A toxicant detection system comprises at least one watertight chamber containing a mollusk. Water to be screened is introduced into the chamber. The mollusk is preferably supported on a mounting structure which facilitates quick mounting of the mollusk in the device, comprising a releasable element affixed to the shell of the mollusk, e.g., a bolt which is screwed into a nut affixed to the mollusk's shell. A sensing apparatus is provided which includes a movable member supported in the chamber so that it engages the shell of the mollusk, and moves when the shell opens and closes. The sensing apparatus detects the position of the mollusk shell based on the position of the movable member. The sensing apparatus preferably includes a Hall effect transducer which co-acts with a magnet carried on the movable member. The system allows for accurate readings of the small movements of the mollusk shell which aid in detecting toxicants, and also provides for installation of mollusks of varying sizes, without the need to move components of the standard configuration of the chamber. Methods and systems for detecting or identifying a toxicant in water being screened are also described.