Tethys vagina

Tethys vagina
With a length exceeding 30 cm, Thetys is truly an impressive member of the zooplankton.   It is the largest species of salp along the West Coast and is relatively easy to distinguish from all others.   Unlike most gelatinous animals, the body is relatively firm due to the thick spiny test (the test, or tunic, is the hard outer covering typical of many tunicates, hence the name for the group).  It retains its shape even when removed from the water.
 
Solitary individuals have 20 partial muscle bands (far more than other salps in the area) that are used for constricting the body while pumping water for feeding and locomotion.  A pair of pigmented posterior projections are very distinctive, as is the darkly colored, compact gut.   Aggregate chains may be several meters in length.  Individuals within the aggregate possess only 5 dorsally positioned muscle bands, and form double-row chains with zooid axes not quite at right angles to the length of the chain.  Like other salps, Thetys continuous pumps water through a mucous net to extract phytoplankton and other small particles.

Salps are common in equatorial, temperate, and cold seas, where they can be seen at the surface, singly or in long, stringy colonies. The most abundant concentrations of salps are in the Southern Ocean (near Antarctica). Here they sometimes form enormous swarms, often in deep water, and are sometimes even more abundant than krill. Since 1910, while krill populations in the Southern Ocean have declined, salp populations appear to be increasing. Salps have been seen in increasing numbers along the coast of Washington State.

References:
http://en.wikipedia.org/wiki/Salp
http://jellieszone.com/thetys.htm

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オオサルパ　Thetys vagina (Tilesius)
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