Phylogeography of sand-burrowing amphipods (Haustoriidae) supports an ancient suture zone in the Gulf of Mexico
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2019 John Wiley & Sons Ltd Aim: To evaluate the impact of a proposed ancient suture zone in the Gulf of Mexico on the distribution and molecular diversity of dispersal-limited, sand-burrowing amphipods of the genera Haustorius and Lepidactylus (Haustoriidae: Amphipoda). Location: Gulf of Mexico, USA and Mexico. Methods: Phylogenetic and population genetic analyses were performed using mitochondrial (COI and 16S) and nuclear (18S and 28S) data from 93 amphipod individuals from 16 sites across the Gulf of Mexico. Bayesian and ML phylogenies were constructed for all genes, divergence times were estimated using a molecular clock for COI (0.0070.013 subs./site per Myr), and four species delimitation methods were used to identify operational taxonomic units (OTUs) within each amphipod species. The mitochondrial COI gene was used to construct haplotype networks and estimate population genetic parameters to evaluate historical changes in effective population sizes. Results: Deep divergences (most estimated to be >4 Ma) were uncovered between sister clades of both amphipod genera on either side of the Mississippi River as well as within Lepidactylus triarticulatus where each sample site was found to harbour a unique genetic lineage. Two cryptic OTUs of L. triarticulatus were identified living sympatrically at Pass Christian, Mississippi. Two distinct OTUs representing western and eastern Haustorius galvezi clades were identified along the Texas and Mexico coastlines with abutting ranges. Population genetic results show some support for recent population expansions for western Gulf OTUs, while eastern Gulf OTUs may have suffered population bottlenecks in the past. Main conclusions: Divergences between sister species of sand-burrowing amphipods exceeds the timing of previous vicariant hypotheses. The split appears to be consistent with Miocene sedimentation levels from the Mississippi River acting as an eastwest barrier to gene flow in the Gulf of Mexico. Given their strong population structure and cryptic diversity, haustoriid amphipods are ideal model organisms for studying open coast biogeography.
