Elements and stable isotopes as tracers for sleeper shark biology and the Iceland marine food web

Stable isotope studies often rely on only two tracers (usually [delta] 13C and [delta]15N) to study marine ecosystems, which are inherently complex. The ability of elements to act as additional tracers of ecological processes in marine organisms and in a marine food web was investigated. The element analysis of two sleeper shark species, Pacific (\u27Somniosus pacificus\u27) and Greenland (\u27Somniosus microcephalus\u27), collected from different ecosystems demonstrated that elements are useful indicators of physiological and exposure differences between closely related species. The Greenland shark\u27s food web about Iceland was more clearly resolved concerning trophic links and carbon sources by combining mercury data with stable isotope and stomach content data. Mercury also indicated that \u27Lycodes\u27 potentially belonged to a different food web than the other fishes. Results from this research demonstrated the value of elemental tracers in food web studies and generated new questions about the application and interpretation of trophic position designations

Effects of seasonal seston and temperature changes on lake zooplankton fatty acids

Abstract We investigated how seston fatty acids (FA) and water temperature explained seasonal variation in cladoceran and copepod FA over three years in pre-alpine, oligotrophic Lake Lunz, Austria. Using the mostly algalderived polyunsaturated FA (PUFA: arachidonic, ARA; eicosapentaenoic, EPA; docosahexaenoic acid, DHA), terrestrial FA (TFA, 22 : 0, 24 : 0), and bacterial FA (BAFA, 15 : 0, 17 : 0 and their branched homologues) as source-specific biomarkers, we show that cladocerans consistently contained more ARA and EPA and copepods more DHA than the available food (seston). None of these physiologically important PUFA were significantly related between zooplankton and seston across the entire study period but copepod DHA increased with seston DHA during the coldest months (< 8 C, based on a significant seston FA*temperature interaction). EPA, conversely, increased with decreasing water temperature in both zooplankton groups. For the nonessential FA, TFA were lower in zooplankton than in seston and not related to dietary supply or water temperature. However, cladoceran and copepod BAFA increased significantly with increasing seston BAFA and decreasing water temperature. These findings suggest that physiological regulation in response to changing water temperature had a significant impact on cladoceran and copepod EPA and the extent of dietary tracking for copepod DHA. TFA available in the seston may not have been consumed or were poorly incorporated by zooplankton, but BAFA were good indicators of available resources throughout multiple seasonal cycles. Based on our study, both FA type and water temperature impact the extent that dietary vs. nondietary processes govern cladoceran and copepod FA in oligotrophic lakes

Comparative Brain Morphology of the Greenland and Pacific Sleeper Sharks and its Functional Implications

In cartilaginous fishes, variability in the size of the brain and its major regions is often associated with primary habitat and/or specific behavior patterns, which may allow for predictions on the relative importance of different sensory modalities. The Greenland (Somniosus microcephalus) and Pacific sleeper (S. pacificus) sharks are the only non-lamnid shark species found in the Arctic and are among the longest living vertebrates ever described. Despite a presumed visual impairment caused by the regular presence of parasitic ocular lesions, coupled with the fact that locomotory muscle power is often depressed at cold temperatures, these sharks remain capable of capturing active prey, including pinnipeds. Using magnetic resonance imaging (MRI), brain organization of S. microcephalus and S. pacificus was assessed in the context of up to 117 other cartilaginous fish species, using phylogenetic comparative techniques. Notably, the region of the brain responsible for motor control (cerebellum) is small and lacking foliation, a characteristic not yet described for any other large-bodied (\u3e3 m) shark. Further, the development of the optic tectum is relatively reduced, while olfactory brain regions are among the largest of any shark species described to date, suggestive of an olfactory-mediated rather than a visually-mediated lifestyle

Advancing Research for the Management of Long-Lived Species: A Case Study on the Greenland Shark

