Oxygen consumption in four species of teleosts from Greenland: no evidence of metabolic cold adaptation

Standard metabolic rate of Greenland cod or uvak, Gadus ogac, polar cod, Boreogadus saida, Atlantic cod, Gadus morhua, and sculpin, Myxocephalus scorpius, caught in the same geographical area on the west coast of Greenland was measured at 4.5°C, the temperature at which the fish were caught. The present data does not support the Metabolic Cold Adaptation theory in the traditional sense of the standard metabolic rate being 2–4 times higher for Arctic fishes than for temperate species. The standard metabolic rate of the two exclusively Arctic species of teleosts was only 10% and 26% higher, respectively, than the two species that occur in temperate as well as Arctic areas. The critical oxygen tension, with respect to oxygen consumption, of resting uvak was between 50 and 60 mmHg, and the lethal oxygen tension 20–25 mmHg at 4.5°C, which is considerably higher than for Atlantic cod from a temperate area measured at the same temperature.     

Is the poleward expansion by Atlantic cod and haddock threatening native polar cod, <Emphasis Type="Italic">Boreogadus saida</Emphasis>?

During a recent period of increased influx of warm Atlantic water to the western coast of Svalbard, we have observed a northward expansion of boreal Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) into areas dominated by the native polar cod (Boreogadus saida). To determine the potential impact of new ecological interactions, we studied the diet of co-occurring juvenile gadoids in fjords, open water, and sea ice around Svalbard. We also reviewed the available literature on polar cod feeding in different habitats across the Arctic to determine whether region, habitat, or fish size may influence diet. Feeding by polar cod in the pelagic zone was size dependent, with small fish primarily consuming Calanus spp. and smaller copepods, with an increasing ration of Themisto spp. at larger sizes. In benthic habitats, diets were more varied and included considerably more unidentified material and sediment. Less than 40% dietary overlap was detected among the three species when they were found together. Stable isotope analyses indicated these patterns were representative of longer-term assimilation. The low interspecific dietary overlap suggests little direct competition. Future increases in abundance and the high predation potential of the boreal taxa, however, may impact the persistence of polar cod on some Arctic shelves.     

Is Ambient Light during the High Arctic Polar Night Sufficient to Act as a Visual Cue for Zooplankton?

The light regime is an ecologically important factor in pelagic habitats, influencing a range of biological processes. However, the availability and importance of light to these processes in high Arctic zooplankton communities during periods of ''complete'' darkness (polar night) are poorly studied. Here we characterized the ambient light regime throughout the diel cycle during the high Arctic polar night, and ask whether visual systems of Arctic zooplankton can detect the low levels of irradiance available at this time. To this end, light measurements with a purpose-built irradiance sensor and coupled all-sky digital photographs were used to characterize diel skylight irradiance patterns over 24 hours at 79°N in January 2014 and 2015. Subsequent skylight spectral irradiance and in-water optical property measurements were used to model the underwater light field as a function of depth, which was then weighted by the electrophysiologically determined visual spectral sensitivity of a dominant high Arctic zooplankter, Thysanoessa inermis. Irradiance in air ranged between 1–1.5 x 10^-5 μmol photons m^-2 s^-1 (400–700 nm) in clear weather conditions at noon and with the moon below the horizon, hence values reflect only solar illumination. Radiative transfer modelling generated underwater light fields with peak transmission at blue-green wavelengths, with a 465 nm transmission maximum in shallow water shifting to 485 nm with depth. To the eye of a zooplankter, light from the surface to 75 m exhibits a maximum at 485 nm, with longer wavelengths (>600 nm) being of little visual significance. Our data are the first quantitative characterisation, including absolute intensities, spectral composition and photoperiod of biologically relevant solar ambient light in the high Arctic during the polar night, and indicate that some species of Arctic zooplankton are able to detect and utilize ambient light down to 20–30m depth during the Arctic polar night.     

