Why not just point a webcam at a bed of them?

You can do image analysis to count the shellfish in the image, and attach an “open” or “closed” status to each. Once you have a reliable field of Boolean values, you can detect anomalies in the close count.

You could then have a grid of tanks with inlets from the clean water supply and outlets to the sewer where you monitored (more or less real time) the reaction to our clean water supply.