Dispersal and Upstream Migration of an Amphidromous Neritid Snail: Implications for Restoring Migratory Pathways in Tropical Streams

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Freshwater Biology


The amphidromous life cycle of several species of neritid snails, shrimp and gobies throughout the tropics includes juveniles that migrate from the ocean to breed in fresh water. In many Hawaiian streams, the decline of Neritina granosa, an endemic gastropod, has been associated with habitat degradation and water withdrawal, which are common factors affecting tropical rivers around the world. We investigated the effects of water withdrawal and density on dispersal and upstream migration of N. granosa using three experimental treatments: (i) reduced flow (RF) owing to a stream diversion, (ii) natural flow (NF) and (iii) natural flow with artificially increased snail density. For each treatment, snails were differentially tagged and released in a stream without a natural, extant population of N. granosa. Capture rates ranged from 17 to 65% over a 63-day period following release. Captures on 2-6days after release measured initial dispersal and migration, whereas longer-term migration rates were calculated from snails captured 16-63 days after release. Snails under NF displayed positive rheotactic behaviour, with only 3-12% demonstrating initial downstream movement. Under RF, 22-77% of snails moved downstream or showed no bias either way. Initial mean upstream migration rate (UMR) was 0.25, 0.66 and 1.16mday-1 under RF, NF and natural flow with increased snail density, respectively. Longer-term migration rates did not differ significantly between treatments, and the overall mean was 0.62mday-1. Principal component analysis and generalised linear models were used to identify habitat characteristics important to UMR, with habitat and reach-scale hydraulics as the most important factors. The relationship between discharge and UMR suggested it would take 11-35 years for snails to migrate past the most upstream water diversion. However, rates from published studies of neritid snail species migrating en masse or in long lines under natural situations suggested that N. granosa could migrate above stream diversions within 72 days-2.5 years (when in an aggregation) and 29 days-1.1 years (when following in long lines of individuals). An understanding of upstream neritid snail migration can be used for the management and conservation of this and other migratory species in tropical streams.




John Wiley & Sons



Peer Reviewed





Amphidromy, Dispersal, Gastropoda, Migration, Water diversion