Gene flow shapes spatial genetic structure as well as the potential for local adaptation of populations. Among marine animals with non-migratory adults, the presence or absence of a pelagic larval stage is thought to be a key determinant in shaping gene flow and the genetic structure of populations. In addition, the spatial distribution of suitable habitats will influence the distribution of biological populations and their pattern of gene flow. We used whole genome sequencing to study demographic history and reduced representation (ddRAD) sequencing data to analyze spatial genetic structure in the broadnosed pipefish (Syngnathus typhle). Its main habitat are seagrass meadows, which along the study coast (SW Norway) have a patchy distribution. Combining the results from several analyses including scans for selection, suggests that stochastic genetic drift has shaped the observed population structure largely due to its patchy habitat distribution. The restricted gene flow is further driven by life history traits such as the presence of parental care combined with no pelagic life stages, resulting in a clear isolation-by-distance pattern spanning 100s of kilometers.

The spatial scale of demographic connectivity was inferred from long-term (~30 year) census population counts that uncovered a sharp decline in spatial correlations in abundance with distance (37% decorrelation over 2 km). These findings were contrasted with data from two other fish species sampled along the same coastline, both having pelagic larval stages lasting ~20 days (corkwing wrasse, Symphodus melops, and black goby, Gobus niger) where the population structure is not that evident. For these species, we found a wider spatial scale of demographic connectivity (decorrelation distances of 14 and 28 km, respectively), and weaker isolation-by-distance except at one point along the coast where both species revealed a strong barrier to gene flow, seemingly due to a lack of suitable habitat. Combined, these findings suggest that habitat fragmentation and absence of a pelagic larval stage in pipefish strongly increases geographic structuring, while the pelagic larvae of wrasse and goby increase genetic and demographic connectivity, except over extensive habitat shifts.