I live in Poland and this is the first time I've heard about this. If anyone is interested in more details, here are some I've found out:
There are actually three stations: one in Poznań like Gus said, but also two in other towns near Poznań. They have 8 clams in each station and they monitor how much they are closed and then calculate average value. They stay in a water coming from filtration system. If the filters fail and the water is not pure, the clams react quickly. If the average score goes too low, the alarm system goes off.
Regarding the clams themselves, they usually work for 2 months, during which they can't be fed (because they need to stay open). After that time they are freed as they become too hungry. The three stations can even use up to 200 clams per year.