The non-indigenous copepod Pseudodiaptomus marinus Sato, 1913 in an Eastern Mediterranean coastal area: day–night pattern and genetic characterization

Abstract The continuously growing number of Non-Indigenous Species (NIS) in marine ecosystems poses a modern threat to biodiversity and ecosystem functioning, which calls for studies to extend beyond the initial recording to how these species evolve and adapt to their new environments over time. Here we present the first annual monitoring of the NIS copepod Pseudodiaptomus marinus in Thessaloniki Bay, revealing a pronounced day and night pattern. The species was absent in most daytime samples but was consistently present at night, reaching abundances as high as 7,334 ind. m⁻³. Higher abundances were recorded between July and November, coinciding with higher water temperatures, although individuals were observed across a broad (8.7–28.6 °C) range, confirming the species’ eurythermal character. Molecular taxonomy analyses using a multimarker approach (18S rRNA, COI and ITS) confirmed species identity and revealed low genetic divergence, supporting the establishment of a genetically homogenous population. The regular occurrence, high abundances, and broad environmental tolerance of P. marinus indicate its successful establishment in this eutrophic coastal system, while its night emergence suggests behavioral strategies probably linked to predator avoidance and niche separation.