Washed ashore

Beachcombers know that it's not uncommon to find small, mysterious bits of gelatinous material washed ashore.  Unfortunately, it's often difficult to identify the animal that the material came from.  

Yesterday, while picking up trash on a local beach, I encountered a large gelatinous animal that seemed almost whole:

 

This is a pelagic tunicate known as a salp (nearshore benthic tunicates are commonly known as sea squirts).  Because they are most often found farther offshore, they're unfamiliar to most people (including me).

With some help from Steve Haddock and Larry Madin, this species has been confirmed as Thetys vagina, one of the largest salps on the West Coast.  In the photo above, there are several features visible: 

- a mouth-like opening on the left end (oral siphon)

- vertical lines across the mid-sections (muscle bands)

- a bulbous feature on the upper right with purplish color inside (internal organs)

- a large atrial siphon at the lower right end

The photo below, with the salp flipped over, shows a better view of the atrial siphon (now on the lower left) and muscle bands.

 

I'm a little uncomfortable describing the life history of Thetys, as it's difficult to find information about them.  But it's important to know that they alternate between a solitary phase (asexually reproducing) and an aggregate phase (sexually reproducing).  The animal in these photos is an individual from the aggregate phase — it's one of many that form a long chain.  There's a picture of a chain here and a nice image of a solitary form here.

In the Light & Smith Manual (2007), Larry Madin writes:

"Although ascidian tunicates are commonly found on rocks, pilings, piers, and other hard substrates largely in the subtidal zone, their planktonic relatives, the pelagic salps, doliolids, pyrosomes, and appendicularians, are rarely seen in the intertidal.  These pelagic tunicates are usually offshore species, distributed widely in the slope water and open ocean.  They may occasionally come into shallow water or even be washed up on beaches when offshore populations are pushed in by currents or wind."

After a relatively calm period, strong northwest winds started on 28 July 2012 and perhaps played a role in driving these salps on shore (I encountered three individuals on 29 July and one individual on 30 July).