Twin-sailed Salp, Thetys vagina

Twin-sailed Salp, Thetys vagina, Oozooids. Collected by hand off the surface about three miles at sea in coastal waters off San Jose del Cabo, Baja California Sur, March 2018.  Length: 12.0 cm (4.7 inches).

Phylogeny:  The Twin-sailed Salp, Thetys vagina (Tilesius, 1802), is a pelagic planktonic tunicate that is a member of the Salpidae Family of Salps. The Genus Thetys  is one of thirteen genera in this family and the Thetys Genus has just one member, this species. In Mexico it is known as sálpido vagina. The specific name vagina is not named after the modern day name for an anatomical feature, but for the earlier Latin word meaning scabbard or sheath.

Morphology:  Twin-sailed Salps are transparent tubular gelatinous animals that are cylindrical in shape. They have a complex life cycle that is completed within forty-eight hours. They alternate life stages between aggregated sexual blastozooids or zooids (aggregate form) and solitary asexual oozoids (solitary form). The zooids possess a thick, rigid tunic that is covered with small protuberances and five body muscle bands. The oozoids possess a very thick, rigid tunic covered with small protuberances and have two conspicuous trailing projections that sometimes appear dark green to black and 20 or more body muscle bands. Both phases have gelatinous bodies that are in constant motion as they feed on phytoplankton. They can be found in large blooms that consist of tens of billions of individual animals. Their digestive system is visible as a dark lump suspended in the translucent body. They have the ability of produce bluish-white luminescence. They can be found in chains that reach 5 m (16 feet) in length or as oozoids that have a maximum of 30 cm (12 inches) in length. As such they are the largest salps found along the West Coast of North America.

Habitat and Distribution:  Twin-sailed Salps reside from the ocean’s surface to depths of 150 m (500 feet). They are vertical migrators that follow their food source, phytoplankton, to various depths within the water column generally moving up in the water column at night for feeding and retreated to the depths for protection during daylight hours. Twin-sailed Salps are found in the temperate and tropical waters of the Atlantic, Indian and Pacific Ocean. In Mexican waters their presence has not been well documented. They are normally found in waters that have a high chlorophyll concentration and their presence is directly related to the availability of food.

Diet:  Twin-sailed salps are filter feeders that move via rhythmic contractions of circular muscles that continuously pump sea water through the body, entering at one end, passing through a filtration network, and expelling the water out the other end. They filter out, and feed on, dinoflagellates, diatoms, copepods, and other plankton. They require silt free waters away from the surge zone. The carbon-containing waste is packed into dense pellets which quickly sink to the ocean floor providing for fertilizer for bottom dwelling organisms.

Predators: Twin-sailed salps have a wide range of predators including marine birds, fish, jellyfish and sea turtles.

Reproduction:  Reproduction in Twin-sailed Salps is complex, depending on the life cycle stage. The zooids develop by asexual budding from an elongated stolon within an oozoid. At that stage they are sequential hermaphrodites that begin life as females. They are fertilized by males within the same chain. The resulting embryos develop into oozoids. This rapid and efficient method of asexual reproduction enables their populations to explode during periods when an abundance of food is present. They can reproduce very quickly producing massive quantities that can quickly deplete the food supply and will then quickly die out.

Ecosystem Interactions:  The bodies of Twin-sailed Salps provide shelter for several pelagic species including: juvenile Medusafish, Icichthys lockington, juvenile Small-Eyed Squaretail, Tetragonurus cuvieri, Hypriid Amphipods, and Tuberculate Pelagic, and the Football Octopus, Ocythoe tuberculata.

Human Interactions:  Twin-sailed Salps have limited direct impact on human activities. They are considered to be harmless to humans, though they have been blamed for the collapse of several localized fisheries. From a conservation perspective the Twin-sailed Salp has not been formally evaluated however they have a wide global distribution and when present they can be found by the billions making them most likely of Least Concern.

Synonyms:  Salpa vagina, Salpa costata, Salpa herculean, Salpa neapolitana, Salpa tilesii, Tetys costata, Tethys tilesii, Thetys costata, and Thetys tilesii.

Productive discussions and identifications courtesy of Linsey Sala, Scripps Institution of Oceanography, La Jolla, California.