1. Amphidromy, thedispersal of larvae to the estuarine or marine environment with juvenilesrecolonising fresh waters to complete development, is common on tropicalislands. This has led to the suggestion that amphidromy is an adaptation tohydrologically unpredictable environments, allowing recolonisation after localextinction (dispersal-limitation hypothesis). Alternatively, amphidromy may bemore common in streams with stronger flow, as high tractive forces flush larvaeinto the estuarine/marine environment, forcing them to recolonise (passivedispersal hypothesis).
2. We tested thesehypotheses in a continental gobiid fish, the Roman Nose Goby (Awaous acritosus)that inhabits hydrologically and physically diverse catchments on the eastcoast of Australia. We measured 87Sr/86Sr along transects of the otoliths of 92adult fish from 12 catchments, identified migratory life histories from thesedata and correlated the proportion of migratory fish with key environmentalvariables. This information was augmented with data from mtDNA (n = 276) andsix microsatellite loci (n = 429) from 19 catchments, to explore gene flowamong catchments and to assess whether local adaptation was likely in the faceof high or restricted gene flow.
3. Estimates of87Sr/86Sr along transects revealed high variation in life history, with 63% ofindividuals showing no evidence of marine/estuarine residence. The slope ofcatchments was the only significant variable associated with the proportion oflarvae retained in catchments, with steep catchments having a greaterproportion of adults exhibiting a marine/estuarine phase during early life.Total panmixia was detected with mtDNA and six microsatellite loci, suggestingthat gene flow is high for this species.
4. These results supportthe passive dispersal hypothesis. Steep slopes are likely to have higher tractiveforces in their flow, leading to a greater proportion of larvae being flushedinto the estuary or sea. In addition, high gene flow would inhibit localadaptation, making the dispersal-limitation hypothesis unlikely.
5. This study provides aframework for understanding how local adaptation may be constrained by geneflow across the landscape so that the adaptive or passive mechanism underlyingfacultative amphidromy can be explored.