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Nonindigenous species pose a major threat for coastal and estuarine ecosystems.Risk management requires genetic information to establish appropriate management units and infer introduction and dispersal routes. We investigated one of the most successful marine invaders, the ctenophore Mnemiopsis leidyi, and used genotypingby-sequencing (GBS) to explore the spatial population structure in its nonindigenous range in the North Sea. We analyzed 140 specimens collected in different environments, including coastal and estuarine areas, and ports along the coast. Single nucleotide polymorphisms (SNPs) were called in approximately 40 k GBS loci. Population structure based on the neutral SNP panel was significant (F ST .02; p < .01), and a distinct genetic cluster was identified in a port along the Belgian coast (Ostend port; pairwise F ST .02-.04; p < .01). Remarkably, no population structure was detected between geographically distant regions in the North Sea (the Southern part of the North Sea vs. the Kattegat/Skagerrak region), which indicates substantial gene flow at this geographical scale and recent population expansion of nonindigenous M. leidyi. Additionally, seven specimens collected at one location in the indigenous range (Chesapeake Bay, USA) were highly differentiated from the North Sea populations (pairwise F ST .36-.39; p < .01). This study demonstrates the utility of GBS to investigate fine-scale population structure of gelatinous zooplankton species and shows high population connectivity among nonindigenous populations of this recently introduced species in the North Sea. K E Y W O R D S genotyping-by-sequencing, invasion biology, Mnemiopsis leidyi, population differentiation, population genomics, Single nucleotide polymorphisms S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section at the end of the article. How to cite this article: Verwimp C, Vansteenbrugge L, Derycke S, et al. Population genomic structure of the gelatinous zooplankton species Mnemiopsis leidyi in its nonindigenous range in the North Sea.

The recognition of cryptic biodiversity provides valuable insights for the management of exploited species. The Atlantic seabob shrimp (Xiphopenaeus kroyeri) is a commercially important fishery resource in the Guianan ecoregion, South America. Previous research in Brazil suggested the presence of cryptic species within the genus. Here, we confirm this presence and delimit the species by applying a multilocus approach based on two mitochondrial (COI and cytb) and two nuclear (PEPCK and NaK) genes. Species boundaries were tested using BPP, GMYC and bPTP delimitation algorithms. These analyses provided strong support for three clades within the genus Xiphopenaeus, including one undescribed clade, which occurs sympatrically with X. kroyeri in the Western Atlantic. Unexpectedly, this undescribed clade is more closely related to the Pacific Xiphopenaeus riveti than to their Atlantic congener. Our DNA-based species delimitation was further supported by new ecological information on habitat and morphology (colour). We also expand the known distribution range of the cryptic species, currently restricted to Brazil, to include French Guiana, Suriname and Colombia. Our findings have important consequences for the management of the species, in terms of both biodiversity management and fisheries management.

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A new probabilistic approach is proposed to assess muricid species population abundances at scales relevant to both Ancient and Modern coastal fisheries. Motivated by the long‐term goal of reconstructing the dynamics of exploited murex populations during Antiquity, the objective was to estimate the population density of the banded dye‐murex, Hexaplex trunculus (Linnaeus, 1758) from successive captures with baited traps, using a method similar to the technique employed in the Mediterranean purple dye industry. The stochastic model developed simulates cumulative captures while accounting for high variability. It was calibrated with data acquired during a field trapping experiment (Crete Island, Greece). Traps’ catchability and Effective Area of Attraction (EAA) were estimated using the individual speed and behavioural response towards bait from laboratory experiments. Average density of H. trunculus was estimated as 2.2 ± 1.4 SE individuals per square metre, with no significant differences between seagrass and rocky habitats. The clearing time of successive capture experiments averaged 84 ± 6 SE hr. Clearing ca. 0.4 ha of subtidal area would be necessary to produce ca. 1.0 g of pure Tyrian purple pigment. The method described is generalizable to making population abundance estimates for similar groups, such as whelks, in modern fisheries.