Rare, long-distance dispersal underpins genetic connectivity in the pink sea fan, Eunicella verrucosa

Characterising patterns of genetic connectivity in marine species is of
critical importance given the anthropogenic pressures placed on the marine
environment. For sessile species, population connectivity can be shaped by
many processes, such as pelagic larval duration, oceanographic boundaries,
and currents. This study combines restriction-site associated DNA
sequencing (RADseq) and passive particle dispersal modelling to delineate
patterns of population connectivity in the pink sea fan, Eunicella
verrucosa, a temperate octocoral. Individuals were sampled from 20 sites
covering most of the species’ northeast Atlantic range, and a site in the
northwest Mediterranean Sea to inform on connectivity across the
Atlantic-Mediterranean transition. Using 7,510 neutral SNPs, a geographic
cline of genetic clusters was detected, partitioning into: Ireland,
Britain, France, Spain (Atlantic), and Portugal and Spain (Mediterranean).
Evidence of significant inbreeding was detected at all sites, a finding
not detected in a previous study of this species based on microsatellite
loci. Genetic connectivity was characterised by an isolation by distance
pattern (IBD) (r2 = 0.78, p<0.001), which persisted across the
Mediterranean-Atlantic boundary. In contrast, exploration of ancestral
population assignment using the program ADMIXTURE indicated genetic
partitioning across the Bay of Biscay, which we suggest represents a
natural break in the species’ range, possibly linked to a lack of suitable
habitat. As the pelagic larval duration (PLD) is unknown, passive particle
dispersal simulations were run for 14 and 21 days. For both modelled PLDs,
inter-annual variations in particle trajectories suggested that in a
long-lived, sessile species, range-wide IBD is driven by rare, longer
dispersal events which act to maintain gene flow. These results suggest
that oceanographic patterns may facilitate range-wide stepping-stone
genetic connectivity in E. verrucosa, and highlight that both oceanography
and natural breaks in a species’ range should be considered in the
designation of ecologically coherent MPA networks.