The impacts of past, present and future ocean chemistry on predatory planktonic snails.

The atlantid heteropods represent the only predatory, aragonite shelled zooplankton. Atlantid shell production is likely to be sensitive to ocean acidification (OA), and yet we know little about their mechanisms of calcification, or their response to changing ocean chemistry. Here, we present the first study into calcification and gene expression effects of short-term OA exposure on juvenile atlantids across three pH scenarios: mid-1960s, ambient and 2050 conditions. Calcification and gene expression indicate a distinct response to each treatment. Shell extension and shell volume were reduced from the mid-1960s to ambient conditions, suggesting that calcification is already limited in today's South Atlantic. However, shell extension increased from ambient to 2050 conditions. Genes involved in protein synthesis were consistently upregulated, whereas genes involved in organismal development were downregulated with decreasing pH. Biomineralization genes were upregulated in the mid-1960s and 2050 conditions, suggesting that any deviation from ambient carbonate chemistry causes stress, resulting in rapid shell growth. We conclude that atlantid calcification is likely to be negatively affected by future OA. However, we also found that plentiful food increased shell extension and shell thickness, and so synergistic factors are likely to impact the resilience of atlantids in an acidifying ocean.

Oceanic dispersal barriers in a holoplanktonic gastropod.

Pteropods, a group of holoplanktonic gastropods, are regarded as bioindicators of the effects of ocean acidification on open ocean ecosystems, because their thin aragonitic shells are susceptible to dissolution. While there have been recent efforts to address their capacity for physiological acclimation, it is also important to gain predictive understanding of their ability to adapt to future ocean conditions. However, little is known about the levels of genetic variation and large-scale population structuring of pteropods, key characteristics enabling local adaptation. We examined the spatial distribution of genetic diversity in the mitochondrial cytochrome c oxidase I (COI) and nuclear 28S gene fragments, as well as shell shape variation, across a latitudinal transect in the Atlantic Ocean (35°N-36°S) for the pteropod Limacina bulimoides. We observed high levels of genetic variability (COI π = 0.034, 28S π = 0.0021) and strong spatial structuring (COI ΦST  = 0.230, 28S ΦST  = 0.255) across this transect. Based on the congruence of mitochondrial and nuclear differentiation, as well as differences in shell shape, we identified a primary dispersal barrier in the southern Atlantic subtropical gyre (15-18°S). This barrier is maintained despite the presence of expatriates, a gyral current system, and in the absence of any distinct oceanographic gradients in this region, suggesting that reproductive isolation between these populations must be strong. A secondary dispersal barrier supported only by 28S pairwise ΦST comparisons was identified in the equatorial upwelling region (between 15°N and 4°S), which is concordant with barriers observed in other zooplankton species. Both oceanic dispersal barriers were congruent with regions of low abundance reported for a similar basin-scale transect that was sampled 2 years later. Our finding supports the hypothesis that low abundance indicates areas of suboptimal habitat that result in barriers to gene flow in widely distributed zooplankton species. Such species may in fact consist of several populations or (sub)species that are adapted to local environmental conditions, limiting their potential for adaptive responses to ocean changes. Future analyses of genome-wide diversity in pteropods could provide further insight into the strength, formation and maintenance of oceanic dispersal barriers.

The origin and diversification of pteropods precede past perturbations in the Earth's carbon cycle.

Pteropods are a group of planktonic gastropods that are widely regarded as biological indicators for assessing the impacts of ocean acidification. Their aragonitic shells are highly sensitive to acute changes in ocean chemistry. However, to gain insight into their potential to adapt to current climate change, we need to accurately reconstruct their evolutionary history and assess their responses to past changes in the Earth's carbon cycle. Here, we resolve the phylogeny and timing of pteropod evolution with a phylogenomic dataset (2,654 genes) incorporating new data for 21 pteropod species and revised fossil evidence. In agreement with traditional taxonomy, we recovered molecular support for a division between "sea butterflies" (Thecosomata; mucus-web feeders) and "sea angels" (Gymnosomata; active predators). Molecular dating demonstrated that these two lineages diverged in the early Cretaceous, and that all main pteropod clades, including shelled, partially-shelled, and unshelled groups, diverged in the mid- to late Cretaceous. Hence, these clades originated prior to and subsequently survived major global change events, including the Paleocene-Eocene Thermal Maximum (PETM), the closest analog to modern-day ocean acidification and warming. Our findings indicate that planktonic aragonitic calcifiers have shown resilience to perturbations in the Earth's carbon cycle over evolutionary timescales.

