Clams are all mouth but have nothing to say.
In this case they appear long dead (presumably by way of the drought or a wading bird's beak,) although in walking on them, as you'll hear in the video, they made quite a crunch.
Biologists at Everglades National Park identified them as the non-native Corbicula clam, most commonly found at inflow points, which in this case would be from the upstream Tamiami Trail. These tiny clams are usually found in areas of high flow, a condition which bivalves prefer since as filter feeders that don't move they rely on the fast-flowing water to bring the food to them.
Just upstream you can hear the occasional roar of the Trail by way of a Tractor Trailer rolling by.
The clams of course are silent.