Multilocus evidence for globally distributed cryptic species and distinct populations across ocean gyres in a mesopelagic copepod

Abstract

Zooplanktonic taxa have a greater number of distinct populations and species than might be predicted based on their large population sizes and open-ocean habitat, which lacks obvious physical barriers to dispersal and gene flow. To gain insight into the evolutionary mechanisms driving genetic diversification in zooplankton, we developed eight microsatellite markers to examine the population structure of an abundant, globally distributed mesopelagic copepod, Haloptilus longicornis, at 18 sample sites across the Atlantic and Pacific Oceans (n = 761). When comparing our microsatellite results with those of a prior study that used a mtDNA marker (mtCOII, n = 1059, 43 sample sites), we unexpectedly found evidence for the presence of a cryptic species pair. These species were globally distributed and apparently sympatric, and were separated by relatively weak genetic divergence (reciprocally monophyletic mtCOII lineages 1.6% divergent; microsatellite FST ranging from 0.28 to 0.88 across loci, P < 0.00001). Using both mtDNA and microsatellite data for the most common of the two species (n = 669 for microsatellites, n = 572 for mtDNA), we also found evidence for allopatric barriers to gene flow within species, with distinct populations separated by continental landmasses and equatorial waters in both the Atlantic and Pacific Ocean basins. Our study shows that oceanic barriers to gene flow can act as a mechanism promoting allopatric diversification in holoplanktonic taxa, despite the high potential dispersal abilities and pelagic habitat for these species.

Contrasting evolutionary patterns between two haplogroups of Haematobia exigua (Diptera: Muscidae) from the mainland and islands of Southeast Asia

“…This hypothesis is supported by the high genetic differentiation and low gene flow in our datasets. Indeed, the oceanographic barrier to dispersal and gene flow has been suggested as a key factor driving diversification in several insects 10, 22 . Nevertheless, the South China Sea did not have a limiting effect on gene flow for H. exigua on the mainland and Borneo.…”

Comparative phylogeography of the ocean planet

“…Phylogeographic studies reveal that many cosmopolitan taxa are composed of multiple cryptic species (138,139), including some that are sympatric over part of their ranges (140). Populations of these cosmopolitan species are subdivided in two ways concordant with the BGCP framework: (i) by continental land masses separating ocean basins, and (ii) by habitat discontinuities in the equatorial region between subtropical gyres in the northern and southern hemispheres (140)(141)(142).…”

“…Phylogeographic studies reveal that many cosmopolitan taxa are composed of multiple cryptic species (138,139), including some that are sympatric over part of their ranges (140). Populations of these cosmopolitan species are subdivided in two ways concordant with the BGCP framework: (i) by continental land masses separating ocean basins, and (ii) by habitat discontinuities in the equatorial region between subtropical gyres in the northern and southern hemispheres (140)(141)(142). The few global-scale phylogeographic studies have been restricted to copepods, but evidence from a diversity of other taxa sampled at ocean basin scales indicate that lineages have diverged both in allopatry and sympatry at much smaller geographic distances than anticipated, with examples drawn from chaetognaths (143), euphausiids (144), and mollusks (145).…”

Cryptic diversity in the Japanese mantis shrimp Oratosquilla oratoria (Crustacea: Squillidae): Allopatric diversification, secondary contact and hybridization

“…The application of molecular methods to species delimitation has uncovered an overwhelming amount of unrecognized cryptic diversity in marine organisms, e.g. copepod 3, 4 , barnacles 5, 6 , bivalves 7, 8 and fishes 9, 10 . These patterns challenged the paradigm that an apparent homogeneity of the marine environments and pelagic larval stages in marine species limit their diversification 11 .…”