Fossil-calibrated molecular phylogeny of atlantid heteropods (Gastropoda, Pterotracheoidea).

BACKGROUND: The aragonite shelled, planktonic gastropod family Atlantidae (shelled heteropods) is likely to be one of the first groups to be impacted by imminent ocean changes, including ocean warming and ocean acidification. With a fossil record spanning at least 100 Ma, atlantids have experienced and survived global-scale ocean changes and extinction events in the past. However, the diversification patterns and tempo of evolution in this family are largely unknown. RESULTS: Based on a concatenated maximum likelihood phylogeny of three genes (cytochrome c oxidase subunit 1 mitochondrial DNA, 28S and 18S ribosomal rRNA) we show that the three extant genera of the family Atlantidae, Atlanta, Protatlanta and Oxygyrus, form monophyletic groups. The genus Atlanta is split into two groups, one exhibiting smaller, well ornamented shells, and the other having larger, less ornamented shells. The fossil record, in combination with a fossil-calibrated phylogeny, suggests that large scale atlantid extinction was accompanied by considerable and rapid diversification over the last 25 Ma, potentially driven by vicariance events. CONCLUSIONS: Now confronted with a rapidly changing modern ocean, the ability of atlantids to survive past global change crises gives some optimism that they may be able to persist through the Anthropocene.

Environmental DNA surveys detect distinct metazoan communities across abyssal plains and seamounts in the western Clarion Clipperton Zone.

The deep seafloor serves as a reservoir of biodiversity in the global ocean, with >80% of invertebrates at abyssal depths still undescribed. These diverse and remote deep-sea communities are critically under-sampled and increasingly threatened by anthropogenic impacts, including future polymetallic nodule mining. Using a multigene environmental DNA (eDNA) metabarcoding approach, we characterized metazoan communities sampled from sediments, polymetallic nodules and seawater in the western Clarion Clipperton Zone (CCZ) to test the hypotheses that deep seamounts (a) are species richness hotspots in the abyss, (b) have structurally distinct communities in comparison to other deep-sea habitats, and (c) that seafloor particulate organic carbon (POC) flux and polymetallic nodule density are positively correlated with metazoan diversity. eDNA metabarcoding was effective at characterizing distinct biotas known to occur in association with different abyssal substrate types (e.g., nodule- and sediment-specific fauna), with distinct community composition and few taxa shared across substrates. Seamount faunas had higher overall taxonomic richness, and different community composition and biogeography than adjacent abyssal plains, with seamount communities displaying less connectivity between regions than comparable assemblages on the abyssal plains. Across an estimated gradient of low to moderate POC flux, we find lowest taxon richness at the lowest POC flux, as well as an effect of nodule size on community composition. Our results suggest that while abyssal seamounts are important reservoirs of metazoan diversity in the CCZ, given limited taxonomic overlap between seamount and plains fauna, conservation of seamount assemblages will be insufficient to protect biodiversity and ecosystem function in regions targeted for mining.

Genetic diversity and novel lineages in the cosmopolitan copepod Pleuromamma abdominalis in the Southeast Pacific.

Across boundary currents, zooplankton are subject to strong oceanographic gradients and hence strong selective pressures. How such gradients interact with the speciation process of pelagic organisms is still poorly understood in the open ocean realm. Here we report on genetic diversity within the pelagic copepod Pleuromamma abdominalis in the poorly known Southeast Pacific region, with samples spanning an ocean gradient from coastal upwelling to the oligotrophic South Pacific Subtropical Gyre. We assessed variation in fragments of the mitochondrial (mt) genes cytochrome c oxidase subunit I (COI) and Cytochrome b as well as in the nuclear internal transcribed spacer (ITS) region and 28 S rRNA. Phylogenetic analyses revealed the presence of 8 divergent lineages occurring across the gradient with genetic distances in the range of 0.036 and 0.44 (mt genes), and GMYC species delimitation methods support their inference as distinct (undescribed) species. Genetic lineages occurring across the zonal gradient showed strong genetic structuring, with the presence of at least two new lineages within the coastal upwelling zone, revealing an unexpectedly high level of endemism within the Humboldt Current System. Multivariate analyses found strong correlation between genetic variation and surface chlorophyll-a and salinity, suggesting an important role for hydrographic gradients in maintaining genetic diversity. However, the presence of cryptic lineages within the upwelling zone cannot be easily accounted for by environmental heterogeneity and poses challenging questions for understanding the speciation process for oceanic zooplankton.

