Reproductive dysfunction in seawater provides an environmental barrier to the range expansion of the invasive Neogobius melanostomus into the North Sea

Salinity can shape species’ distributions via physiological costs. Understanding how and when these costs occur can help predict colonisation patterns, for example during species invasions. To understand the impacts of high salinity environments on the ongoing expansion of the invasive round goby ( Neogobius melanostomus ) towards the North Sea, fish from this invasion front were spawned in a common-garden setup at different salinities. Reproductive adults were acclimated to a control (20 PSU) salinity representing the current invasion front conditions, and a North Sea coastal salinity (30 PSU). In these salinities they were sorted into spawning groups, and spawning attempts and egg development was tracked during 1.5 months. The results showed a strong impact of the 30 PSU salinity treatment, with a complete developmental failure of all clutches spawned in this salinity. The failed clutches resulted in a significant increase of egg consumption by the nest-guarding males. In addition, sperm velocity showed a decrease with increasing salinity when tested in both spawning salinities and at an intermediate level. Few differences in adult physiology were found as a consequence of the salinity treatments. These results show that, despite adult acclimation to North Sea coastal conditions, the gametes of N. melanostomus cannot as of now cope with the salinities into which their spread is currently predicted. Though adaptation or transgenerational acclimation might allow for reproduction in salinities higher than the species currently occupy, we suggest current models to incorporate a reproductive limit for N. melanostomus at a salinity of 30 PSU.