Rockpools at low tide.
large acorn barnacles on bottom of tire
Tightly closed shell and white in color. These barnacles are very abundant on rocky beaches.
For more information on the habitat, vegetation, and weather of the area where I made this observation, please see the journal entry for April 28, 2012 here on iNaturalist. These barnacles were all over the rocks covering the beach's ground. It was nearly impossible to walk without stepping on them. They varied in size from less than an inch across to two inches across and lived both outside and inside the tide pools. The ones inside the tide pools were particularly interesting because I got to observe them sending out their thoracic appendages (the cirri) to filter feed, searching for zooplankton and detritus in the water. I had never seen them do that before, I had only read about them doing it, so I was glad to get to witness it.
I saw many of these larger barnacles attached to the larger rocks in the intertidal zone