Resolving species boundaries in the Atlanta brunnea species group (Gastropoda, Pterotracheoidea).

Atlantid heteropods are a family of holoplanktonic marine gastropods that occur primarily in tropical and subtropical latitudes. Atlantids bear a delicate aragonitic shell (<14 mm) and live in the upper ocean, where ocean acidification and ocean warming have a pronounced effect. Therefore, atlantids are likely to be sensitive to these ocean changes. However, we lack sufficiently detailed information on atlantid taxonomy and biogeography, which is needed to gain a deeper understanding of the consequences of a changing ocean. To date, atlantid taxonomy has mainly relied on morphometrics and shell ornamentation, but recent molecular work has highlighted hidden diversity. This study uses an integrated approach in a global analysis of biogeography, variation in shell morphology and molecular phylogenies based on three genes (CO1, 28S and 18S) to resolve the species boundaries within the Atlanta brunnea group. Results identify a new species, Atlanta vanderspoeli, from the Equatorial and South Pacific Ocean, and suggest that individuals of A. brunnea living in the Atlantic Ocean are an incipient species. Our results provide an important advance in atlantid taxonomy and will enable identification of these species in future studies of living and fossil plankton.

Larval assemblages over the abyssal plain in the Pacific are highly diverse and spatially patchy.

Abyssal plains are among the most biodiverse yet least explored marine ecosystems on our planet, and they are increasingly threatened by human impacts, including future deep seafloor mining. Recovery of abyssal populations from the impacts of polymetallic nodule mining will be partially determined by the availability and dispersal of pelagic larvae leading to benthic recolonization of disturbed areas of the seafloor. Here we use a tree-of-life (TOL) metabarcoding approach to investigate the species richness, diversity, and spatial variability of the larval assemblage at mesoscales across the abyssal seafloor in two mining-claim areas in the eastern Clarion Clipperton Fracture Zone (CCZ; abyssal Pacific). Our approach revealed a previously unknown taxonomic richness within the meroplankton assemblage, detecting larvae from 12 phyla, 23 Classes, 46 Orders, and 65 Families, including a number of taxa not previously reported at abyssal depths or within the Pacific Ocean. A novel suite of parasitic copepods and worms were sampled, from families that are known to associate with other benthic invertebrates or demersal fishes as hosts. Larval assemblages were patchily distributed at the mesoscale, with little similarity in OTUs detected among deployments even within the same 30 × 30 km study area. Our results provide baseline observations on larval diversity prior to polymetallic nodule mining in this region, and emphasize our overwhelming lack of knowledge regarding larvae of the benthic boundary layer in abyssal plain ecosystems.

Copepod nauplii use phosphorus from bacteria, creating a short circuit in the microbial loop.

In subtropical oceans phytoplankton carbon: phosphorus (C : P) ratios are high, and these ratios are predicted to increase further with rising ocean temperatures and stratification. Prey stoichiometry may pose a problem for copepod zooplankton nauplii, which have high phosphorus demands due to rapid growth. We hypothesised that nauplii meet this demand by consuming bacteria. Naupliar bacterial and phytoplankton carbon and phosphorus ingestion, assimilation and incorporation were traced using 33 P and 14 C radioisotopes. Bacterial carbon was incorporated four times less efficiently into biomass than phytoplankton carbon. In contrast, bacterial and phytoplankton phosphorus were incorporated at similar efficiencies, and bacteria could meet a substantial amount of naupliar phosphorus requirements. As parts of the ocean become more oligotrophic, bacteria could help sustain naupliar growth and survival under suboptimal stoichiometric conditions. Thus, nauplii may be a shortcut for phosphorus from the microbial loop to the classical food web.

Biogeography and genetic diversity of the atlantid heteropods.

