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Sunday, March 10, 2013

The one who wears a large robe

This is a somewhat unusual limpet, encountered infrequently on Bodega Head.  It's commonly known as a Two-spotted Keyhole Limpet (Fissurellidea bimaculata, formerly known as Megatebennus bimaculata).

Its shell is extremely small for its body size.  In the photo above, the pale brown shell with the relatively large oval opening in the center is the entire shell.  The dark gray body extends away from the shell on all sides.  (Although this individual is gray, others may be red, orange, or yellow.)  I haven't looked up the history of who came up with the genus Megatebennus, but it's a fun name in that I think it means "a large robe."  I'm guessing it probably refers to how significant the body of this limpet is in comparison to its shell.

Here's a close-up of the shell.  [Note that there is a smaller red-brown limpet (a different species) on the lower edge of the Two-spotted Keyhole Limpet's shell.]  The size of the opening (or keyhole) in the center of the shell is distinctive — it's up to a third of the entire length of the shell.

A side view shows numerous golden-brown spots scattered across the body surface.  I'm wondering if they might be glands that release bad-tasting toxins if the limpet is disturbed?

A slight angle in the photo below revealed the head and a tentacle with an eyespot at the base. Keyhole limpets breathe by drawing a current of water in over the head which passes by internal gills and then out through the keyhole in the top.

Two-spotted Keyhole Limpets haven't been well studied, but I've read that they eat tunicates and sponges.


Peter said...

The side view does look like a large robe. But which two, of many, spots is it named for?

Jackie Sones said...

Ahh, I was wondering if someone would ask about the two spots! My best guess is that it refers to the two darker stripes on the shell itself. If you scan around the edges of the shell (especially when viewed from above), you'll see that there are two places where the rays are more concentrated creating two darker bands.

I read one of Dall's earlier descriptions of this species which seemed to refer to these shell markings rather than the spots on the body. It would be more satisfying to call this species a Two-striped Keyhole Limpet, but so it goes. Or perhaps we should start a new trend! ;)