Seascape genomics identify adaptive barriers correlated to tidal amplitude in the shore crab Carcinus maenas

Most marine invertebrates disperse during a planktonic larval stage that may drift for weeks with ocean currents. A challenge for larvae of coastal species is to return to coastal nursery habitats. Shore crab (Carcinus maenas L.) larvae are known to show tidal rhythmicity in vertical migration in tidal areas and circadian rhythmicity in microtidal areas, which seems to increase successful coastal settlement. We studied genome-wide differentiation based on 24,000 single nucleotide polymorphisms of 12 native populations of shore crab sampled from a large tidal amplitude gradient from macrotidal (~8 m) to microtidal (~0.2 m). Dispersal and recruitment success of larvae was assessed with a Lagrangian biophysical model, which showed a strong effect of larval behaviour on long-term connectivity, and dispersal barriers that partly coincided with different tidal environments. The genetic population structure showed a subdivision of the samples into three clusters, which represent micro-, meso- and macrotidal areas. The genetic differentiation was mostly driven by 0.5% outlier loci, which showed strong allelic clines located at the limits between the three tidal areas. Demographic modelling suggested that the two genetic barriers have different origins. Differential gene expression of two clock genes (cyc and pdp1) further highlighted phenotypic differences among genetic clusters that are potentially linked to the differences in larval behaviour. Taken together, our seascape genomic study suggests that tidal regime acts as a strong selection force on shore crab population structure, consistent with larval behaviour affecting dispersal and recruitment success.