The atlantid heteropods are regularly encountered, but rarely studied marine planktonic gastropods. Relying on a small (<14 mm), delicate aragonite shell and living in the upper ocean means that, in common with pteropods, atlantids are likely to be affected by imminent ocean changes. Variable shell morphology and widespread distributions indicate that the family is more diverse than the 23 currently known species. Uncovering this diversity is fundamental to determining the distribution of atlantids and to understanding their environmental tolerances. Here we present phylogenetic analyses of all described species of the family Atlantidae using 437 new and 52 previously published cytochrome c oxidase subunit 1 mitochondrial DNA (mtCO1) sequences. Specimens and published sequences were gathered from 32 Atlantic Ocean stations, 14 Indian Ocean stations and 21 Pacific Ocean stations between 35°N and 43°S. DNA barcoding and Automatic Barcode Gap Discovery (ABGD) proved to be valuable tools for the identification of described atlantid species, and also revealed ten additional distinct clades, suggesting that the diversity within this family has been underestimated. Only two of these clades displayed obvious morphological characteristics, demonstrating that much of the newly discovered diversity is hidden from morphology-based identification techniques. Investigation of six large atlantid collections demonstrated that 61% of previously described (morpho) species have a circumglobal distribution. Of the remaining 39%, two species were restricted to the Atlantic Ocean, five occurred in the Indian and Pacific oceans, one species was only found in the northeast Pacific Ocean, and one occurred only in the Southern Subtropical Convergence Zone. Molecular analysis showed that seven of the species with wide distributions were comprised of two or more clades that occupied distinct oceanographic regions. These distributions may suggest narrower environmental tolerances than the described morphospecies. Results provide an updated biogeography and mtCO1 reference dataset of the Atlantidae that may be used to identify atlantid species and provide a first step in understanding their evolutionary history and accurate distribution, encouraging the inclusion of this family in future plankton research.

Pseudoliparis swirei sp. nov.: A newly-discovered hadal snailfish (Scorpaeniformes: Liparidae) from the Mariana Trench.

Pseudoliparis swirei sp. nov. is described from 37 individuals collected in the Mariana Trench at depths 6898-7966 m. The collection of this new species is the deepest benthic capture of a vertebrate with corroborated depth data. Here, we describe P. swirei sp. nov. and discuss aspects of its morphology, biology, distribution, and phylogenetic relationships to other hadal liparids based on analysis of three mitochondrial genes. Pseudoliparis swirei sp. nov. is almost certainly endemic to the Mariana Trench, as other hadal liparids appear isolated to a single trench/ trench system in the Kermadec, Macquarie, South Sandwich, South Orkney, Peru-Chile, Kurile-Kamchatka and Japan trenches. The discovery of another hadal liparid species, apparently abundant at depths where other fish species are few and only found in low numbers, provides further evidence for the dominance of this family among the hadal fish fauna.

Vertical gradients in species richness and community composition across the twilight zone in the North Pacific Subtropical Gyre.

Although metazoan animals in the mesopelagic zone play critical roles in deep pelagic food webs and in the attenuation of carbon in midwaters, the diversity of these assemblages is not fully known. A metabarcoding survey of mesozooplankton diversity across the epipelagic, mesopelagic and upper bathypelagic zones (0-1500 m) in the North Pacific Subtropical Gyre revealed far higher estimates of species richness than expected given prior morphology-based studies in the region (4,024 OTUs, 10-fold increase), despite conservative bioinformatic processing. Operational taxonomic unit (OTU) richness of the full assemblage peaked at lower epipelagic-upper mesopelagic depths (100-300 m), with slight shoaling of maximal richness at night due to diel vertical migration, in contrast to expectations of a deep mesopelagic diversity maximum as reported for several plankton groups in early systematic and zoogeographic studies. Four distinct depth-stratified species assemblages were identified, with faunal transitions occurring at 100 m, 300 m and 500 m. Highest diversity occurred in the smallest zooplankton size fractions (0.2-0.5 mm), which had significantly lower % OTUs classified due to poor representation in reference databases, suggesting a deep reservoir of poorly understood diversity in the smallest metazoan animals. A diverse meroplankton assemblage also was detected (350 OTUs), including larvae of both shallow and deep living benthic species. Our results provide some of the first insights into the hidden diversity present in zooplankton assemblages in midwaters, and a molecular reappraisal of vertical gradients in species richness, depth distributions and community composition for the full zooplankton assemblage across the epipelagic, mesopelagic and upper bathypelagic zones.

