Sundials are just bad news all around, these little guys are oportunistic feeders so just assume you need to get rid of them ASAP. I've had some attack my clams a few years ago, the clams are alive but the damage was already done. They attacked from the byssal and tried to work their way in from beneath. I got rid of them by doing a freshwater dip with my clams and the zoos they we;re feeding on.
good luck