Restricted dispersal in a sea of gene flow

How far do marine larvae disperse in the ocean? Decades of population
genetic studies have revealed generally low levels of genetic structure at
large spatial scales (hundreds of km). Yet this result, which is typically
based on discrete sampling designs, does not necessarily imply extensive
dispersal. Here, we adopt a continuous sampling strategy along 950 km of
coast in the Northwestern Mediterranean Sea to address this question in
four species. In line with expectations, we observe weak genetic structure
at large spatial scale. Nevertheless, our continuous sampling strategy
uncovers a pattern of genetic isolation by distance at small spatial
scales (few tens of km). Individual-based simulations indicate that this
signal is an expected signature of restricted dispersal. At the other
extreme of the connectivity spectrum, two pairs of individuals that are
closely related genetically were found up to 250 km apart, indicating
long-distance dispersal. Such a combination of restricted dispersal with
rare long-distance dispersal events is supported by a high-resolution
biophysical model of larval dispersal in the study area, and we posit that
it may be common in marine species. Our results bridge population genetic
studies with direct dispersal studies and have implications for the design
of marine reserve networks.