Gender Specific Reproductive Strategies of an Arctic Key Species (Boreogadus saida) and Implications of Climate Change

The Arctic climate is changing at an unprecedented rate. What consequences this may have on the Arctic marine ecosystem depends to a large degree on how its species will respond both directly to elevated temperatures and more indirectly through ecological interactions. But despite an alarming recent warming of the Arctic with accompanying sea ice loss, reports evaluating ecological impacts of climate change in the Arctic remain sparse. Here, based upon a large-scale field study, we present basic new knowledge regarding the life history traits for one of the most important species in the entire Arctic, the polar cod (Boreogadus saida). Furthermore, by comparing regions of contrasting climatic influence (domains), we present evidence as to how its growth and reproductive success is impaired in the warmer of the two domains. As the future Arctic is predicted to resemble today''s Atlantic domains, we forecast changes in growth and life history characteristics of polar cod that will lead to alteration of its role as an Arctic keystone species. This will in turn affect community dynamics and energy transfer in the entire Arctic food chain.     

The oxygen transport system in three species of the boreal fish family Gadidae. Molecular phylogeny of hemoglobin

The Arctic and Antarctic marine faunas differ by age and isolation. Fishes of the two polar regions have undergone different regional histories that have driven the physiological diversities. Antarctic fish are highly stenothermal, in keeping with stable water temperatures, whereas Arctic fish, being exposed to seasonal temperature variations, exhibit higher physiological plasticity. This study reports the characterization of the oxygen transport system of three Arctic species of the family Gadidae, namely the Arctic cod Arctogadus glacialis, the polar cod Boreogadus saida, and the Atlantic cod Gadus morhua. Unlike Antarctic notothenioids, the blood displays high multiplicity, i.e. it has three hemoglobins, similar to many other acanthomorph teleosts. In the most abundant hemoglobin, oxygen binding is modulated by heterotropic effectors, with marked Bohr and Root effects. Remarkably, in two species (A. glacialis and B. saida), the Hill coefficient is very close to one in the whole pH range, indicating the apparent absence of cooperativity. The amino acid sequences have been used to gain insight into the evolution history of globins of polar fish. The results indicate that Arctic and Antarctic globins have different phylogenies and lead us to suggest that the selective pressure of environment stability allows the phylogenetic signal to be maintained in the Antarctic sequences, whereas environmental variability would tend to disrupt this signal in the Gadidae sequences.     

New encounters in Arctic waters: a comparison of metabolism and performance of polar cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) and Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) under ocean acidification and warming

Oceans are experiencing increasing acidification in parallel to a distinct warming trend in consequence of ongoing climate change. Rising seawater temperatures are mediating a northward shift in distribution of Atlantic cod (Gadus morhua), into the habitat of polar cod (Boreogadus saida), that is associated with retreating cold water masses. This study investigates the competitive strength of the co-occurring gadoids under ocean acidification and warming (OAW) scenarios. Therefore, we incubated specimens of both species in individual tanks for 4 months, under different control and projected temperatures (polar cod: 0, 3, 6, 8 °C, Atlantic cod: 3, 8, 12, 16 °C) and PCO₂ conditions (390 and 1170 µatm) and monitored growth, feed consumption and standard metabolic rate. Our results revealed distinct temperature effects on both species. While hypercapnia by itself had no effect, combined drivers caused nonsignificant trends. The feed conversion efficiency of normocapnic polar cod was highest at 0 °C, while optimum growth performance was attained at 6 °C; the long-term upper thermal tolerance limit was reached at 8 °C. OAW caused only slight impairments in growth performance. Under normocapnic conditions, Atlantic cod consumed progressively increasing amounts of feed than individuals under hypercapnia despite maintaining similar growth rates during warming. The low feed conversion efficiency at 3 °C may relate to the lower thermal limit of Atlantic cod. In conclusion, Atlantic cod displayed increased performance in the warming Arctic such that the competitive strength of polar cod is expected to decrease under future OAW conditions.     

Aspects of reproductive biology of wild-caught polar cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) from Svalbard waters

Polar cod (Boreogadus saida) is considered a key species in the Arctic marine ecosystems. Yet detailed or even basic knowledge regarding its biology and adaptations, especially during the polar night, is in many cases poor. Data are presently unavailable in Western literature on the gonad development of polar cod and its reproductive biology in wild specimens. Accordingly, gonad development of wild-caught polar cod from fjords of the Svalbard archipelago was studied across seasons (April, August, September, November and January). Histological analyses of polar cod showed strong indication of a group-synchronous oocyte development with determinate fecundity and iteroparity. Females started gonadal development prior to April and had not yet reached the final stage of maturation in January. Testes matured more rapidly, with males ready to spawn in January. Furthermore, our data show that polar cod were able to reach sexual maturity at age 1+. Based on our data and previous reports, we hypothesise that polar cod is a total spawner.     

