We have clams! We researched them online and found out that what we found were clams. They have a leg to help them burrow into the sand or mud and siphons for helping them breathe and eat. They eat plankton. We've found it very fascinating!

And we found they they can spit at you when you take them out of the water while they are still open. If you count the ridges on the outer shells, it will give you a good estimate of how old they are. Ours range in age from 6 years to 19 years old.