Replacing the locals? Distribution and expansion dynamics of native versus non-native Rockpool shrimp (Palaemon elegans Rathke, 1837) lineages in the North and Baltic Sea

Abstract Cryptic introductions often remain unnoticed, but can have unforeseen consequences for the invaded ecosystems. A sudden eastward range expansion of the Rockpool shrimp Palaemon elegans into the low-salinity part of the Baltic Sea in the 2000s could genetically be identified as a cryptic introduction of a distinct Black Sea lineage. To understand the risk of its expansion westwards into the North Sea and Atlantic and competitive displacement of the native Atlantic lineage, we surveyed the distribution of both mitochondrial lineages at different salinities along the Baltic and North Sea, investigated the temporal stability of this pattern over 15 years and the pre-introduction distribution of the native Atlantic lineage in the Baltic Sea. Salinity correlated significantly with the frequency of the lineages, while the time series did not indicate an increase in relative abundance over time and the historical distribution of the assumed Atlantic lineage aligns with its current distribution. We conclude that the non-native Black Sea lineage is unlikely to expand further into the Atlantic, and has not replaced the Atlantic lineage.