A new record of the rapidly spreading calanoid copepod Pseudodiaptomus marinus (Sato, 1913) in the Levantine Sea using multi-marker metabarcoding

Over the last decade, the calanoid copepod Pseudodiaptomus marinus—native to the Indian Ocean—has rapidly spread throughout the European Seas. Here we report its first occurrence in the southern Levantine Sea. Zooplankton samples were collected monthly by vertical net hauls in a coastal monitoring station at the Israeli Mediterranean Sea during 2019–2021. The samples were analyzed using mitochondrial COI and 18S rRNA metabarcoding, revealing the occurrence of P. marinus in winter and spring. Following the molecular detection, two individuals of P. marinus were observed in the samples and identified morphologically, indicating a low population abundance (0.4 ind. m^-3) and confirming its status as widespread but rare, as reported in former colonized areas. Rare species often go undetected in zooplankton assemblages using morphological examination, whereas DNA metabarcoding is a sensitive, rapid, and cost-effective method that can provide valuable presence/absence data of such species. We further show that the use of both mitochondrial and nuclear gene markers provides a robust and comprehensive non-indigenous species (NIS) early-detection system, and stress that combining DNA metabarcoding with morphological examination is necessary for biodiversity monitoring in marine ecosystems that undergo significant transformations due to climate and/or anthropogenic forcing.