Evidence for female-biased juvenile dispersal in corophiid amphipods from a New Zealand estuary

This study examined the distribution and dispersal of corophiid amphipods (Paracorophium spp.) in a temperate New Zealand estuary. Individuals in the sediment (resident population) and those caught in bed-load (local-scale movement) and suspended-load (larger-scale movement) were counted, measured, and sexed, during two tidal cycles at 4-6 week intervals spanning 13 months. Gravid females and juveniles were present in the resident population on all sample dates suggesting near continuous reproduction. Juveniles (< 2.2 mm body length) accounted for 49.8% of individuals in the sediment, compared to 65.4% of those caught in the bed-load and suspended-load traps. For the majority of sample dates, there was a net export of juveniles from the estuary. The total number of juveniles captured in suspended-load traps was significantly greater during the ebb tide relative to the flood tide (n = 541 versus 256), whereas no significant differences were found for adults. Similarly, numbers in the bed-load traps were significantly greater for juveniles during the ebb versus the flood tide (n = 900 versus 484), and also for adult females (n = 180 versus 86). Juvenile sex ratios were female biased in the sediment (9.8:1) and in suspended-load traps (7.6:1) and both were significantly more female-biased than that recorded for juveniles caught in bed-load traps (5.8:1). By contrast, there was a significantly lower proportion of adult females to adult males found in bed-load traps (1.7:1) and in suspended-load traps (0.98:1) compared to the resident population in the sediment (3.6:1). The mean size of juvenile females caught on a flood tide was significantly larger compared to those on the ebb tide, yet juvenile males were not significantly different, suggesting that smaller females were not returning on the flood tide. We suggest that female-biased juvenile dispersal for estuarine Paracorophium spp. lowers the risk of inbreeding and reduces competition. This may have evolved in response to a polygamous breeding system with little or no social organisation where a female produces more female offspring to maximise her inclusive fitness.