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Changes in the community size structure of Arctic copepods toward smaller and less fat individuals or species have been linked to environmental changes. The underpinning mechanisms are, however, poorly understood. We use a two‐step hurdle regression model to analyze spatially resolved, long‐term survey data of the Barents Sea mesozooplankton community along gradients of water mass properties, sea ice, and bottom depth. We test the hypothesis that reduced visual predation, and hence increased survival in dim habitats, explains the distribution of large copepods. We expect the presence and biomass of large copepods to increase with increasing bottom depth and the occurrence of seasonal ice‐cover. The patterns and drivers that emerge from our analysis support our hypothesis: in the Barents Sea large copepods were predominantly found in deep troughs that intersect the shelf south of the polar front, or at shallower depths in seasonally ice‐covered waters northeast of Svalbard. On the banks, large copepods are largely absent whereas smaller copepods appear to survive. Top‐down control provides one plausible explanation for these distributions. Large copepods survive where sea‐ice shades the water or deep habitats permit escape from visual predators through vertical migrations. However, when upwelled onto shallow banks or flushed out from below the ice they are decimated by visual foragers. Therefore, advection and topographic blockage of vertical zooplankton distributions are key mechanisms for the efficient energy transfer and productivity in subarctic and Arctic shelf seas. New prolific foraging grounds may open up for planktivores where the ice‐edge recedes under a changing climate. [ABSTRACT FROM AUTHOR]

Aim: Lanternfishes (Myctophidae) are one of the most species‐rich families of mid‐water fishes. They inhabit the mesopelagic zone, where physical barriers to dispersal and gene flow are permeable. Thus, modes of speciation that rely exclusively on geographical separation are potentially of less importance than those that rely more prominently on evolution of assortative mating through divergent habitat use and/or sexual signals, including visual signals from bioluminescent light organs. Here we used phylogenetic, ecological and morphological data to investigate the roles of geography, habitat use and lateral photophores in lanternfish speciation. Location: Global. Time period: Data collected between 1950 and 2015. Major taxa studied: Lanternfishes (Myctophidae). Methods: Ecological niche models (ENMs) were developed for 167 species, enabling the community composition of 33 mesopelagic ecoregions to be determined. Sequence data for seven protein‐coding regions from 175 species were used to reconstruct a phylogenetic tree of Myctophidae. Age‐overlap correlation tests were conducted using this phylogeny with outputs from ENMs (pairwise geographical overlap and pairwise ecological niche overlap, n = 136), in addition to matrices of pairwise depth overlap (n = 158) and photophore pattern dissimilarity (n = 161). Results: Communities assembled according to nine broad climatic regions, and recently diverged species pairs possessed greater geographical and ecological niche overlap than more distantly related species, indicating that sympatric or parapatric speciation might be dominant modes of divergence. Differences in photophore patterns increased with the relative age of speciation events, suggesting that photophore patterns are largely constrained by phylogeny. Main conclusions Based on this evidence, we suggest that large‐scale oceanographic features structure the diversity of lanternfish communities and that speciation within this family of deep‐water fishes might not have required geographical isolation. [ABSTRACT FROM AUTHOR]

Biological communities in the Arctic are changing through the climate‐driven encroachment of subarctic species. This "Atlantification" extends to keystone Calanoid copepods, as the small‐bodied Calanus finmarchicus increases in abundance in areas where it overlaps with larger Arctic congeners. The environmental factors that are facilitating this shift, whether related to optimal conditions in temperature or seasonality, remain unclear. Assessing these drivers at an Arctic‐wide scale is necessary to predict future ecosystem change and impacts. Here we have compiled range‐wide occurrences of C. finmarchicus and a suite of seasonal biophysical climatologies to build a boreo‐Arctic ecological niche model. The data set was divided into two eras, 1955–1984 and 1985–2017, and an optimized MaxEnt model was used to predict the seasonal distribution of the abiotic niche of C. finmarchicus in both eras. Comparing outputs between eras reveals an increase in habitat suitability at the Arctic range edge. Large and significant increases in suitability are predicted in the regions of the Greenland, Labrador, and Southern Barents Seas that have experienced reduced sea‐ice cover. With the exception of the Barents Sea, these areas also show a seasonal shift in the timing of peak habitat suitability toward an earlier season. Our findings suggest that the Atlantification of Arctic zooplankton communities is accompanied by climate‐driven phenology changes. Although seasonality is a critical constraint to the establishment of C. finmarchicus at Arctic latitudes, earlier sea‐ice retreat and associated productivity is making these environments increasingly favorable for this subarctic species. [ABSTRACT FROM AUTHOR]

Aim: Lanternfish (Myctophidae) are one of the most abundant and ecologically important families of pelagic teleosts, yet how these species will respond to climate change is unclear, especially within polar regions. The aim of this study was to predict the impact of climate change on the distribution of Southern Ocean lanternfish and to relate these predicted responses to species traits. Location: Circumpolar, 35–75°S. Methods: We used MaxEnt ecological niche models to estimate the present and predict the future distributions of 10 biomass‐dominant lanternfish species throughout the region. Future conditions were simulated using eight climate models, in both stabilizing (RCP 4.5) and rising (RCP 8.5) emission scenarios, for the time periods 2006–2055 and 2050–2099. Species responses were then related to their realized thermal niche (i.e., thermal tolerance range), latitudinal preference and body size. Results: Despite large variation between climate model simulations, all but one species are consistently predicted to undergo a poleward distribution shift. Species show contrasting projections relating to a gain or loss of suitable habitat which was best explained by their thermal niche. Overall, high‐latitude Antarctic species were found to have narrower thermal niches and a higher likelihood of losing habitat than sub‐Antarctic species. Main conclusions: The direction of a species response was dependent on the interplay between physiology (realized thermal niche) and biogeography (latitudinal preference). Antarctic species with restricted thermal niches and limited available habitat in which to disperse will be the most vulnerable group of Southern Ocean lanternfish in the face of climate change. Predicted range shifts may alter the size structure of the myctophid community as smaller, sub‐Antarctic species reach further south. This could have implications for trophic interactions and thus the wider Southern Ocean ecosystem. [ABSTRACT FROM AUTHOR]