As “broadcast spawners”, bivalves release their gametes (sperm and eggs) into the water around them in unison. Eggs are fertilized by sperm in the surrounding water, and grow into a plankton that looks like a microscopic clam or mussel. Like other plankton, these bivalve larvae are too small to fight strong currents caused by tides and wind, and are typically carried by currents as passive (neutral and free floating) particles. Larvae can, however, swim up and down in the water when vertical currents are weak. Understanding these behaviors allows scientists to predict when larvae act as free floating particles, and when, how, and why larvae alter their movement and direction.