DOI: 10.5061/dryad.573n5tb4n
title: First come, first served: possible role for priority effects in marine populations under different degrees of dispersal potential

description: Aim Studying clearly delineated populations in marine lakes, islands of
 sea, we investigate the interplay of habitat size, immigration, and
 priority effects in shaping marine population genetic structure. Location
 Marine lakes and coastal locations in Indonesia, Palau, Papua New-Guinea
 and Australia. Taxon Mussels (Mytillidae, Brachidontes spp.) Methods
 Populations were sampled from four coastal locations and 22 marine lakes
 of similar age (~8,000 years), yet differing in size (0.04 - 4.7 km2) and
 degree of connection to the adjacent sea. While some lakes are highly
 connected, allowing potential influx of larvae from the sea, others have
 very limited water exchange. We assessed the phylogeographic structure and
 demographic history using mitochondrial and nuclear DNA sequence data, and
 combined this with geometric morphometrics. The effects of lake
 characteristics on population genetic diversity and structure were tested
 using linear regression and Mantel tests. Results Each lake contained one
 of six distinct genetic lineages, which were characterized by deep
 phylogenetic splits and significant morphometric differences. These
 lineages likely represent separate species. The lineages showed similar
 demographic patterns, with lakes containing founder populations that
 rapidly expanded and diverged. Genetic diversity within lake populations
 was significantly correlated with lake area, but not with physical
 connection to the adjacent sea. Within lineages that occurred in multiple
 lakes there was strong population structure (average ΦST 0.65), which did
 not conform to an isolation-by-distance pattern or to the degree of
 dispersal potential. Main Conclusions Marine lakes across a gradient of
 physical isolation show strong population structure and evidence for in
 situ divergence. We hypothesize that the observed genetic structure is the
 result of priority effects. In addition, reduction of habitat size appears
 to reduce genetic diversity, even at very small spatial scales. Our
 findings are relevant in the context of ongoing alterations to coastal
 hydrodynamics, which lead to habitat reduction and influence migration
 among populations at fine spatial scales.
description: To identify morphospecies, geometric morphometric analyses were
 processed for a total of 419 digital images of 13 mussel populations in
 Indonesia (lineage A–D, F). The shells of lineage E were not available for
 analysis. The analyses were performed following the procedure described in
 Becking et al. (2016). Briefly, mussels were positioned in a standardized
 way and photographed. Shell outlines were drawn via curves starting from
 the beak of the mussel using tpsDig (Rohlf, 2010). Curves were converted
 to semi-landmarks and standardized for size and orientation in tpsRelw
 (Rohlf, 2010). We obtained centroid size and relative warp scores for each
 specimen. Furthermore, repeatability was confirmed for the first three
 relative warp axes; thus, subsequent analyses were performed only on the
 first three axes. To study differences among groups, we performed a
 nonparametric permutational multivariate analysis of variance (PERMANOVA,
 1,000 permutations) implemented in past 2.11 based on Euclidean distances
 (Hammer, Harper, & Ryan, 2001). We corrected for multiple testing
 by calculating Bonferroni-corrected P values.