Well, if I had to take a wild guess, I'd say the clam smashes into the rocks with a speed that's a shell of a lot faster than a sloth on roller skates. As for the speed relative to the gull, well, let's just say the gull might have a bird's-eye view of the clam going splat! But hey, at least they'll both have something interesting to talk about at the next clam and gull gathering, right?