Key to Class Ascidiacea (Tunicates, sea squirts)

Taken primarily from  Kozloff, 1987, 1996 p. 467 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)

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1a	Solitary Ascidians (not reproducing by budding) or social ascidians (reproducing by budding and connected, at least initially, by stolons or sheets of tissue and tunic material), but not embedded in a common tunic	2
1b	Compound ascidians, the several to many zooids of the colony embedded in a common tunic, which is usually gelatinous in texture	33
2a	Solitary ascidians (if the animals are in clusters, it is because they have settled on one another, not because they have reproduced by budding individuals); usually more than 1 cm in diameter	3
2b	Social ascidians (members of an aggregation connected, at least initially, by stolons or sheets of tissue and tunic material); members usually less than 1 cm in diameter	28
3a	Tunic transparent or translucent (in Molgula Pacifica, however, considerable foreign material is incorpated into the translucent tunic)	4
3b	Tunic opaque (in Bathypera, the opacity is due to crowded calcareous concretions just beneath the surface of the tunic)	13
4a	Both the oral and atrial apertures situated on a flattened disk consisting of several distinct plates (photo)	5
4b	Oral and atrial apertures not situated on a flattened disk that consits of several distinct plates	6
5a	Plates of disk usually showing concentric growth lines; muscle strands connecting the 2 central plates of the disk not visible through the tunic; no intermediary plates between the central and marginal plates; diameter of disk often exceeding 1.5 cm; intertidal, subtidal, and on floats	Chelyosoma productum
5b	Plates of disk without concentric growth lines; muscle strands connecting the two central plates of the disk visible through the tunic in preserved specimens; 1-3 intermediary plates between the central and marginal plates; diameter of disk rarely exceeding 1.5 cm; subtidal	Chelyosoma columbianum
6a	Not permanently attached to a firm substratum; tunic covered with short, hairlike projections	Molgula pugetiensis
6b	Attached to a firm substratum; tunic smooth or with papillae, but without hairlike projections	7
7a	Body low, somewhat flattened	8
7b	Body taller than wide	9
8a
8b
9a	Tunic with considerable foreign material, even pieces of algae, incorporated into it; atrial siphon about twice as long as the oral siphon; both siphons orange-red or pinkish red; height up to about 2 cm (open coast, intertidal and subtidal)	Molgula pacifica
9b	Tunic generally free of foreign material; oral and atrial siphons about the same length; siphons not some shade of red; height of mature specimens usually more than 2 cm	10
10a	Tunic with scattered large papillae	11
10b	Tunic smooth or irregulary wrinkled	Ascidia paratropa
11a	Body much taller than wide; tunic transparent, but tinted yellow-green; longitudinal muscle bands distinctly visible beneath the tunic; oral and atrial apertures borne on short siphons	Ciona instestinalis
11b	Body not much taller than wide; tunic transparent, but tinted yellow-green; longitudinal muscle bands not obvious beneath the tunic; oral and atrial apertures not borne on short siphons	12
12a	Rectum less than half the height of the body; a portion of the atrium expanded into a pocket in which embryos are brooded	Corella inflata
12b	Rectum more than three-fourths the height of the body; atrium not expanded into a pocket for brooding embryos (this species does not brood)	Corella willmeriana
13a	Surface of tunic underlain by stratum of conspicuous, closely spaced calcareous spicules (these with a long central spine and several lateral spines) (body usually not as tall as wide; diameter up to about 4 cm, but generally much smaller; color usually grayish white, sometimes pinkish; subtidal)	Bathypera feminalba
13b	Surface of tunic not underlain by a stratum of conspicuous calcareous concretions	14
14a	Tunic covered with spinelike projections (these are usually branched)	15
14b	Tunic smooth or wrinkled, but without spinelike projections	17
15a	Body attached to the substratum by a distinct stalk; spinelike projections generally brahched, but the branches are not arranged in circles (low intertidal and subtidal, common on floats in some parts of our region)	Boltenia villosa
15b	Body not attached to the substratum by a distinct stalk; branches of spinelike projections arranged in one or more circles	16
16a	Spinelike projections almost completely obscuring the rest of the tunic, each projection encircled by several rings of recurved, thornlike secondary spinelets; height up to about 10 cm	Halocynthia igaboja
16b	Spinelike projections not obscuring the rest of the tunic, each projection with a single irregular circle of several (usually 4-8) secondary spinelets; height not often exceeding 4 cm	Boltenia echinata
17a	Both the oral and atrial apertures situated on a flattened disk that consists of several distinct plates	18
17b	Oral and atrial apertures not situated on a flattened disk that consists of several distinct plates	19
18a	Plates of disk usually showing concentric growth lines; muscle strands connecting the two central plates not visible through the tunic; no intermediary plates between the central and marginal plates; diameter of disk often exceeding 1.5 cm; low intertidal and subtidal, common on floats	Chelysoma productum
18b	Plates of disk without concentric growth lines; muscle strands connecting the two central plates visible through the tunic in preserved specimens; 1-3 intermediary plates between the central and marginal plates; diameter of disk rarely exceeding 1.5 cm; subtidal	Chelysoma columbianum
19a	Body elongated, attached to the substratum by a rather small area of the tunic, and sometimes with a distinct stalk	20
19b	Body not elongated, attached to the substratum by a rather large are of the tunic	22
