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Tuesday, August 26, 2014

Diamonds in the drift

Last night, on 25 August 2014, I was waiting for someone and decided to pick up some trash on the beach.  I glanced along the water line and noticed something that reminded me of the few times I've seen hail being concentrated in driftlines by the incoming waves:

I bent down for a closer look:

Salps!  Although I've seen other species of salps washing ashore on Bodega Head, I believe this is the first time I've seen Thalia democratica stranding on local beaches.

Here's an image with a ruler for scale (below).  When they're in this chain-like stage, they remind me of diamond bracelets (but note if you try to pick them up, the individuals in the chains separate very easily):

Thalia democratica has a somewhat complicated life cycle.  There are two stages: an aggregate stage (as above) and a solitary stage.  The aggregate stage is sexualeach individual in the chain produces one embryo that will develop into a solitary individual.  The solitary individual is asexual each solitary individual will bud off a chain of aggregates.  This life cycle is illustrated below:

 Example of a salp life cycle modified from Alldredge, A.L. and L.P. Madin. 1982.  Pelagic tunicates: Unique herbivores in the marine plankton.  Bioscience 32: 655-663.

Since observing Thalia democratica on Bodega Head has been rare, I brought a few salps in for documentation.

This is a picture of an aggregate individual under the microscope.  The purplish oval spot at the bottom is called a nucleus and the structure just to the right of the nucleus is a developing embryo (it will become a solitary individual).

The next image shows the posterior end of a full-grown solitary individual.  Note the two long projections (pointing backwards).  There is a developing chain of aggregates wrapped around the gut.

While looking through the specimens, I noticed one that looked different and had me puzzled for a moment.  Most of the individuals were easy to sort into either aggregate forms (rounded or tear-drop shaped) or solitary forms (with two long projections).  Here was a small salp that  looked like it had two little "fists" and two "triangular bumps" at the back end:

When I looked very closely at those little "fists," I realized what I was probably looking at:

I'm not sure what the structure is, but there's something in the little fist that looks funnel-shaped (a bit like Shrek's ears)!  When I saw that, I remembered seeing the same thing in the full-grown solitary individual: 

Above, you can see the "Shrek-like" funnel structures at the base of the two long posterior projections.  Also note the two curved spikes, closer to the top of the picture.  I believe the mystery animal I observed is a very small, juvenile solitary individual, and that eventually the "fists" will become long posterior projections and the triangular bumps will become curved spikes.  Although I've seen Thalia democratica before, I've never encountered such a small solitary individual.

One more note: I checked the seawater temperature after finding these salps.  It was approximately 17.5°C.  I'm wondering if their presence this summer has something to do with the warm water?  I'd love to hear about any other sightings of Thalia democratica.

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