Tunicate
Marine chordates with a tunic and tadpole-like larvae.
Tunicates are marine invertebrates that form the subphylum Tunicata within the phylum Chordata, a group that also includes all vertebrates. Their close evolutionary link to vertebrates is firmly established despite their deceptively simple adult forms. This kinship is revealed during their mobile larval stage, when they possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, a post-anal tail, and an endostyle, giving them a distinctly tadpole-like appearance. Notably, tunicates are the only chordates to have lost myomeric segmentation, though the gill slits may retain some seriation, and doliolids still exhibit segmented muscle bands. The name "Tunicata" was coined in 1760 by nurseryman James Lee, inspired by the creatures' unique outer covering, which he fancifully described as being "enclosed in a tunic." This tunic, composed of proteins and carbohydrates, functions as an exoskeleton; in some species it is thin and gelatinous, while in others it is thick and tough. Tunicates can live as solitary individuals or form colonies through budding, with each unit called a zooid. They are filter feeders, drawing water in through an incurrent siphon and expelling it through an excurrent siphon. Adult ascidians are sessile, permanently attached to hard surfaces, while thaliaceans and larvaceans swim in the open sea. The subphylum was established by Jean-Baptiste Lamarck in 1816, and though Francis Maitland Balfour later proposed the name "Urochordata," the original term Tunicata takes precedence and is now standard in modern science.
- subphylum
- Tunicata
- number_of_species
- about 3,000
- habitat
- marine, mostly shallow water
- key_feature
- tunic (exoskeleton of proteins and carbohydrates)
- larval_stage
- mobile, tadpole-like, with chordate features
- adult_form
- sessile or pelagic filter feeders with siphons
Lore & Background
Tunicates are marine invertebrates that belong to the subphylum Tunicata, a group within the phylum Chordata. Despite their simple adult form, their close relationship to vertebrates is evident during their mobile larval stage, when they possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, a post-anal tail, and an endostyle, resembling a tadpole. As adults, they are the only chordates to have lost myomeric segmentation, though doliolids retain segmented muscle bands. Their defining characteristic is a "tunic," an exoskeleton formed from proteins and carbohydrates. This covering can be thin, translucent, and gelatinous in some species, or thick, tough, and stiff in others. The name Tunicata, coined in 1760, means "clothed with a tunic." Tunicates are marine filter feeders with a water-filled, sac-like body and two siphons: an incurrent siphon for drawing in water and an excurrent siphon for expelling it. Some live as solitary individuals, while others form colonies through budding, with each unit called a zooid. Adult ascidians are sessile, permanently attached to rocks, shells, coral, seaweed, or ship hulls, while thaliaceans and larvaceans swim in the pelagic zone. They appear in a range of solid or translucent colors and may resemble seeds, grapes, peaches, barrels, or bottles. One of the largest, the stalked sea tulip *Pyura pachydermatina*, can grow over one meter tall. Common names include sea squirts, named for their habit of contracting and squirting water when disturbed, and sea liver or sea pork, due to the appearance of their dead colonies.
Reader's Guide
Tunicates are significant as the closest invertebrate relatives of vertebrates, providing insight into the evolution of chordates. Their larval stage exhibits all key chordate features, while adults often lose these traits, making them a model for studying developmental and evolutionary transitions. The subphylum includes about 3,000 species, with ascidians being the most numerous. Tunicates are the only chordates that have lost myomeric segmentation, though doliolids retain muscle band segmentation. Their tunic, containing cellulose (tunicin), is unique among invertebrate exoskeletons in that it grows with the animal. Tunicates are also notable for their colonial forms, where zooids can integrate into complex structures, and for their filter-feeding lifestyle using siphons.
Did You Know?
- Tunicates are the only chordates that have lost myomeric segmentation, except possibly in the seriation of gill slits.
- The tunic of tunicates is unique among invertebrate exoskeletons because it can grow as the animal enlarges and does not need to be shed.
- Some tunicates, such as sea squirts, are named for their habit of contracting their bodies sharply and squirting out water when disturbed.
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