| Thetys vagina
Tilesius, 1802 Phylum Chordata / Subphylum Urochordata / Class Thaliacea / Order Salpida / Family Salpidae |
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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 continuous pumps water through a mucous net
to extract phytoplankton and other small particles. Although relatively
uncommon in Monterey Bay, this widespread species can be found in temperate and tropical
waters of the Pacific, Atlantic and Indian Oceans, to depths of about 150 meters.
All photographs © David Wrobel and may not be used or copied without permission!