Temporal Stability of Genetic Structure in a Mesopelagic Copepod.

Although stochasticity in oceanographic conditions is known to be an important driver of temporal genetic change in many marine species, little is known about whether genetically distinct plankton populations can persist in open ocean habitats. A prior study demonstrated significant population genetic structure among oceanic gyres in the mesopelagic copepod Haloptilus longicornis in both the Atlantic and Pacific Oceans, and we hypothesized that populations within each gyre represent distinct gene pools that persist over time. We tested this expectation through basin-scale sampling across the Atlantic Ocean in 2010 and 2012. Using both mitochondrial (mtCOII) and microsatellite markers (7 loci), we show that the genetic composition of populations was stable across two years in both the northern and southern subtropical gyres. Genetic variation in this species was partitioned among ocean gyres (FCT = 0.285, P < 0.0001 for mtCOII, FCT = 0.013, P < 0.0001 for microsatellites), suggesting strong spatial population structure, but no significant partitioning was found among sampling years. This temporal persistence of population structure across a large geographic scale was coupled with chaotic genetic patchiness at smaller spatial scales, but the magnitude of genetic differentiation was an order of magnitude lower at these smaller scales. Our results demonstrate that genetically distinct plankton populations persist over time in highly-dispersive open ocean habitats, and this is the first study to rigorously test for temporal stability of large scale population structure in the plankton.

Global biogeography and evolution of Cuvierina pteropods.

BACKGROUND: Shelled pteropods are planktonic gastropods that are potentially good indicators of the effects of ocean acidification. They also have high potential for the study of zooplankton evolution because they are metazoan plankton with a good fossil record. We investigated phenotypic and genetic variation in pteropods belonging to the genus Cuvierina in relation to their biogeographic distribution across the world's oceans. We aimed to assess species boundaries and to reconstruct their evolutionary history. RESULTS: We distinguished six morphotypes based on geometric morphometric analyses of shells from 926 museum and 113 fresh specimens. These morphotypes have distinct geographic distributions across the Atlantic, Pacific and Indian oceans, and belong to three major genetic clades based on COI and 28S DNA sequence data. Using a fossil-calibrated phylogeny, we estimated that these clades separated in the Late Oligocene and Early to Middle Miocene. We found evidence for ecological differentiation among all morphotypes based on ecological niche modelling with sea surface temperature, salinity and phytoplankton biomass as primary determinants. Across all analyses, we found highly congruent patterns of differentiation suggesting species level divergences between morphotypes. However, we also found distinct morphotypes (e.g. in the Atlantic Ocean) that were ecologically, but not genetically differentiated. CONCLUSIONS: Given the distinct ecological and phenotypic specializations found among both described and undescribed Cuvierina taxa, they may not respond equally to future ocean changes and may not be equally sensitive to ocean acidification. Our findings support the view that ecological differentiation may be an important driving force in the speciation of zooplankton.

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

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.

High cryptic diversity across the global range of the migratory planktonic copepods Pleuromamma piseki and P. gracilis.

Although holoplankton are ocean drifters and exhibit high dispersal potential, a number of studies on single species are finding highly divergent genetic clades. These cryptic species complexes are important to discover and describe, as identification of common marine species is fundamental to understanding ecosystem dynamics. Here we investigate the global diversity within Pleuromamma piseki and P. gracilis, two dominant members of the migratory zooplankton assemblage in subtropical and tropical waters worldwide. Using DNA sequence data from the mitochondrial gene cytochrome c oxidase subunit II (mtCOII) from 522 specimens collected across the Pacific, Atlantic and Indian Oceans, we discover twelve well-resolved genetically distinct clades in this species complex (Bayesian posterior probabilities >0.7; 6.3-17% genetic divergence between clades). The morphologically described species P. piseki and P. gracilis did not form monophyletic groups, rather they were distributed throughout the phylogeny and sometimes co-occurred within well-resolved clades: this result suggests that morphological characters currently used for taxonomic identification of P. gracilis and P. piseki may be inaccurate as indicators of species' boundaries. Cryptic clades within the species complex ranged from being common to rare, and from cosmopolitan to highly restricted in distribution across the global ocean. These novel lineages appear to be ecologically divergent, with distinct biogeographic distributions across varied pelagic habitats. We hypothesize that these mtDNA lineages are distinct species and suggest that resolving their systematic status is important, given the ecological significance of the genus Pleuromamma in subtropical-tropical waters worldwide.