The phytogeography ofCladophora (Chlorophyceae) in the northern Atlantic Ocean,in comparison to that of other benthic algal species

About 43Cladophora species inhabit the coasts of the northern Atlantic Ocean. These can be subdivided into seven distribution groups: (a) the tropical western Atlantic group (16 species); (b) the warm temperate Mediterranean-Atlantic group (9 species); (c) the warm temperate North American group (1 species); (d) the Arctic group (1 species); (e) the amphiatlantic tropical to warm temperature group (7 species); (f) the amphiatlantic tropical to temperate group (4 species), and (g) the amphiatlantic temperate group (5 species). These groups agree with general phytogeographic patterns. Thus, the high numbers of species restricted to the tropical western Atlantic region and the warm temperate Mediteranean-Atlantic region are in agreement with the richness and high degree of endemism of these regions. The fact that all species occurring in northeast America also occur in Europe agrees with the high floristic similarity of the boreal areas in America and Europe. The sediment coasts of the Carolinas are an efficient barrier to the south-north dispersal of benthic algae. The temperature bound phytogeographic limits are set in most cases by the species ability to survive adverse temperatures; for “northern” species to survive a high summer temperature in the south, and for “southern” species to survive a low winter temperature in the north. The limits in the Arctic region are all set by the species ability for sufficient growth and reproduction in summer. Conversely, only few northern species have a southern limit which is set by a winter temperature that is not too high for sufficient growth and reproduction. Most species of this group are winter-annuals at their southern limit, and summer-annuals at their northern limit. A comparatively small number of species with a tropical-to-warm temperate distribution have a northern limit at temperate latitudes which is set by a sufficiently high summer temperature for growth and reproduction. A high proportion of this group are lagoonal or quiet water species, which profit by higher summer temperatures in sheltered waters.C. vagabunda is an example.C. rupestris andC. sericea have an amphiboreal distribution and also occur in the southern temperate belt. They probably used a Pleistocene temperature drop to disperse, through the Atlantic along the African coast, from one hemisphere to the other. In the Pacific temperatures were not sufficiently low for this dispersal; and hence these two species reached the Pacific probably by way of the Bering Strait.     

From polar night to midnight sun: photoperiod, seal predation, and the diel vertical migrations of polar cod (Boreogadus saida) under landfast ice in the Arctic Ocean

The winter/spring vertical distributions of polar cod, copepods, and ringed seal were monitored at a 230-m station in ice-covered Franklin Bay. In daytime, polar cod of all sizes (7–95 g) formed a dense aggregation in the deep inverse thermocline (160–230 m, −1.0 to 0°C). From December (polar night) to April (18-h daylight), small polar cod <25 g migrated into the isothermal cold intermediate layer (90–150 m, −1.4°C) at night to avoid visual predation by shallow-diving immature seals. By contrast, large polar cod (25–95 g), with large livers, remained below 180 m at all times, presumably to minimize predation by deep-diving mature seals. The diel vertical migration (DVM) of small polar cod was precisely synchronized with the light/dark cycle and its duration tracked the seasonal lengthening of the photoperiod. The DVM stopped in May coincident with the midnight sun and increased schooling and feeding. We propose that foraging interference and a limited prey supply in the deep aggregation drove the upward re-distribution of small polar cod at night. The bioluminescent copepod Metridia longa could have provided the light needed by polar cod to feed on copepods in the deep aphotic layers.     

