A global assessment of genetic diversity and distributions of medusozoans
Jellyfish ‘blooms’ are enigmatic due in part to poor understanding of underlying demographic causes and the geographic extent of populations. I aimed to answer two questions regarding the geographic scales of medusozoan genetic diversity: (1) do congeneric individuals found within the same regional boundary correspond with a single evolutionarily significant unit (ESU), and (2) do individuals from different geographic regions correspond to different ESUs? I used DNA barcoding of cytochrome c subunit I (COI) to estimate genetic heterogeneity in 18 medusozoan genera. I quantified the distribution of genetic variation within- and between-regions: either large marine ecosystems (LMEs) or Longhurst biogeographical provinces (Provinces). For both within- and between-region analyses, I estimated K2P pairwise sequence divergence (PSD) and total ESUs present by genus. For between-region analyses, I also estimated pairwise ΦST. Additionally, I correlated genetic and geodesic distances for three genera to establish an estimate of the geographic scales of ESUs as a continuous-scale alternative to the categorical framework explicit in definitions of LMEs and Provinces. I found that of 3,750 and 5,649 within-region comparisons 73-80% constituted a single ESU. Likewise, within-regions, ~51-67% of comparisons revealed ΦST > 0.2. I found that of 3,859 and 5,460 between-region comparisons, ~16% of comparisons have unique ESUs present between regions (LMEs and Provinces, respectively). Genetic and geodesic distances revealed patterns with life-history; meroplanktonic taxa exhibit species-level differences at 1,000 km, whereas the holoplanktonic taxon was genetically homogenous up to 17,500 km. For the majority of regions, individuals found within the same region do not correspond with a single ESU, and few taxa have unique ESUs present within LMEs or Provinces. This greater understanding of the number, distributions, and differences of ESUs allows us to develop better hypotheses about local adaptation and the causes of population dynamics for these taxa. Taxon and life-history have important effects on genetic structuring, and regional differences may elicit different dynamics. This analysis is a step toward an ecological, genetic framework through which I can begin to explore the impacts of global change elicited on, and via, medusozoan dynamics.