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Thursday, May 9, 2013

Out for a stroll

A couple of weeks ago, Eric spotted this Reticulate Button Snail (Trimusculus reticulatus) out for a stroll.  Note the circular shell and the network of radiating lines.

We were very surprised for a number of reasons. These marine pulmonate snails are found in crevices and shaded overhangs in the rocky intertidal zone.  They're often in gregarious clusters (see next image), and are known to be relatively sessile (i.e., they don't move very much).

In our experience it was unusual to encounter a single individual, especially one that was out walking around!  In fact, we'd never seen the oral lobes extending in front of the shell before.  The lobes are an unusual shape (unlike the true tentacles of other snails), so we took some time to study them and to wonder about how they would be used.

I haven't been able to find anything about how the lobes might function at low tide.  At high tide the snail uses them for feeding.  Reticulate Button Snails are filter-feeders.  They produce mucous nets that collect phytoplankton from the water and then gather the nets with their oral lobes.

A few years ago I was also surprised when I encountered a very large aggregation of this species depositing eggs.  When I looked closely I noticed many egg masses adjacent to the shells.  (Note that often the reticulate pattern for which the species is named is eroded or overgrown.)

Intertidal Invertebrates of California says, "The eggs masses are gelatinous and petal-like, and are attached in rosettes on the rock around the animal that laid them.  Egg deposition occurs in April at Tomales Head [sic]..."

I took these pictures in July on Bodega Head:

You'll probably notice that some of the egg masses are white and others are brown.  I'm not sure what's going on with the colors, but am wondering if the brown masses are older?

Back to the animal we saw in late April.  It's hard not to wonder about its story.  Will it find an aggregation to join, or will it start one of its own?

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