Under-ice distribution of polar cod <Emphasis Type="Italic">Boreogadus saida</Emphasis> in the central Arctic Ocean and their association with sea-ice habitat properties

In the Arctic Ocean, sea-ice habitats are undergoing rapid environmental change. Polar cod (Boreogadus saida) is the most abundant fish known to reside under the pack-ice. The under-ice distribution, association with sea-ice habitat properties and origins of polar cod in the central Arctic Ocean, however, are largely unknown. During the RV Polarstern expedition ARK XXVII/3 in the Eurasian Basin in 2012, we used for the first time in Arctic waters a Surface and Under Ice Trawl with an integrated bio-environmental sensor array. Polar cod was ubiquitous throughout the Eurasian Basin with a median abundance of 5000 ind. km^?2. The under-ice population consisted of young specimens with a total length between 52 and 140 mm, dominated by 1-year-old fish. Higher fish abundance was associated with thicker ice, higher ice coverage and lower surface salinity, or with higher densities of the ice-amphipod Apherusa glacialis. The fish were in good condition and well fed according to various indices. Back-tracking of the sea-ice indicated that sea-ice sampled in the Amundsen Basin originated from the Laptev Sea coast, while sea-ice sampled in the Nansen Basin originated from the Kara Sea. Assuming that fish were following the ice drift, this suggests that under-ice polar cod distribution in the Eurasian Basin is dependent on the coastal populations where the sea-ice originates. The omnipresence of polar cod in the Eurasian Basin, in a good body condition, suggests that the central Arctic under-ice habitats may constitute a favourable environment for this species survival, a potential vector of genetic exchange and a recruitment source for coastal populations around the Arctic Ocean.     

The aggregation of polar cod (Boreogadus saida) in the deep Atlantic layer of ice-covered Amundsen Gulf (Beaufort Sea) in winter

During the Circumpolar Flaw Lead System Study (CFL, 2007–2008), large aggregations of polar cod were detected in winter in the Amundsen Gulf (Western Canadian Arctic) using the EK60 echosounder of the CCGS Amundsen research icebreaker. Biomass estimated over 10 months reached a maximum of 0.732 kg m⁻² in February. Aggregations were encountered only in the presence of an ice cover from December to April. The vertical extent of the aggregations was dictated by temperature and zooplankton prey distribution. In winter, polar cod generally occupied the relatively warm deep Atlantic Layer (>0°C), but a fraction of the densest aggregations occasionally followed zooplankton prey up into the cold Pacific Halocline (−1.6 to 0°C). The diel vertical migration of polar cod was precisely synchronized with the seasonally increasing photoperiod. Throughout winter, polar cod aggregations migrated to progressively deeper regions (from 220 to 550 m bottom depths) in response to increasing light intensity, presumably to avoid predation by visual predators such as the ringed seal. Comparing Amundsen Gulf and Franklin Bay indicates that the entrapment of polar cod in embayments during winter is an important mechanism to provide marine mammal predators with dense concentrations of their main prey within their diving range.     

Molecular Evidence that Only Two Opsin Subfamilies,the Blue Light- (SWS2) and Green Light-Sensitive (RH2), Drive Color Vision in Atlantic Cod (Gadus morhua)

Teleosts show a great variety in visual opsin complement, due to both gene duplication and gene loss. The repertoire ranges from one subfamily of visual opsins (scotopic vision) including rod opsin only retinas seen in many deep-sea species to multiple subfamilies of visual opsins in some pelagic species. We have investigated the opsin repertoire of Atlantic cod (Gadus morhua) using information in the recently sequenced cod genome and found that despite cod not being a deep sea species it lacks visual subfamilies sensitive towards the most extreme parts of the light spectra representing UV and red light. Furthermore, we find that Atlantic cod has duplicated paralogs of both blue-sensitive SWS2 and green-sensitive RH2 subfamilies, with members belonging to each subfamily linked in tandem within the genome (two SWS2-, and three RH2A genes, respectively). The presence of multiple cone opsin genes indicates that there have been duplication events in the cod ancestor SWS2 and RH2 opsins producing paralogs that have been retained in Atlantic. Our results are supported by expressional analysis of cone opsins, which further revealed an ontogenetic change in the array of cone opsins expressed. These findings suggest life stage specific programs for opsin regulation which could be linked to habitat changes and available light as the larvae is transformed into an early juvenile. Altogether we provide the first molecular evidence for color vision driven by only two families of cone opsins due to gene loss in a teleost.     

