A few summers ago I was exploring the tidepools with a group of Junior Rangers when I came across a rock scallop (Crassedoma giganteum). The scallop was sitting on an otherwise bare rock situated in the middle of a tidepool that was about five feet in diameter. The water was more than a foot deep, and the scallop was fully submerged. I assumed it was an empty shell that had been picked up and then dropped into the deep pool by another tidepool explorer or a bird.
I waded into the deep pool in my knee-high boots and reached to pick up the scallop to show the kids…and nearly fell over when it snapped its shells together and jetted away!
I watched with my jaw hanging open in amazement as it rapidly squeezed its shells shut half-a-dozen times, squirting water to propel itself several feet, where it took shelter at the edge of the pool.
By the time the Junior Rangers arrived at the tidepool, the scallop had retreated into a crevice, and all we could see were the rows of its tiny blue eyes along the edge of its mantle.
I found this image, by Christopher Bradford, at http://www.coastal.ca.gov/publiced/photos/2009/gallery1-2009.html