A perfect fit

  

I like the warm colors in this photo, so it seemed like a good opportunity to introduce Lottia instabilis.  

This interesting limpet is stenotopic — meaning that it lives in a narrow range of conditions or a restricted habitat.  Lottia instabilis lives on kelps, especially on the stipes (or stems).  In the Bodega Bay area I've seen it on Walking Kelp (Pterygophora californica).  I don't have a great photo of this species on kelp, but here's one from 2012 so you can visualize where it lives:

One of the unusual things about Lottia instabilis is the shape of the bottom edge of the shell.  To fit tightly against the kelp stipe, which is rounded, the shell is curved upward at both ends:

 

 

So the shell is "unstable" (or rocks back and forth) if you place it on a flat, horizontal surface (hence the "instabilis" species name).  But if the limpet is on a rounded kelp stipe, it fits perfectly!