eat small plants (phytoplankton). Others trap small animals, and some, especially in the tropics, have a symbiotic relationship with unicellular plants and blue-green algae that provide them with food. Appendicularians secrete a mucous house around themselves and use tail undulations to create a feeding current, filtering microscopic organisms (nannoplankton) with a fine mucous sheet.
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What is unique about the life cycle of tunicates?
Tunicates, members of the subphylum Tunicata (Urochordata) within the phylum Chordata, are small marine animals found throughout the world's seas.
What makes the tunicate life cycle unique:
* They have a tadpole larva in their life cycle, sharing features with other chordates. However, most of these chordate features disappear during metamorphosis.
* The larval stage is brief, as the larva focuses on finding a place for the adult to live. The larva does not feed.
* Tunicates can reproduce sexually or asexually (budding). They are mostly hermaphrodites and generally avoid self-fertilization.
* Some tunicates form colonies through asexual reproduction. Thaliaceans have alternating phases, where the first phase reproduces by budding, and the resulting individuals may release sperm and eggs.2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How are tunicates related to other chordates, like fish or humans?
Tunicates are members of the subphylum Tunicata (Urochordata) within the phylum Chordata, which also includes vertebrates like fish and humans, as well as cephalochordates. Chordates are characterized by features such as a notochord, a dorsal nerve cord, gill slits, and a tail that extends behind the anus.
Tunicate larvae possess chordate features like a notochord, dorsal nerve cord, and tail, but these are lost during metamorphosis into the adult form. Adult tunicates are marine animals that are typically benthic or pelagic. Vertebrates, including fish and humans, share a common ancestry with tunicates and cephalochordates.
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Why are tunicates important in studying the evolution of animals with backbones?
Tunicates, members of the subphylum Tunicata (Urochordata) within the phylum Chordata, are crucial in studying the evolution of animals with backbones because they possess features common to other chordates during their larval stage. The debate over whether the first ancestral chordate resembled a tunicate or a cephalochordate is ongoing. The classical theory suggests a cephalochordate-like ancestor, while an alternative proposes a tunicate-like ancestor, with other subphyla arising from modifications of the tadpole larva. Tunicates likely branched off before the common ancestor of cephalochordates and vertebrates, as the latter share similarities in neuroanatomy and biochemistry.
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tunicate, any member of the subphylum Tunicata (Urochordata) of the phylum Chordata. Small marine animals, they are found in great numbers throughout the seas of the world.
Adult members are commonly embedded in a tough secreted tunic containing cellulose (a glucose polysaccharide not normally found in animals). The less modified forms are benthic (bottom-dwelling and sessile), while the more advanced forms are pelagic (floating and swimming in open water). A characteristic tadpole larva develops in the life cycle, and in one group (the appendicularians, or larvaceans) the adult closely resembles this larva, which has many features in common with other chordates. Most chordate features disappear at metamorphosis.
General features
Size range and diversity of structure
The tunicates are divided into three classes: Ascidiacea (ascidians, or sea squirts), Appendicularia (Larvacea), and Thaliacea. Ascidians are largely benthic animals. They often form colonies, comprising a few to many individuals (zooids), which reach up to two metres in length. Solitary (noncolonial) forms range from one millimetre to over 20 centimetres in length. The adult appendicularian resembles the tadpole larva of other tunicates. The body is enveloped in a “house,” with which the animal nets food. Small (usually around five millimetres in length, including the tail) and simple, appendicularians do not form colonies. They spend their entire lives in the open sea. The thaliaceans (pyrosomes, dolioloids, and salps) are also pelagic. Their structure suggests that they are ascidians modified in adaptation to conditions in open water. They have specialized modes of reproduction, sometimes with a complicated alteration of sexual and asexual phases. Pyrosomes form long, tubular colonies. Dolioloids and salps occur both as solitary individuals and as chains.
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Distribution and abundance
Tunicates are distributed in ocean waters from the polar regions to the tropics. Free-swimming tunicates are found throughout the oceans as plankton, while sessile forms grow mainly on solid surfaces such as wharf piles, ship hulls, rocks, and the shells of various sea creatures.
Importance
Although rarely eaten by humans, tunicates are an important link in the food chain and thus indirectly provide humans with a source of food. Tunicates contain some unusual chemicals, and some of these may prove useful as drugs. Some tunicates are fouling organisms that grow on ships’ hulls. Their main interest to humans is in providing clues to the possible ancestry of vertebrates.
Natural history
Reproduction and life cycle
With rare exceptions, tunicates are hermaphrodites, but reproduction may be by sexual or asexual (budding) means. In general, hermaphroditic animals do not self-fertilize (i.e., provide both the male and female gametes) if they can avoid doing so, a rule that seems also to be true of tunicates. In primitive forms the eggs are fertilized, and development takes place, in the surrounding water, but often embryos are retained in the female’s atrium or elsewhere until the larva is developed.
The larval stage is brief; the larva does not feed, but concentrates on finding an appropriate place for the adult to live. In keeping with this motile phase, the muscular tail comprises two-thirds of the larval body; it is supported by a notochord and contains a nerve cord. Gravity- and light-sensitive sensory vesicles along the dorsal surface of the larval body orient the animal as it swims. After a period of up to a few days, the larva will settle and attach itself to a surface using three anterior adhesive papillae. As the larva metamorphoses into an adult, the tail resorbs, providing food reserves for the developing animal. Free-swimming tunicates metamorphose without attachment.
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Colonies are formed by asexual reproduction, with zooids usually being formed by budding. In thaliaceans, two groups (dolioloids and salps) have a complex system of alternating phases; the first phase reproduces by budding, and the resulting individuals may release sperm and eggs.