"Tide pools are rocky pools by oceans that are filled with seawater. Many of these pools exist as separate entities only at low tide." ~ wikipedia
I really wanted to get a picture of a barnacle with it's legs sticking out into the water, so I took a bunch of pictures in a row. Here are two of them. They look very much alike, except if you look closely (you can click on them to see a bigger picture), you can see that the snails have moved a little and the barnacles are opening and closing.
The next picture is an animation made by putting a cropped section of the above two pictures together. They aren't exactly the same size and they don't fit exactly together, but they are close enough that you can see the barnacles opening and that the snails have moved! I'm not sure how far apart in time the pictures were taken, but I'm guessing maybe about 6-10 seconds (with three other pictures taken in between).