Importance of the natural flow regime to an amphidromous shrimp: a case study

Amphidromous shrimp inhabiting large river systems are thought to migrate downstream to hatch larvae close to the estuary or sea. Recent work on Macrobrachium spinipes in northern Australia challenged this hypothesis, with the discovery of abundant females in reproductive condition over 400km upstream. We investigated the early life history of M. spinipes by using a combination of field sampling, laboratory experiments and modelling. We determined the spatial and temporal production of larvae, larval salinity requirements, and, finally, the potential for transport of larvae to the estuary during the wet season. Larvae occurred in high abundances across a 400-km river reach. However, successful larval development was contingent on access, by drift, to saltwater within 7 days. This interval is longer than that found for the majority of amphidromous shrimp species and we propose that larvae in the study river are able to successfully moult through the early stages in freshwater. Large, wet-season discharge events were critical for transporting larvae from all sampled sites to the estuary. The present study confirmed the amphidromous life history of M. spinipes, highlighted the importance of natural flows for maintaining existing populations and provided evidence of a novel type of amphidromous life-history strategy.