Past and present drivers of population structure in a small coastal fish, the European long snouted seahorse Hippocampus guttulatus

The effective design of species conservation programs is reliant on information such as extant geographic distribution, taxon-specific life-history characteristics, and the relative influence of historic processes and contemporary environmental parameters in shaping population genetic diversity. Seahorses are weak swimmers and have a brooded young, limiting their dispersal potential. They live in sheltered locations, which are physically isolated from each other. Therefore panmixia across their geographic range is unlikely. Hippocampus guttulatus, a seahorse inhabiting European waters, has a geographic range spanning a number of contemporary oceanographic features that are proposed barriers to gene flow. Thus this fish is well-placed to test the contributions of environment and life-history factors in shaping population structuring. This study found that mitochondrial DNA and nuclear DNA (microsatellite) genotype data are concordant in suggesting that, like many other small fishes in European waters, H. guttulatus extant populations expanded from at least one southern European refugial population. Subsequent population differentiation of four geographic lineages reflects contemporary oceanographic barriers to gene flow. Demographic analyses suggest a northward, and long-term isolation between Black Sea and Mediterranean Sea populations. Moreover H. guttulatus contemporary population distribution and population structure are predominately explained by historic and oceanographic influences. These findings suggest that conservation of genetic diversity in H. guttulatus may be aided by a network of marine protected areas (MPAs), implemented to conserve coastal habitats, but the species' unusual life history and gamete retaining behaviours should be considered as part of management decisions including MPA design and fisheries management plans.