Home and Away: Variation in Signal Crayfish (Pacifastacus leniusculus) Morphology and Environmental Conditions Between its Native and Invasive Ranges

The North American signal crayfish (Pacifastacus leniusculus) is native to the Pacific North West, with introduced populations extending to southern California. The species has also been widely introduced across Europe, with multiple detrimental impacts on native biota and abiotic environments. Understanding the differences between native and invasive populations of invading species may provide an insight into why some species are successful invaders, and may also aid in predicting future invaders and their impacts. Previous work shows that native British crayfish are morphologically adapted to local environments. Here we present evidence that invasive populations of North American signal crayfish in the UK show the same morphological adaptations; carapaces are narrower in running water and wider in still water, and areolae show the opposite correlation. This relationship may reflect differences in dissolved oxygen concentrations between lentic and lotic environments. No such relationship was found in native populations of the species from the same latitude in British Columbia, Canada; although all Canadian crayfish were smaller than their British counterparts. The lack of morphological correlation with flow rate may result from generally more favourable oxygen conditions in Canadian stillwaters. Alternatively, the inevitable population bottleneck following introduction to the UK may have increased the degree of local adaptation, possibly through phenotypic plasticity. Further investigation of genetic diversity and other European populations of P. leniusculus could shed further light on the situation.