ABSTRACT
Abstract Selective Tidal-Stream Transport (STST) is used by invertebrates and fishes for horizontal movement. In general, animals ascend from the bottom and are carried by tidal currents during one phase of the tide. During slack water, at the end of this tidal phase, they return to the bottom and remain there during the opposite tidal phase. Through this sequence, horizontal movement takes place in a series of saltatory steps. In coastal and estuarine areas, STST can be characterised as ebb-or flood-tide transport depending upon which phase of the tide is used for transport. Modelling studies indicate STST is a highly effective means of horizontal movement for life-history stages that have weak swimming abilities and for energy conservation by adults with strong swimming ability. Frequently, the direction of STST reverses within a species, especially at different physiological or life-cycle stages. For example, larvae of estuarine crabs undergo ebb-tide transport for migration out of estuaries for development offshore, whereas older post-larvae use flood-tide transport for movement up estuaries to nursery areas. The behaviour underlying STST is ascribed to (a) a tidal rhythm in activity or vertical migration or (b) behavioural responses to environmental cues associated with tides. If behavioural responses are involved, then recent studies suggest that no single cue is used for STST but that animals respond to a sequence of cues during transport. Collectively, STST is well documented in the field, but underlying behaviours need future study.
