This solitary tunicate
has a translucent or mostly opaque, light brown tunic. The
is mostly smooth or slightly wrinkled, without spinelike
The oral and atrial apertures, both of which are short, are located on
a flattened disk. Each aperture is surrounded by 6 plates,
plates occur on the disk as well. The plates usually have
growth lines. The muscle strands connecting the two central
plates are not visible through the tunic. There are no
plates between the central and marginal plates of the disk.
to 6 cm; disk diameter to 2.5 cm. The tunic is whitish, gray,
brownish and appears slightly translucent. It is smooth in
individuals but becomes opaque and more wrinkled in older
Individuals are often covered with fouling diatoms. The
by flaps (photo).
The oral siphon has 75-125
Common name(s): Flattop sea squirt, horseshoe ascidian
75 m depth, San Juan Channel. Width of upper flattened disk
the two apertures are located is 2.5 cm. This large
numerous diatoms and other fouling organisms attached to the tunic.
|(Photo by: Dave Cowles,
July 2008 )
How to Distinguish from
columbianum has disk plates without concentric
the muscle strands connecting the two central disk plates are visible
least in preserved specimens), there are one to 3 intermediary plates
the plates surrounding the apertures and the margin of the disk, and
disk diameter rarely is greater than 1.5 cm.
Sound, Alaska to San Diego, CA. Those in the southern end of
range are usually smaller.
Low intertidal and
subtidal to 50 (75) m depth
Rocks; common on floats
and pilings. Often several individuals are clumped
Mostly in quiet waters.
This species is known to feed on barnacle and copepod nauplii
eggs, and the larvae of gastropods and other ascidians. It
in spring, releasing its gametes in the early morning (it does not
its young). Fertilized eggs, which may be tan, yellow, or
hatch into tadpole larvae in 29-40 hours at 11C. After
a few hours to a few days, the larvae settle, preferring to settle on
tunics of individuals of the same species.
Predators include the seastar Orthasterias
koehleri and the gastropodFusitron
oregonensis (Oregon triton), which eats mostly
(over 1.4 cm diameter). Several commensal copepod species may
found inside the branchial chamber.
This species may contain up to 800 ppm of the metal vanadium
tunic by weight.
Flora and Fairbanks, 1966
Kozloff, 1987, 1996
Smith and Carlton, 1975
Johnson and Snook, 1955
Lamb and Hanby, 2005
Morris et al., 1980
O'Clair and O'Clair, 1998
General Notes and
abundances, unusual behaviors:
In this closeup view , the two siphons can be seen opening on the
upper surface. Several of the fouling organisms are also
such as the entoprocts seen above the siphon on the left.
Authors and Editors of Page:
Dave Cowles (2008): Created original page
CSS coding for page developed by Jonathan Cowles (2007)