A salp, a pyrosome and a doliolid walked into a bar.... - Merry Passage

Thetys vagina

T. vagina is the largest species of salp

• has a rigid tunic composed of tunicin, a cellulose-like compound

• water intake simultaneously propels it along as muscle bands pump out the water

• tremendously efficient filter feeder

• secretes a fine, mucous net that can sieve and/or trap particles as small as a bacterium or smaller

• net is rolled-up and transported to the esophagus for consumption, then replaced

• along with other salps, its dense fecal pellets play an important role in carbon cycling by removing carbon from surface waters and sending it to the depths

• reproduces by Alternation of Generations

• can increase numbers rapidly when conditions are favorable

Oral and Aboral Ends


The stolon produces a double chain of hundreds of individual salps. Because this is the solitary generation of Thetys, it is asexual,  so it can only reproduce by budding off clones of itself.  The mature clones will be linked together in an aggregate chain.  

Pyrosoma atlanticum

Pyrosoma is a pelagic, colonial tunicate / salp made up of hundreds of individual zooids.  The individual units function just like a single, free-living animal, but remain embedded in a common cylinder.  The blue projections on the side of the cylinder create still water around the colony, which may make it easier for the zooid to draw water into its pharyngeal basket for filtration. Water expelled from hundreds of zooids jets out the common lumen, propelling the colony along.

How do the zooids arrange themselves to form a perfectly symmetrical tube?

Why don't we ever see a rogue pyrosome that's shaped like a saguaro cactus?

Water is expelled from a common lumen and propells the pyrosome at the same time.


Doliolids are real oddballs.  They are very small plankton grazers with a multi-stage life cycle that is atypical compared to other members of the Tunicata.  The doliolids below have already transformed into a colonial organism known as a "nurse". 

A nurse doliolid develops a tail with feeding structures that sustain the entire colony.  Growth of the nurse is dependent on the energy supply from the trophozooids. 

Young Nurse Doliolid (~2.5 mm)

Older Nurse Doliolid

Trophozooids filter-feed plankton particles for the nurse doliolid.

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