Abstract
Invasive species may quickly colonize novel environments, which could be attributed to both phenotypic plasticity and an ability to locally adapt. Reproductive traits are expected to be under strong selection when the new environment limits reproductive success of the invading species. This may be especially important for external fertilizers, which release sperm and eggs into the new environment. Despite adult tolerance to high salinity, the invasive fish Neogobius melanostomus (round goby) is absent from fully marine regions of the Baltic Sea, raising the possibility that its distribution is limited by tolerance during earlier life stages. Here, we investigate the hypothesis that the spread of N. melanostomus is limited by sperm function in novel salinities. We sampled sperm from two invasion fronts with higher and lower salinities in the Baltic Sea and tested them across a range of salinity levels. We found that sperm velocity and percentage of motile sperm declined in salinity levels higher and lower than those currently experienced by the Baltic Sea populations, with different performance curves for the two fronts. Sperm velocity also peaked closer to the home salinity conditions in each respective invasion front, with older localities showing an increased fit to local conditions. By calculating how the sperm velocity has changed over generations, we show this phenotypic shift to be in the range of other fish species under strong selection, indicating ongoing local adaptation or epigenetic acclimation to their novel environment. These results show that while immigrant reproductive dysfunction appears to at least partly limit the distribution of invasive N. melanostomus in the Baltic Sea, local adaptation to novel environments could enable future spread beyond their current boundaries.
Highlights
For populations in novel environments, selection can contribute to adaptive trait change and with moderate to low gene flow, can result in local adaptations (Westley, Ward, & Fleming, 2013)
Our results show that salinity affects sperm velocity and sperm motility in N. melanostomus, but that sperm were still active at the high‐ and low‐end salinities that the Baltic N. melanostomus is ex‐ panding towards
The sperm of males sampled at the high salinity invasion front peaked in velocity at 15–20 PSU
Summary
For populations in novel environments, selection can contribute to adaptive trait change and with moderate to low gene flow, can result in local adaptations (Westley, Ward, & Fleming, 2013). Given the potential for phenotypic change to enable or possi‐ bly even promote colonization of invasive species into new envi‐ ronments (Hudson, McCurry, Lundgren, McHenry, & Shine, 2016; Westley, 2011), and the wide‐reaching effects of N. melanostomus on the ecosystem level (Kornis et al, 2012; Ojaveer et al, 2015), there is a strong argument to investigate possible signs of local adaptation in sperm performance, and evidence of reproductive restraint to their current range of salinity conditions
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