A genetic investigation into the population structure and invasion dynamics of Charybdis japonica (Crustacea: Decapoda: Portunidae) within New Zealand

Abstract

Invasive species are an increasing global problem, with their environmental costs having been documented and researched for many years. Although considerable theoretical work has been undertaken, there is still a conspicuous lack of practical means of predicting the impact of established species, or which species are likely to arrive in a new location. Ecological paradigms provide a wealth of information and discrete hypotheses for specific situations, yet lack generality. Genetic approaches are increasing in number and complexity but are yet to provide a complete answer. Invasive populations founded by few individuals are predicted to show distinct patterns of reduced genetic diversity and should therefore suffer from slowed population growth and diminished adaptive capacity. However the success of invaders often contradicts these assumptions, and thus investigations into the actual genetic diversity of invasive populations has become more widespread. Such investigations will not only benefit eradication attempts but are also valuable for testing aspects of evolutionary theory and, paradoxically, application to conservation of endangered species. Currently invasion biology literature is focussed on the unexpectedly high genetic diversity in many invasive populations. Those demonstrating this so far are mainly in close proximity to native populations or have been persistent in the location for a prolonged period of time. The current investigation looked at the genetic patterns in a recent invader, recognised for only seven years. Samples from two and seven years post-establishment have allowed comparisons of the short term changes in a newly invasive population. Charybdis japonica is a large marine portunid species native to East Asia and first observed in New Zealand in 2000. New Zealand samples were obtained from three contemporary Hauraki Gulf populations and compared with samples collected in 2002. Additional samples from the native range (Japan and China) were also obtained for comparison. Strong evidence for a bottleneck on colonisation was revealed through mitochondrial control region (mtCR) analysis; only three mt CR haplotypes were found in the 140 New Zealand samples, compared with eight haplotypes in 17 native range samples. Nuclear ITS-1 diversity was not significantly different between the two populations and 71 % of alleles were shared. This suggests that the size and duration of the bottleneck were insufficient to significantly reduce nuclear diversity. Genetic data shows small scale changes in the frequencies of alleles in New Zealand between 2002 and 2007 samples, and circumstantial evidence of a secondary invasion exists. Geographic range expansion has occurred relatively rapidly since the population was first observed in 2000; genetic data and water current modelling patterns support an outward radiation of individuals spreading from the Waitemata Harbour, and a relatively high degree of larval mixing. Population differentiation measures suggest that the most southern population at Whitford is recently established. In contrast to recently reported data from some invasive species, C. japonica does not have a higher than expected genetic diversity. Small scale fluctuations are evident and highlight the facts that short term patterns influence invasions, and that these patterns may be missed by the assumption that a single sample is wholly representative of a population over time. The genetic data presented here also supports existing biological suggestions of ‘favourable’ life history characteristics being a main contributor to invasive success and reiterates that these characters (such as high gametic output) can release populations from the predicted effects of low genetic diversity. C. japonica possesses many of these and combined with the open niche provided by a distinct lack of comparable native crab species, the impacts may potentially be immense. Results from this study are directly relevant to eradication attempts for this species; localised attempts will be futile unless all populations are simultaneously targeted, otherwise highly dispersive larvae will recolonise the area. Currently, eradication may be a possibility given the limited geographical distance separating populations but the rapid proliferation indicates action must be immediate to prevent further range expansion of this potentially destructive species.