Female swimming crabs (Callinectes sapidus) have an internal circatidal clock that tells them when to swim so that they can move easily up or downstream in estuaries to spawn. After spawning they migrate back into the estuary. By swimming near the surface when the tide is ebbing (moving out) the crabs take advantage of the water flow which is greatest at the surface. The reverse happens (swimming as the tide floods (comes in)) for post-spawn females. This strategy maximises the speed of migration and saves energy! This phenomenon is called Selective Tidal Stream Transport and is used by various species, including fish.