A global estimate of genetic and geographic differentiation in macromedusae - implications for identifying the causes of jellyfish blooms

Jellyfish blooms are enigmatic, in part due to uncertainty surrounding the geographic extents of populations and underlying causes and effects of demographic change. We aimed to promote understanding of likely drivers of macromedusae blooms by addressing 2 questions about patterns of genetic diversity: (1) Do congeneric individuals found within the same large marine ecosystem (LME) comprise a single species? (2) Do congeneric individuals from different LMEs represent different species? We DNA-barcoded (cytochrome c oxidase subunit I) 804 specimens in 16 medusozoan genera across 32 LMEs. We calculated K2P pairwise sequence divergence between congeneric individuals and estimated geodesic distance between all sample locations within and between LMEs; additionally, we calculated pairwise Phi(ST) among conspecific samples within LMEs. While LMEs reasonably served as a proxy for species in similar to 76% of between-LME comparisons, LME boundaries did not match species boundaries in similar to 24% of inter-regional comparisons. Moreover, similar to 19% of within-LME comparisons showed cryptic species and similar to 67% showed substantial intra-specific phylogeographic structure. The overall rate of mismatch of the scale of LMEs and the scale of genetic structure in macromedusae is likely > 70%, because the barcoding and phylogeographic analyses employed here cannot yet distinguish even finer-scale ecologically important population structure. These results were mirrored in analyses using Longhurst's Biogeographical Provinces. Meroplanktonic species often were genetically structured on scales of 10s to 100s of km, though holoplanktonic species may be eurymictic across 1000s to 10000s of km. We also found tentative evidence of onshore-offshore, depth, and latitudinal trends in population structure. When studying the causes and consequences of jellyfish blooms, more accurate descriptions of genetic and geographic differentiation are crucial.