Long-lived species share life history traits such as slow growth, late maturity, and low fecundity, which lead to slow recovery rates and increase a population’s vulnerability to disturbance. The Greenland shark (Somniosus microcephalus) has recently been recognized as the world’s longest-lived vertebrate, but many questions regarding its biology, physiology, and ecology remain unanswered. Here we review how current and future research will fill knowledge gaps about the Greenland shark and provide an overall framework to guide research and management priorities for this species. Key advances include the potential for specialized aging techniques and demographic studies to shed light on the distribution and age-class structure of Greenland shark populations. Advances in population genetics and genomics will reveal key factors contributing to the Greenland shark’s extreme longevity, range and population size, and susceptibility to environmental change. New tagging technologies and improvements in experimental and analytical design will allow detailed monitoring of movement behaviors and interactions among Greenland sharks and other marine species, while shedding light on habitat use and susceptibility to fisheries interactions. Interdisciplinary approaches, such as the combined use of stable isotope analysis and high-tech data-logging devices (i.e., accelerometers and acoustic hydrophones) have the potential to improve knowledge of feeding strategies, predatory capabilities, and the trophic role of Greenland sharks. Measures of physiology, including estimation of metabolic rate, as well as heart rate and function, will advance our understanding of the causes and consequences of long lifespans. Determining the extent and effects of current threats (as well as potential mitigation measures) will assist the development of policies, recommendations, and actions relevant for the management of this potentially vulnerable species. Through an interdisciplinary lens, we propose innovative approaches to direct the future study of Greenland sharks and promote the consideration of longevity as an important factor in research on aquatic and terrestrial predators

Impacts of food web structure and feeding behavior on mercury exposure in Greenland Sharks (Somniosus microcephalus)

Benthic and pelagic food web components in Cumberland Sound, Canada were explored as sources of total mercury (THg) to Greenland Sharks (Somniosus microcephalus) via both bottom-up food web transfer and top-down shark feeding behavior. Log10THg increased significantly with δ15N and trophic position from invertebrates (0.01±0.01μg·g-1 [113±1ng·g-1] dw in copepods) to Greenland Sharks (3.54±1.02μg·g-1). The slope of the log10THg vs. δ15N linear regression was higher for pelagic compared to benthic food web components (excluding Greenland Sharks, which could not be assigned to either food web), which resulted from THg concentrations being higher at the base of the benthic food web (i.e., in benthic than pelagic primary consumers). However, feeding habitat is unlikely to consistently influence shark THg exposure in Cumberland Sound because THg concentrations did not consistently differ between benthic and pelagic shark prey. Further, size, gender and feeding behavior (inferred from stable isotopes and fatty acids) were unable to significantly explain THg variability among individual Greenland Sharks. Possible reasons for this result include: 1) individual sharks feeding as generalists, 2) high overlap in THg among shark prey, and 3) differences in turnover time between ecological tracers and THg. This first assessment of Greenland Shark THg within an Arctic food web revealed high concentrations consistent with biomagnification, but low ability to explain intra-specific THg variability. Our findings of high THg levels and consumption of multiple prey types, however, suggest that Greenland Sharks acquire THg through a variety of trophic pathways and are a significant contributor to the total biotic THg pool in northern seas

Similarity between predator and prey fatty acid profiles is tissue dependent in Greenland sharks (Somniosus microcephalus): Implications for diet reconstruction

Fatty acid (FA) analysis is increasingly being applied to study the feeding ecology of sharks. However, very little knowledge exists regarding how sharks alter dietary FAs prior to incorporation into their tissues, or which tissue provides the most accurate representation of diet. To provide insight into these questions, we compared FAs of muscle, liver and blood (plasma) of 18 individuals of a large elasmobranch, the Greenland shark Somniosus microcephalus (Bloch and Schneider, 1801), to FA profiles of several known prey. Greenland sharks fed predominantly on Greenland halibut (Reinhardtius hippoglossoides, Walbaum 1792) and ringed seal (Pusa hispida, Schreber, 1775) based on stomach contents. Shark muscle FA profiles were the most similar to prey FA profiles, both in relative proportions (e.g. muscle 18:1n-7=7.1±1.0%, ringed seal blubber=7.2±1.2%) and on a μgmg -1 basis (e.g. shark muscle 22:1n-11=47.9±12.0μgmg -1 dw, Greenland halibut muscle =59.9±18.5μgmg -1 dw), indicating direct incorporation of most FAs from the diet. Shark blood plasma FAs also corresponded to prey FAs, and were more similar to shark muscle than liver, which supports the suggestion that muscle FAs were of dietary origin. Shark liver had the most variable FA profiles among individuals and retained higher amounts of long-chain monounsaturated FAs (e.g. 20:1n-9) than were observed in prey. As a consequence, shark liver FAs differed the most from known prey like ringed seal. Our results indicate that the FA profiles of shark tissues will not always match those of dominant prey items, and highlight the use of non-lethal tissues, like muscle and plasma, for studying shark diet using FA analysis. Further work is required to unravel what mechanisms underlie the observed differences in FA profiles and shark-prey FA relationships among different shark species, but data presented here will aid future researchers in more accurately applying FAs to study the diet of large, mobile sharks. © 2012 Elsevier B.V

