This research focused on understanding population genetic structure within marine zooplankton species, the invertebrate animals that drift in the open ocean throughout their life cycle (Figure 1). Marine zooplankton make up the intermediate trophic levels of pelagic marine food webs, and play an important role in biogeochemical cycling of the upper ocean. Although many aspects of zooplankton ecology are well studied, very little is known about their population genetic structure at regional to global spatial scales. Understanding the genetic structure of these populations is important, because it will strongly influence their potential for adaptation to environmental change. High levels of migration among populations are known to inhibit adaptation to local environmental conditions, and historically, these planktonic drifters were thought to experience very high levels of dispersal among populations in the open ocean. Although early studies on the population genetics of marine zooplankton found little genetic differentiation among populations, recent work has challenged this view. We now know that the dispersal capacity of these species may not be as high as initially expected, and that it may be moderated by particular ecological traits. But which ecological traits are important determinants of dispersal in open ocean plankton species? The traits of importance, and how they interact with ocean circulation, are entirely unknown. The goal of this research was to test whether habitat depth specialization is the primary trait driving large-scale population genetic structure of zooplankton species. We conducted comparative studies of the large-scale population structure of five planktonic copepod species that utilize strikingly different depth-related habitats, in order to test key predictions about how organismal depth interacts with oceanography and bathymetry to control genetic structure of open ocean species. Under the first objective of the research, we developed new molecular markers to assess the genetic composition and structure of our target species, using Next Generation Sequencing (NGS) techniques. These new markers allowed us to determine population genetic structure across a global collection of material in the Indian, Pacific and Atlantic Oceans using a multilocus approach. As part of that effort, we made field collections on a cruise that transited the entire Atlantic Ocean in 2012 (> 90° latitude, Atlantic Meridional Transect Cruise 22). We found that oceanic zooplankton species often are characterized by strong genetic structure among ocean biomes, with genetic breaks among populations typically occurring at water mass boundaries (Figure 2). In two species, Haloptilus longicornis and Pleuromamma xiphias, lower population abundance in equatorial waters of the Atlantic Ocean coincided with a strong genetic break within each species, suggesting that this equatorial region acts as a dispersal barrier for populations in northern and southern subtropical gyres. Additional modeling work by graduate student Emily Norton also demonstrated that this dispersal barrier must occur due to biophysical mechanisms, rather than being solely due to a lack of physical transport among ocean regions. Our results provide the first well-documented case of a dispersal barrier for oceanic zooplankton. Our work has important implications for understanding the scale over which plankton populations may be adapted to distinct ocean habitats. In addition, we discovered many species that are new to science, and fully characterized several cryptic species complexes across their global distributions (Figures 3, 4). Because we focused on zooplankton species that are abundant and common in the upper ocean, these cryptic species complexes were found within species that are ecologically important in pelagic food webs. Our findings from this research have been published in 7 peer-reviewed publications. Two additional publication...
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