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Subphylum Urochordata

Urochordata (L., uro = tail, L., chorde = cord)

Important features of the group:

Members of the subphylum Urochordata are commonly known as tunicates or sea squirts. As adults, tunicates are sessile, marine filter feeders that are ecological and morphologically similar to sea sponges. Although adult urochordates share relatively little with other chordates, larval tunicates are free-swimming and have a "tadpole-like" body plan.

Note that while urochordate larvae exhibit all four of the chordate diagnostic characteristics (notochord, dhnc, pharyngeal slits, post-anal tail) most of these features are lost during the metamorphosis into the adult form. The adult urochordate retains only the pharyngeal slits (used in filter feeding) to hint at its membership in the phylum Chordata.

The Adult Tunicate

Tunicates are often found on the pilings, hawser, and breakwaters of clean ports and may be extremely numerous; at the end of their growing season they may be the most numerous organisms encrusting the habitat. Tunicates such as the ones in the plastic mounts in lab are best observed underwater since the body has a diaphanous appearance and collapses when brought out of the supporting water. Underwater it is normally fully expanded to form a translucent, elongated cylinder four to six inches long, with two short siphons extending from the animal’s anterior (top) region. The siphon at the anterior end is the incurrent siphon and slightly below it and dorsally is the excurrent siphon. You can determine which siphon is which by looking for pharyngeal slits below the incurrent siphon, and locating the intestine and anus that empty into the excurrent siphon.

The diagram and photograph below show a typical tunicate. Required structures are in bold face.

1. incurrent siphon
2. excurrent siphon
3. eye spots
4. neural gland
5. tunic
6. attachments to substrate
7. incurrent siphon tentacles
8. branchial chamber
9. mantle
10. atrial chamber
11. branchial basket with
pharyngeal slits
12. endosytle
13. dorsal lamina
14. esophagus
15. stomach
16. intestine
17. female and male gonads
18. reproductive ducts
19. heart

Subphylum Urochordata

Tunicates are chordates and are a primitive link between invertebrates and vertebrates. Adult sea squirts (a common name for tunicates) live a sedentary life attached to a sub-strate such as rock. Their body shape is plump and saccular; an inhalent siphon admits water into the body; an exhalent siphon allows the water current, filtered of food, to pass out of the body (in the center of the flower shape). Photo: Ali Pivoda

A larval tunicate - note the chordate characteristics in the diagrammatic view.

Metamorphosis of a larval urochordate. Note the loss of all chordate characteristics except the pharyngeal slits. Click image to enlarge.

Adult urochordates. Note the inhalant (larger) and exhalant siphons and the striking convergence with sea sponges.

Subphylum Urochordata: a tunicate. (a) An adult tunicate, or sea squirt, is a sessile animal commonly arranged in a U shape (photo is approximately life-sized). (b) In the adult, prominent pharyngeal slits function in suspension feeding, but the other chordate characteristics are not obvious. (c) In the tunicate larva, which is a free-swimming but nonfeeding "tadpole," the chordate characteristics are evident: It has a notochord, dorsal nerve tube, and a tail with muscle segments. (From Campbell 6th edition's Fig 34-3.)

Examples:

The subphylum Urochordata is made up of three distinct classes:

Ascidiacea - “sea squirts”, free-swimming larvae, sessile adults.
Larvacea / Appendicularia - pelagic, drift w/ currents, adults resemble ascidian larvae living in gelationous "houses". Follow these links for more on these weird creatures:
Thaliacea - pelagic, drift w/ currents, similar to adult ascidians.

Examples of each class are shown at right. You are not expected to learn the class names or know anything about the separate classes of urochordates. Any information about them is strictly optional and supplementary.

Class Ascidiacea

Sea squirts are chordates and are a primitive link between invertebrates and vertebrates. Adult sea squirts live a sedentary life attached to a sub-strate such as rock, as seen here nuzzled between sea urchins. Their body shape is plump and saccular; an inhalent siphon (here, on the left) admits water into the body; an exhalent siphon allows the water current, filtered of food, to pass out of the body (here, to the right). Many ascidian species, such as this one, are known for their bright color. Photo credit: NOAA Damage Assessment and Restoration Program

Class Larvacea (Appendicularia)

Class Larvacea. Oikopleura has the typical larvacean tadpole-like appearance with a tail that is 3 to 4 times the length of the body. When undisturbed, it produces 5 to 10 pea-sized mucous webs (the "house") a day that are used for feeding, protection and buoyancy. The house concentrates tiny food particles, which are transferred to the mouth for ingestion. Credit: jellieszone.com.

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