Diel vertical migration (DVM) of zooplankton is a global phenomenon, characteristic of both marine and limnic environments. At high latitudes, patterns of DVM have been documented, but rather little knowledge exists regarding which species perform this ecologically important behaviour. Also, in the Arctic, the vertically migrating components of the zooplankton community are usually regarded as a single sound scattering layer (SSL) performing synchronized patterns of migration directly controlled by ambient light. Here, we present evidence for hitherto unknown complexity of Arctic marine systems, where zooplankton form multiple aggregations through the water column seen via acoustics as distinct SSLs. We show that while the initiation of DVM during the autumnal equinox is light mediated, the vertical positioning of the migrants during day is linked more to the thermal characteristics of water masses than to irradiance. During night, phytoplankton biomass is shown to be the most important factor determining the vertical positioning of all migrating taxa. Further, we develop a novel way of representing acoustic data in the form of a Sound Image (SI) that enables a direct comparison of the relative importance of each potential scatterer based upon the theoretical contribution of their backscatter. Based on our comparison of locations with contrasting hydrography, we conclude that a continued warming of the Arctic is likely to result in more complex ecotones across the Arctic marine system.

Many dominating phytoplankton form chains of attached cells. Chain length strongly influences how the organism interacts with its environment, but the factors driving the evolution of chain formation and chain length plasticity are not entirely clear. We tested the hypothesis that chain formation in diatoms is a grazer avoidance strategy. We modelled the effect of chain length plasticity on grazing mortality in Skeletonema marinoi over a temperate year, based on empirical data on grazer densities, induced chain length plasticity, and grazing rates. The predicted optimal chain length strategy was compared with field data of S. marinoi and copepod biomass. We found that low copepod densities, corresponding to spring conditions in the field, induced chain length reduction in S. marinoi. Modelled grazing risk over an annual cycle showed that fixed traits with either single cells or long chains have respectively 31 % and 36 % higher mortality than S. marinoi with grazer induced chain length plasticity. Field measurements of chain length and grazer abundances also agree well with chain length plasticity as a grazer defense strategy. We conclude that grazer regime could be a major driving force in the evolution of chain length plasticity in phytoplankton organisms.

Copepods are a fundamental trophic link in the marine food web. While much attention has been devoted to the role of temperature and food for copepod development and growth, little is known about how marine copepods adjust their life history according to the prevailing predation risk. This is striking, considering the potential advantage of risk-sensitive life history, and the many reports of freshwater zooplankton showing strong effects of risk cues on growth and development. Here, we measured growth and development in nauplii of the marine copepod Temora longicornis. We incubated newly hatched nauplii individually with or without a predator chemical cue. Individuals were followed and measured repeatedly over time, generating highresolution data. We estimated treatment-specific stage transition probabilities from daily molting frequencies. The nauplii showed an increased growth rate when exposed to fish kairomones. However, the corresponding response in development differed between stages, with the later naupliar stages generally displaying a higher molting probability and higher body mass (ash-free dry weight) per stage. These results suggest that development and growth in marine copepods is flexible and sensitive to predation risk. Our findings also indicate that investment in growth might be beneficial in copepods despite higher visibility.

In three-spined stickleback (Gasterosteus aculeatus) colonisation of freshwater has led to rapid evolution in various morphological and behavioural traits. Examples are reduction in lateral plate number and increased benthic foraging. These changes are believed to result from natural selection, but the evolutionary mechanisms driving behavioural changes and how such changes are associated with phenotypes, are poorly understood. Here, we study three different lateral plate morphs from a brackish water lake and one morph from a river upstream. We investigate if the lateral plate morphs differ in overall body shape and then experimentally test how foraging behaviour differ among morphs and with body shape. Foraging behaviour is measured as the consumption efficiency of benthic and pelagic prey types. The results show that lateral plate morphs differ in overall body shape and that body shape significantly co-varies with foraging behaviour.

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Copepods are ubiquitous in aquatic habitats. They exude bioactive compounds that mediate mate finding or induce defensive traits in prey organisms. However, little is known about the chemical nature of the copepod exometabolome that contributes to the chemical landscape in pelagic habitats. Here we describe the development of a closed loop solid phase extraction setup that allows for extraction of exuded metabolites from live copepods. We captured exudates from male and female Temora longicornis and analyzed the content with high resolution LC-MS. Chemometric methods revealed 87 compounds that constitute a specific chemical pattern either qualitatively or quantitatively indicating copepod presence. The majority of the compounds were present in both female and male exudates, but nine compounds were mainly or exclusively present in female exudates and hence potential pheromone candidates. Copepodamide G, known to induce defensive responses in phytoplankton, was among the ten compounds of highest relative abundance in both male and female extracts. The presence of copepodamide G shows that the method can be used to capture and analyze chemical signals from living source organisms. We conclude that solid phase extraction in combination with metabolic profiling of exudates is a useful tool to develop our understanding of the chemical interplay between pelagic organisms.

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Copepod mating is a complex behavioral interplay between males and females. But successful mating also requires a physiological component in terms of egg-and spermatophore production. In striking contrast to the extensive literature on copepod egg production, costs of spermatophore production has largely been ignored. This article explores the energetic demands in male copepod reproduction, testing how food and predation risk influence spermatophore production. Moreover, we analyse swimming behavior to test if predation risk and food act on reproductive investments via behavioral responses. We found that spermatophore production depends strongly on food availability. The copepods increased grazing rates with predation risk when food was limited, and video analyses revealed that males spend less time swimming in presence of a risk cue. A review of the few available spermatophore production rates suggests that food limits spermatophore production for copepods at large. Apparently, potential encounter rates with mates or predators did not influence spermatophore production. However, the reduced activity indicates a trade-off between risk avoidance and mate encounter, potentially influencing mating rates. This study shows that males invest considerable resources in each spermatophore, with implications for mating strategies and population dynamics in copepods, the most abundant metazoans on earth.