Thetys vagina Tilesius, 1802
Phylum Chordata / Subphylum Urochordata / Class Thaliacea / Order Salpida / Family Salpidae
Solitary salp (pelagic tunicate), Thetys vagina, Point Lobos CA, Pacific Ocean
Aggregate salp, Thetys vagina, Class Thaliacea, Monterey Bay, Pacific Ocean

Among the more bizarre visitors to the waters of Monterey Bay is this pelagic tunicate. 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 continuously pumps water through a mucous net to extract phytoplankton and other small particles (see video below of an aggregate chain). Although relatively uncommon in Central California, this widespread species can be found in temperate and tropical waters of the Pacific, Atlantic and Indian Oceans, to depths of about 150 meters.

Thetys vagina, video courtesy of Patrick Anders Webster, Carmel Bay CA, September 2014

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