Population dynamics of marine species that are sessile as adults are driven by oceanographic dispersal of larvae from spawning to nursery grounds. This is mediated by life‐history traits such as the timing and frequency of spawning, larval behaviour and duration, and settlement success. Here, we use 1725 single nucleotide polymorphisms (SNPs) to study the fine‐scale spatial genetic structure in the commercially important cockle species Cerastoderma edule and compare it to environmental variables and current‐mediated larval dispersal within a modelling framework. Hydrodynamic modelling employing the NEMO Atlantic Margin Model (AMM15) was used to simulate larval transport and estimate connectivity between populations during spawning months (April–September), factoring in larval duration and interannual variability of ocean currents. Results at neutral loci reveal the existence of three separate genetic clusters (mean FST = 0.021) within a relatively fine spatial scale in the north‐west Atlantic. Environmental association analysis indicates that oceanographic currents and geographic proximity explain over 20% of the variance observed at neutral loci, while genetic variance (71%) at outlier loci was explained by sea surface temperature extremes. These results fill an important knowledge gap in the management of a commercially important and overexploited species, bringing us closer to understanding the role of larval dispersal in connecting populations at a fine geographic scale.

Marine heatwaves (MHWs) are emerging as forceful agents of ecosystem change and are increasing in frequency, duration, and intensity with climate change. During MHWs, physiological thresholds of native species may be exceeded while the performance of invasive species with warm affinities may be enhanced. As a consequence, MHWs could significantly alter an ecosystem's invasive dynamics, but such interactions are poorly understood. Following a 10-d acclimation period, we investigated the physiological resistance and resilience of an intertidal rock pool assemblage invaded by the seaweed Sargassum muticum to realistic 14-d marine heatwave scenarios (+1.5°C, +2.0°C, +3.5°C) followed by a 14-d recovery period. We conducted mesocosm experiments in both summer and winter to investigate temporal variability of MHWs. MHW treatments had clear negative impacts on native seaweeds (Fucus serratus and Chondrus crispus) while enhancing the performance of S. muticum. This pattern was consistent across season indicating that acclimation to cooler ambient temperatures results in winter MHWs having significant impacts on native species. As climate warming advances, this may ultimately lead to changes in competitive interactions and potentially exclusion of native species, while invasive species may proliferate and become more conspicuous within temperate rocky shore environments.

Invasive non-native species and global warming are two of the greatest components of global ecosystem change. The Pacific oyster, Crassostrea gigas, is the world most cultivated shellfish and was introduced throughout the Northwest European Shelf (NWES) under the premise it could not complete its life cycle. Recent warming trends have changed this and wild populations can be found as far north as Nordic Scandinavia. Under the RCP8.5 concentration pathway, we predict that the majority of NWES coastline will be within C. gigas’s thermal recruitment niche by 2100. Given the widespread occurrence of current naturalized C. gigas populations, its large larval dispersal potential and a lack of feasible management solutions, C. gigas will likely undergo a considerable range expansion this century. The time taken to reach maturity is predicted to decrease by up to 60 days, which may lead to precocious spawning events, facilitating expansion further. Crassostrea gigas can form extensive reefs completely transforming native systems. This may compromise native biodiversity, protected habitats, and commercial species. However, naturalization can also deliver a number of beneficial ecosystem goods and services to human society. Whether naturalization is deemed positive or negative will depend on biogeographic context, the perceptions of stakeholders, and the wider management priorities.

5 6 Running title: Seascape genomics of the common cockle 7 8ABSTRACT 26 Population dynamics of marine species that are sessile as adults are driven by oceanographic 27 dispersal of larvae from spawning to nursery grounds. This is mediated by life-history traits 28 such as the timing and frequency of spawning, larval behaviour and duration, and settlement 29 success. Here, we use 1725 single nucleotide polymorphisms (SNPs) to study the fine scale 30 spatial genetic structure in the commercially important cockle species Cerastoderma edule 31 and compare it to environmental variables and current-mediated larval dispersal within a 32 modelling framework. Hydrodynamic modelling employing the NEMO Atlantic Margin 33 Model (AMM15) was used to simulate larval transport and estimate connectivity between 34 populations during spawning months (April -September), factoring in larval duration and 35 seasonal variability of ocean currents. Results at neutral loci reveal the existence of three 36 separate genetic clusters (mean FST=0.021) within a relatively fine spatial scale in the 37 northwest Atlantic. Environmental association (EA) analysis indicates that oceanographic 38 currents and geographical distance between the populations explain over 20% of the variance 39 observed at neutral loci, while genetic variance (71%) at outlier loci was explained by sea 40 surface temperatures extremes. These results fill an important knowledge gap in the 41

Knowledge on how environmental factors shape the genome of marine species is crucial for sustainable management of fisheries and wild populations. The edible cockle (Cerastoderma edule) is a marine bivalve distributed along the Northeast Atlantic coast of Europe and is an important resource from both commercial and ecological perspectives. We performed a population genomics screening using 2b-RAD genotyping on 9,309 SNPs localised in the cockle’s genome on a sample of 536 specimens pertaining to 14 beds in the Northeast Atlantic to ascertain its genetic structure regarding environmental variation. Larval dispersal modelling considering species behaviour and interannual variability in ocean conditions was carried out, as an essential background to compare genetic information with. Cockle populations in the Northeast Atlantic were shown to be panmictic and displayed low but significant geographical differentiation across populations (FST = 0.0240; P < 0.001), albeit not across generations. We identified 441 outlier SNPs related to divergent selection, sea surface temperature being the main environmental driver following a latitudinal axis. Two main genetic groups were identified, northwards and southwards of French Brittany, in accordance with our modelling, which demonstrated a barrier for larval dispersal linked to the Ushant front. Further genetic subdivision was observed using outlier loci and considering larval behaviour. The northern group was divided into the Irish/Celtic Seas and the English Channel/North Sea, while the southern group was divided into three subgroups. This information represents the baseline for management of cockles, designing conservation strategies, founding broodstock for depleted beds, and producing suitable seed for aquaculture production.