Preliminary assessment of Greenland halibut diet in Cumberland Sound using stable isotopes

We provide preliminary carbon (δ13C) and nitrogen (δ15N) stable isotope assessment of the Greenland halibut (Reinhardtius hippoglossoides) diet in Cumberland Sound, with focus on two possible prey sources: pelagic represented by capelin (Mallotus villosus) and epibenthic represented by shrimp (Lebbeus polaris). The δ13C for the Greenland halibut stock indicated a pelagic carbon source in Cumberland Sound while stable isotope mixing models, IsoSource and MixSIR, indicated a 99% dietary composition of capelin relative to the shrimp. The δ15N did not vary across Greenland halibut size ranges and placed them at a fourth trophic position relative to a primary herbivore. This study provides the starting point for more elaborate Cumberland Sound research on the local Greenland halibut feeding ecology by confirming pelagic feeding and establishing relative trophic position as well as identifying stable isotopes as a useful tool for the study of diet in cold water fish species. © 2009 Springer-Verlag

Diet and resource use among Greenland sharks (somniosus microcephalus) and teleosts sampled in icelandic waters, using δ\u3csup\u3e13\u3c/sup\u3eC, δ\u3csup\u3e15\u3c/sup\u3eN, and mercury

Stable carbon (δ13C) and nitrogen (δ15N) isotopes and total mercury (Hg) were used to investigate diet and resource use among Greenland sharks (Somniosus microcephalus) and 14 teleosts inhabiting Icelandic waters. Greenland shark stomachs contained 11 of the teleosts sampled, along with other fishes and marine mammal tissues. Teleost resource use ranged from pelagic (e.g., Argentina silus) to benthic (e.g., Anarhichas lupus) based on δ13C, and relative trophic positions (TP, based on δ15N) ranged from 3.0 (Mallotus villosus) to 3.8 (e.g., Brosme brosme). Greenland shark δ13C indicated feeding on benthic and pelagic resources, with a high input of pelagic carbon, and δ15N indicated a relative TP of 4.3. Log[Hg] increased with δ15N (i.e., TP) from teleosts to Greenland sharks and was higher in offshore vs. inshore teleosts. Linear regressions revealed that log[Hg] was better described by both δ15N and δ13C-assigned resource use than by δ15N alone. Hg was useful for supporting the TPs suggested by δ15N, and the higher Hg in offshore fishes could help explain the high Hg of Greenland sharks. Results from this study demonstrated the potential use of Hg as a dietary tracer in marine fishes

Food web structure of a coastal Arctic marine ecosystem and implications for stability

There is little doubt that Arctic ecosystems will continue to face unprecedented change in the coming decades. The identification of food web structures that confer stability to these systems is, therefore, a priority. Here, we use stable isotopes and fatty acids to resolve the food web structure of a seasonally ice-covered fjord (Cumberland Sound, Baffin Island, Canada) sampled in late summer. We show that the food web is structured such that upper trophic levels couple separate energy channels (based on phytoplankton or macroalgae), a previously documented food web structure that has been linked with stability in temperate ecosystems, but never established in a seasonally dynamic, ice-covered ecosystem. Herbivorous zooplankton (e.g. Calanus hyperboreus) relied exclusively on phytoplankton, whereas herbivorous benthos used either phytodetritus (e.g. Hiatella arctica) or macroalgae (e.g. Tectura testudinalis), supporting the existence of separate energy channels. Upper trophic level fishes and marine mammals relied more heavily on phytoplankton- than macroalgal-derived carbon (58 to 100% reliance on phytoplankton), but 6 out of 8 species sampled derived energy from both carbon sources. Since benthic invertebrate predators used both phytodetrital- and macrolgal-based resources, the coupling of separate energy channels was also iterated within the benthos. The temporally pulsed nature of phytoplankton production, characteristic of Arctic seas, indicates that Arctic consumers also act as couplers of resources in time because phytoplankton- and detrital-based carbon would likely reach upper trophic levels earlier and later in the season, respectively. Potential changes in the relative production of macroalgae and phytoplankton under climate change scenarios could impact the stability-promoting food web structure reported here. © Inter-Research 2013