Microsatellites for next-generation ecologists: a post-sequencing bioinformatics pipeline.

Microsatellites are the markers of choice for a variety of population genetic studies. The recent advent of next-generation pyrosequencing has drastically accelerated microsatellite locus discovery by providing a greater amount of DNA sequencing reads at lower costs compared to other techniques. However, laboratory testing of PCR primers targeting potential microsatellite markers remains time consuming and costly. Here we show how to reduce this workload by screening microsatellite loci via bioinformatic analyses prior to primer design. Our method emphasizes the importance of sequence quality, and we avoid loci associated with repetitive elements by screening with repetitive sequence databases available for a growing number of taxa. Testing with the Yellowstripe Goatfish Mulloidichthys flavolineatus and the marine planktonic copepod Pleuromamma xiphias we show higher success rate of primers selected by our pipeline in comparison to previous in silico microsatellite detection methodologies. Following the same pipeline, we discover and select microsatellite loci in nine additional species including fishes, sea stars, copepods and octopuses.

High evolutionary potential of marine zooplankton.

Open ocean zooplankton often have been viewed as slowly evolving species that have limited capacity to respond adaptively to changing ocean conditions. Hence, attention has focused on the ecological responses of zooplankton to current global change, including range shifts and changing phenology. Here, we argue that zooplankton also are well poised for evolutionary responses to global change. We present theoretical arguments that suggest plankton species may respond rapidly to selection on mildly beneficial mutations due to exceptionally large population size, and consider the circumstantial evidence that supports our inference that selection may be particularly important for these species. We also review all primary population genetic studies of open ocean zooplankton and show that genetic isolation can be achieved at the scale of gyre systems in open ocean habitats (100s to 1000s of km). Furthermore, population genetic structure often varies across planktonic taxa, and appears to be linked to the particular ecological requirements of the organism. In combination, these characteristics should facilitate adaptive evolution to distinct oceanographic habitats in the plankton. We conclude that marine zooplankton may be capable of rapid evolutionary as well as ecological responses to changing ocean conditions, and discuss the implications of this view. We further suggest two priority areas for future research to test our hypothesis of high evolutionary potential in open ocean zooplankton, which will require (1) assessing how pervasive selection is in driving population divergence and (2) rigorously quantifying the spatial and temporal scales of population differentiation in the open ocean. Recent attention has focused on the ecological responses of open ocean zooplankton to current global change, including range shifts and changing phenology. Here, we argue that marine zooplankton also are well poised for evolutionary responses to global change.

Population differentiation in the open sea: insights from the pelagic copepod Pleuromamma xiphias.

Although a number of recent studies of marine holoplankton have reported significant genetic structure among populations, little is currently known about the biological and oceanographic processes that influence population connectivity in oceanic plankton. In order to examine how depth preferences influence dispersal in oceanic plankton, I characterized the genetic structure of a copepod with diel vertical migration (DVM) (Pleuromamma xiphias), throughout its global distribution, and compared these results to those expected given the interaction of this species' habitat depth with ocean circulation and bathymetry. Mitochondrial COI sequences from 651 individuals from 28 sites in the Indian, Pacific, and Atlantic Oceans revealed highly significant genetic differentiation both within and among ocean basins. Limited dispersal among distinct pelagic provinces seems to have played a major role in population differentiation in this species, with strong genetic breaks observed across known oceanographic fronts or current systems in all three ocean basins. The Indo-West Pacific (IWP) holds a highly distinct genetic population of this species that was sampled in both the western Pacific and eastern Indian Oceans. This suggests that the IWP does not act as a strong barrier to gene flow between basins, as expected, despite the relatively shallow water depth (<200 m) and vertically extensive (>400 m) diel migration of this species. A pattern of isolation by distance was observed in the Indian Ocean with genetic differentiation among samples down to spatial scales of ∼800 km, indicating that realized dispersal in P. xiphias occurs over much smaller spatial scales than in previously reported oceanic holoplankton. Given its highly regionalized population genetic structure, P. xiphias may have some capacity to adapt to local oceanographic conditions, and it should not be assumed that populations of this species in distinct pelagic biomes will respond in the same way to shared physical or climatic forcing.