The Sedentary Habits of Intertidal Purple Sea Urchins
Over the course of one year, I collected digital photos of 21 permanent marked plots at South Cove of Cape Arago. I was surprised to learn that sea urchins rarely move around in the intertidal, when inside or outside pits. In fact, of the 583 urchins that were in the last set of digital photos, 549 (94%) had not moved AT ALL during the year! That is, since they appeared in the same location in every photograph, I concluded that they had not moved.
Of course, the other 6% of sea urchins in those photos had moved to their locations inside the tidepools. Very rough calculations would indicate that if 6% of the individuals move in a given year, then any one sea urchin would move once per 16 years. This is probably an ultra-conservative estimate of movement in sea urchins, but since some movement does occur, we can suppose that movement of pit urchins out of pits most likely contributes to the size difference between pit and nonpit urchins.