|Title||Barriers to gene flow in common seadragons (Syngnathidae: Phyllopteryx taeniolatus)|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Wilson NG, Stiller J, Rouse GW|
|Type of Article||Article|
|Keywords||australia; Bassian Isthmus; biogeographic barrier; coastal waters; conservation; mitochondrial genome; Movement; ocean currents; phylogeography; population-structure; seahorses; Syngnathid; Weedy seadragon|
The common seadragon is an iconic fish with presumed limited dispersal, because juveniles hatch directly from the tail of the male parent. Nothing is presently known of their phylogeographic structure, despite conservation concerns and a distribution spanning southern Australia. Here, we sequenced mitochondrial genes from 201 common seadragons in Western Australia, South Australia, Victoria, Tasmania and New South Wales. We show that common seadragon populations are highly structured geographically, and that genetic variation varies significantly. The historical Bassian Isthmus appears to have left a strong imprint on population structure. Populations east of the Bassian Isthmus are low in diversity and appear more connected than those in the west, although this is likely caused by a recent expansion from a common source after the Last Glacial Maximum. All individuals from Eden, Bicheno and Hobart are represented by only two haplotypes. Populations west of the Bassian Isthmus are more diverse, with the highest diversity indices shown by Western Australian and Spencer Gulf populations. A large sampling gap across the Great Australian Bight is yet to be resolved; the west versus east break here may be an artifact of this gap. Almost all sampled populations can be inferred to have limited gene flow among them, which has implications for recovery after local extinction. Populations thought to be in decline (Sydney, Hobart) showed low genetic diversity, which may make them vulnerable to further reductions.
Currently, common seadragons are categorized as Near Threatened by the IUCN (Connolly 2006). Habitat loss and collection for trade have been evaluated as the major extrinsic threats to seadragon populations and intrinsically, their limited dispersal potential (Connolly 2006). The criteria that have proved harder to evaluate are associated with population trends. Our study indicates that the assumption of a continuous distribution along the southern coast needs to be re-evaluated, and tested with multi-locus data. If the genetic gap detected at the GAB is a real distribution break for common seadragons, the adjusted distribution, and thus area of occupancy would impact population estimates. We demonstrate strong geographic signal in population structure, which is likely a manifestation of the reduced dispersal in this species. The low genetic diversity of several populations in New South Wales and Tasmania may warrant attention given the predicted changes of the eastern Australian coastline due to climate chang