Borealization of nearshore fishes on an interior Arctic shelf over multiple decades

Borealization is a type of community reorganization where Arctic specialists are replaced by species with more boreal distributions in response to climatic warming. The process of borealization is often exemplified by the northward range expansions and subsequent proliferation of boreal species on the Pacific and Atlantic inflow Arctic shelves (i.e., Bering/Chukchi and Barents seas, respectively). But the circumpolar nearshore distribution of Arctic-boreal fishes that predates recent warming suggests borealization is possible beyond inflow shelves. To examine this question, we revisited two nearshore lagoons in the eastern Alaska Beaufort Sea (Kaktovik and Jago lagoons, Arctic National Wildlife Refuge, Alaska, USA), a High Arctic interior shelf. We compared summer fish species assemblage, catch rate, and size distribution among three periods that spanned a 30-year record (baseline conditions, 1988–1991; moderate sea ice decline, 2003–2005; rapid sea ice decline, 2017–2019). Fish assemblages differed among periods in both lagoons, consistent with borealization. Among Arctic specialists, a clear decline in fourhorn sculpin (Myoxocephalus quadricornis, Kanayuq in Iñupiaq) occurred in both lagoons with 86%–90% lower catch rates compared with the baseline period. Among the Arctic-boreal species, a dramatic 18- to 19-fold increase in saffron cod (Eleginus gracilis, Uugaq) occurred in both lagoons. Fish size (length) distributions demonstrated increases in the proportion of larger fish for most species examined, consistent with increasing survival and addition of age-classes. These field data illustrate borealization of an Arctic nearshore fish community during a period of rapid warming. Our results agree with predictions that Arctic-boreal fishes (e.g., saffron cod) are well positioned to exploit the changing Arctic ecosystem. Another Arctic-boreal species, Dolly Varden (Salvelinus malma, Iqalukpik), appear to have already responded to warming by shifting from Arctic nearshore to shelf waters. More broadly, our findings suggest that areas of borealization could be widespread in the circumpolar nearshore.     

A Focus on the Optical Properties of the Regenerated Newt Lens

Lens regeneration studies in the adult newt suggest that molecular aspects of lens regeneration are complete within 5 weeks of lentectomy. However, very little is known about the optical properties of the regenerated lens. In an aquatic environment, the lens accounts for almost all of the refractive power of the eye, and thus, a fully functional lens is critical. We compared the optical properties of 9- and 26-week regenerated lenses in the red spotted newt, Notophthalmus viridescens, with the original lenses removed from the same eyes. At 9 weeks, the regenerated lenses are smaller than the original lenses and are histologically immature, with a lower density of lens proteins. The 9 week lenses have greater light transmission, but significantly reduced focal length and refractive index than the original lenses. This suggests that following 9 weeks of regeneration, the lenses have not recovered the functionality of the original lens. By 26 weeks, the transmission of light in the more mature lens is reduced, but the optical parameters of the lens have recovered enough to allow functional vision.     

Congeneric amphipods show differing abilities to maintain metabolic rates with latitude

Metabolic variability across latitudinal populations of gammarid amphipods was examined in the summer by determining whole-animal rates of oxygen uptake (M(o)?) in four species with overlapping distribution patterns in the northeast Atlantic and Arctic oceans. Comparisons were made between an arctic/boreal species, Gammarus setosus, a subarctic/boreal species, Gammarus oceanicus, a boreal/temperate species, Gammarus duebeni duebeni, and a temperate species, Gammarus locusta. Measurements included acclimatized M(o)? in all four species and M(o)? after acclimation to 10°C in two populations of G. oceanicus and G. locusta. In G. oceanicus, acclimatized M(o)? declined with latitude (13° to 5°C) so that metabolic rates were lower in subarctic (79°N) relative to temperate (58°N) populations and similar to the values in G. setosus at 79°N. Consequently, there was no evidence for metabolic rate compensation in the colder-water, high-latitude populations in the summer. Further examination of the specific effects of temperature revealed similarities in M(o)? between populations of G. oceanicus acclimated at 10°C and similarities in thermal sensitivity (Q(10)) and activation energies (E(a)) on exposure to acute temperature change. In sharp contrast, there was no variation in summer acclimatized M(o)? with latitude in either G. d. duebeni between 48° and 70°N or G. locusta between 38° and 53°N. Instead, the two species maintained relatively high metabolic rates across latitudes, which were associated in G. locusta with differences in M(o)? and with Q(10) and E(a) values in amphipods acclimated at 10°C. The ability to compensate metabolic rate with latitude in the summer suggests greater metabolic flexibility, which predicts a greater capacity for survival during climate change of the temperate/boreal over the subarctic and arctic gammarid species.     