20a	With a slender, furrowed stalk that is about half the total height of the body; restricted to the open coast	Styela montereyensis
20b	Without a slender, furrowed stalk; not restricted to the open coast	21
21a	Body barrel-shaped; tunic peach-colored, smooth or with inconspicuous furrows and wrinkles; height up to about 6 cm	Halocynthia aurantium
21b	Body elongate, cylindrical, shaped much like a short cucumber; tunic brown or brownish red, with conspicuous lengthwise wrinkles; height not often more than 4 cm	Styela gibbsi
22a	Body broadened in such a way that the oral and atrial apertures are widely separated and on opposite sides of the body	Pyrua mirabilis
22b	Body not broadened in such a way that the oral and atrial apertures are widely separated and on opposite sides of the body	23
23a	Entire animal bright pinkish red or orange-red; tunic smooth and shiny	Cnemidocarpa finmarkiensis
23b	Animal not entirely red (if some bright red coloration is present, it is restricted to the areas surrounding the oral and atrial siphons); tunic usually wrinkled to at least some extent, not so smooth as to be shiny	24
24a	Body taller than wide; siphons red (tunic tough, leathery, coarsely wrinkled, mostly orange-brown; height up to about 5 cm; common intertidal and subtidal species)	Pyura haustor
24b	Body low, approximately hemispherical or appreciably flatter than hemispherical; siphons not red	25
25a
25b
26a
26b
27a
27b
28a	Individuals not much, if any, taller than wide	29
28b	Individuals much taller than wide	31
29a	Individuals translucent, greenish, attached to the substratum by a small portion of the tunic	Perophora annectens
29b	Individuals opaque and usually orange-red or brick-red (sometimes yellowish), attached to the substratum by a considerable portion of the tunic	30
30a	Contiguous individuals joined by slender stolons or by thin sheets of tunic material; the connections often indistinct (on rocks, intertidal and subtidal)	Metandrocarpa taylori
30b	Contiguous individuals joind to the extent that they nearly resemble zooids of a compound ascidian (usually on kelp, but also on rocks, intertidal and subtidal)	Metandrocarpa dura
31a	Individuals widely separated, less than 1 cm tall when mature; pharynx yellow-orange	Pycnoclavella stanleyi
31b	Individuals clustered, usually more than 1 cm tall when mature; pharynx, if pigmented, not yellow-orange (caution: the larvae, which may be brooded in the pharynx, are yellow)	32
32a	Dorsal lamina and endostyle bright fluorescent pink; margins of oral and atrial apertures not lobed	Clavelina huntsmani
32b	Dorsal lamina and endostyle not fluorescent pink; margin of oral aperture irregularly lobed, the atrial aperture with 2 lips, each divided into 3 small lobes	Clavelina sp.
33a	Colonies forming thin encrusting sheets, thick encrusting slabs (often irregular and lobed), or low mounds attached for most of their diameter to the substratum (the colony may be very lumpy, but it does not consist of club-shaped, mushroomlike, leaflike, or globular masses that have distinct stalks	34
33b	Colonies forming club-shaped, mushroomlike, leaflike, or globular masses, usually with distinct stalks (even when the masses are moundlike, they have narrow stalks)	43
34a	Colony forming encrustations not more than 3 mm thick and not lumpy (in Diplosoma listerianum, however, the common atrial apertures are on prominent elevations)	35
34b	Colony forming cakes, low mounds, or slabs (these often irregular, lobed, and lumpy) 5 mm to 1 cm or more in thickness	37
35a
35b
36a
36b
37a	Tunic with large, densely packed bladder cells and with disk-shaped calcareous concretions (both visible at a magnification of 15x) (color whitish tan or pink)	Cystodytes lobatus
37b	Tunic without bladder cells or disk-shaped calcareous concretions	38
38a	Most or all zooids arranged in systems (the individual zooids of a system have their oral apertures at the surface, but their atrial apertures join a cavity that has a single opening) (photo)	39
38b	Zooids not arranged in systems (the oral and atrial apertures of each zooid are at the surface)	41
39a	Tunic tough, leathery, encrusted with sand and with sand embedded in it; zooids with 2 body regions; pharynx of zooids with 3 rows of stigmata	Archidistoma psammion
39b	Tunic gelatinous or fleshy, sometimes encrusted with sand but not with sand embedded in it; zooids with 3 body regions; pharynx of zooids with at least 8 rows of stigmata	40
40a	Colonies forming smooth or irregular sheets 1-3 cm thick; zooids tan, yellowish, or orange-brown; pharynx with 7-15 (usually 8-12) rows of stigmata	Aplidium californicum
40b	Colonies forming flat-topped slabs up to 5 cm thick, often massive and sometimes encrusted with sand; zooids generally red or orange-brown; pharynx with 12-16 (usually 13-15) rows of stigmata	Aplidium solidum
41a
41b
42a
42b
43a	Pharynx with 4 rows of stigmata; colonies not sand-encrusted	44
43b	Pharynx with 5 or more stigmata; colonies often sand-encrusted	45
44a	Colony consisting of club-shaped or mushroomlike masses, sometimes broad mounds (but these have narrow stalks); color ranging from pale orange to dark purplish red; found in protected situations, especially on floats and pilings, as well as on the open coast	Distaplia occidentalis
44b	Colony consisting of a cluster of leaflike or paddlelike lobes; color ranging from cream to light orange-brown; restricted to the open coast	Distaplia smithi
45a
45b
46a
46b
47a
47b
48a
48b
49a
49b
50a
50b
51a
51b

  Aplidium californicum
  Aplidium solidum
  Ascidia paratropa
  Boltenia echinata
  Boltenia villosa
  Chelyosoma columbianum
  Chelysoma productum
  Clavelina huntsmani
  Cnemidocarpa finmarkiensis
  Corella inflata
  Distaplia occidentalis
  Distaplia smithi
  Halocynthia igaboja
  Metandrocarpa taylori
  Perophora annectens
  Pyura haustor
  Styela gibbsi
  Styela motereyensis