Coquinas like to hang out with each other. Where you find one, you can find many others forming colonies of clams. They align themselves vertically in the sand, which means they can pack themselves into the sand quite nicely."
And from the Shell Museum.org I learned the following:
"In order to remain in their terrain near the surf line where the sand is always wet, the coquinas move up and down the beach as the tide goes in and out. They accomplish this by monitoring the vibration of waves on the sand. When the tide is coming in they emerge from the sand and hitch a ride to shore, digging in to prevent being washed back out to sea. If they end up too far from the water, they sense that the sand is dry and hop back on the next wave to allow themselves to be pulled further out to sea.