Dietary characteristics of co-occurring polar cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) and capelin (<Emphasis Type="Italic">Mallotus villosus</Emphasis>) in the Canadian Arctic,Darnley Bay

Reduction in sea ice due to climate change is expected to have a negative impact on habitat availability for Arctic marine fishes and induce range expansion of species from southern environments. Such an effect will likely be observed in the abundance of polar cod, Boreogadus saida (Lepechin, 1774), as well as interspecific interactions of this intermediate-level trophic taxon, particularly in more southerly fringing seas in the Arctic. Polar cod and capelin, Mallotus villosus (Müller, 1776), are pelagic, planktivorous forage fishes, which occupy similar dietary niches and are the primary prey of marine predators. Co-occurring polar cod and capelin were collected at seven stations in Darnley Bay, NT, during August 2013. Standard length (SL), used as a proxy for age, suggested that polar cod (mean ± 1 SD: 71.1 ± 10.3 mm) were predominantly age 1+ and capelin (96.2 ± 13.4 mm) were mostly age 2+. Stomach content analyses indicated that both species feed extensively on calanoid copepods (Calanus hyperboreus, C. glacialis, Metridia longa) and amphipods (Themisto libellula). There was high dietary overlap between capelin and polar cod, evidenced by Schoener’s index (0.80). Additionally the quantity of dietary items, biomass and energetic content consumed differed among size classes in both capelin (SL, 70.5–132.0 mm) and polar cod (SL, 42.1–114.4 mm). This study illustrates that the diets of these sympatric forage fishes in an Arctic ecosystem are very similar, indicating a high potential for interspecific competition as the sub-Arctic capelin expands its range into Arctic regions with climate change.     

Distribution and seasonal dynamics of <Emphasis Type="Italic">Boroecia maxima</Emphasis> (Ostracoda: Halocypridinae) in the Arctic basin and adjacent Atlantic waters

Based on materials obtained during Soviet and Russian expeditions to the Arctic basin and adjacent Atlantic waters during the period from 1929–1993, as well as data from the literature, we examined the distribution range of the common deep-sea ostracod Boroecia maxima. This species occurs throughout the pelagic water column (down to 3000 m depth and below) and prefers low water temperatures (surface Arctic and intermediate water masses). During the year, at all depths, the ratio between age groups of B. maxima remains virtually unchanged. Breeding apparently continues year-round. During the polar night, B. maxima makes no migration into warm Atlantic water, where wintering predatory zooplankton occur in large numbers, and thus avoids grazing pressure.     

Spatial variation and low diversity in the major histocompatibility complex in walrus (Odobenus rosmarus)

Increased global temperature and associated changes to Arctic habitats will likely result in the northward advance of species, including an influx of pathogens novel to the Arctic. How species respond to these immunological challenges will depend in part on the adaptive potential of their immune response system. We compared levels of genetic diversity at a gene associated with adaptive immune response [Class II major histocompatibility complex (MHC), DQB exon 2] between populations of walrus (Odobenus rosmarus), a sea ice-dependent Arctic species. Walrus was represented by only five MHC DQB alleles, with frequency differences observed between Pacific and Atlantic populations. MHC DQB alleles appear to be under balancing selection, and most (80 %; n = 4/5) of the alleles were observed in walruses from both oceans, suggesting broad scale differences in the frequency of exposure and diversity of pathogens may be influencing levels of heterozygosity at DQB in walruses. Limited genetic diversity at MHC, however, suggests that walrus may have a reduced capacity to respond to novel immunological challenges associated with shifts in ecological communities and environmental stressors predicted for changing climates. This is particularly pertinent for walrus, since reductions in summer sea ice may facilitate both northward expansion of marine species and associated pathogens from more temperate regions, and exchange of marine mammals and associated pathogens through the recently opened Northwest Passage between the Atlantic and Pacific Oceans in the Canadian high Arctic.