Chelyosoma productum Stimpson, 1864

Common name(s):  Flattop sea squirt, horseshoe ascidian

Synonyms:  Chelyosoma productum
Phylum Chordata 
Subphylum Urochordata
Order Phlebobranchia
Family Corellidae
Chelyosoma productum from 75 m depth, San Juan Channel.  Width of upper flattened disk on which the two apertures are located is 2.5 cm.  This large individual has numerous diatoms and other fouling organisms attached to the tunic.
(Photo by: Dave Cowles, July 2008 )
Description:   This solitary tunicate has a translucent or mostly opaque, light brown tunic.  The tunic is mostly smooth or slightly wrinkled, without spinelike projections.  The oral and atrial apertures, both of which are short, are located on a flattened disk.  Each aperture is surrounded by 6 plates, and other plates occur on the disk as well.  The plates usually have concentric growth lines.  The muscle strands connecting the two central disk plates are not visible through the tunic.  There are no intermediary plates between the central and marginal plates of the disk.  Height to 6 cm; disk diameter to 2.5 cm.  The tunic is whitish, gray, or brownish and appears slightly translucent.  It is smooth in small individuals but becomes opaque and more wrinkled in older specimens.  Individuals are often covered with fouling diatoms.  The siphons close by flaps (photo).  The oral siphon has 75-125 tentacles.

How to Distinguish from Similar Species:Chelyosoma columbianum has disk plates without concentric growth lines, the muscle strands connecting the two central disk plates are visible (at least in preserved specimens), there are one to 3 intermediary plates between the plates surrounding the apertures and the margin of the disk, and the disk diameter rarely is greater than 1.5 cm.

Geographical Range:  Prince William Sound, Alaska to San Diego, CA.  Those in the southern end of the range are usually smaller.

Depth Range:  Low intertidal and subtidal to 50 (75) m depth

Habitat:  Rocks; common on floats and pilings.  Often several individuals are clumped together.  Mostly in quiet waters.

Biology/Natural History:   This species is known to feed on barnacle and copepod nauplii (larvae),  eggs, and the larvae of gastropods and other ascidians.  It reproduces in spring, releasing its gametes in the early morning (it does not brood its young).  Fertilized eggs, which may be tan, yellow, or purple, hatch into tadpole larvae in 29-40 hours at 11C.  After swimming for a few hours to a few days, the larvae settle, preferring to settle on the tunics of individuals of the same species.
Predators include the seastar Orthasterias koehleri and the gastropodFusitron oregonensis (Oregon triton), which eats mostly large individuals (over 1.4 cm diameter).  Several commensal copepod species may be found inside the branchial chamber.

This species may contain up to 800 ppm of the metal vanadium in the tunic by weight.



Dichotomous Keys:
  Flora and Fairbanks, 1966
  Kozloff, 1987, 1996
  Smith and Carlton, 1975

General References:
  Harbo, 1999
  Johnson and Snook, 1955
  Kozloff, 1993
  Lamb and Hanby, 2005
  Morris et al., 1980
  O'Clair and O'Clair, 1998

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

In this closeup view , the two siphons can be seen opening on the tunicate's upper surface.  Several of the fouling organisms are also visible, 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)