We describe a novel sensor based on fiber optic technology for fine scale (∼0.1 mm) laboratory measurements of valve gape in bivalve molluscs. We have illustrated the operation of this sensor by measuring valve gape in conjunction with depletion rate assays in the Eastern oyster (Crassostrea virginica, Gmelin 1791). The sensor is capable of accurate and repeatable measurements, with few of the drawbacks of other valve gape sensing methods (e.g., levers, strain gauges, electromagnetic sensors). This sensor technology can be applied to other systems requiring the measurement of axial distances while immersed in seawater or other harsh environments.