Diet- and tissue-specific incorporation of isotopes in the shark Scyliorhinus stellaris, a North Sea mesopredator

Abstract

14Elucidating predator-prey relationships is an important part of understanding and assessing the 15 structure and function of ecosystems. Sharks are believed to play a significant role in marine 16 ecosystems, although their specific trophic ecology is largely unexplored. Stable isotopes of 17 nitrogen (δ 15 N) and carbon (δ 13 C) are a widely applied tool in food web studies but there is a 18 need to quantify stable isotope dynamics in animals, particularly sharks. In this study, diet-tissue 19 discrimination factors (DTDF = stable isotope in consumer tissue -stable isotope in diet) and 20 turnover rates (time for the isotope to be assimilated into the consumer's tissue) of stable 21 isotopes were estimated in blood, fin, and muscle tissue for the shark species Scyliorhinus 22 stellaris fed two diets with different isotope values. Subsequently, these diet-and tissue-specific 23DTDFs were used in isotopic mixing models to quantify the diet of Scyliorhinus canicula caught 24 in the North Sea and compared with stomach content data. DTDFs for δ 15 N (∆ 15 N) and δ 13 C 25 (∆ 13 C) ranged from -1.95‰ to 3.49‰ and from 0.52‰ to 5.14‰, respectively, and varied with 26 tissue and diet type. Isotope turnover rates in plasma and red blood cells, expressed as half-lives, 27 range from 39 to 135 days. A majority of the variability of DTDFs reported in this and other 28 studies with sharks can be explained by linear relationships between DTDF and dietary isotopic 29 values. From these relationships, we propose a method for isotope mixing models that uses diet-30 specific DTDFs, which improves diet reconstruction estimates of animals, particularly 31 mesopredator sharks that consume a large range of prey types. 32 33

Short-term shifts of stable isotope (δ13C, δ15N) values in juvenile sharks within nursery areas suggest rapid shifts in energy pathways

“…Our findings from sequentially sampled individuals, however, suggest a longer time period is needed to detect such changes, and that stable isotope data from a tissue with faster turnover rates (e.g. plasma δ 15 N half-life =~33 days, liver δ 15 N half-life =~39 days, whole blood δ 15 N half-life =~62 days; Caut et al, 2013;Kim et al, 2012;MacNeil et al, 2006;Malpica-Cruz et al, 2012), and/or using modeling approaches to test a priori hypotheses (e.g. Araujo et al, 2007;Lee et al, 2012;Matich and Heithaus, 2014) is more appropriate for detecting short-term shifts in elasmobranch trophic interactions.…”

“…Hussey et al, 2012;Kinney et al, 2011;McMeans et al, 2010), yet relatively slow turnover times (e.g. muscle N250 days, Kim et al, 2012;fin N500 days, MacNeil et al, 2006) and few lab studies quantifying turnover rates and discrimination rates (Caut et al, 2013;Hussey et al, 2010a;Kim et al, 2012;Logan and Lutcavage, 2010;MacNeil et al, 2006;Malpica-Cruz et al, 2012) hinders our ability to interpret stable isotopes of elasmobranchs, especially among juveniles in which "maternal meddling" can greatly affect δ 13 C and δ 15 N values (Olin et al, 2011). Indeed, in placentatrophic sharks, embryos tend to have enriched isotopic values relative to their mothers, which persists into at least their first few months after birth, and may indicate that sharks are reliant on maternal energy reserves at the time of capture, despite feeding independently for weeks or even months prior to capture (e.g.…”

Trophic interactions of common elasmobranchs in deep-sea communities of the Gulf of Mexico revealed through stable isotope and stomach content analysis

“…A scaled discrimination factor framework, where higher dietary δ 15 N values result in lower trophic discrimination factors (Caut et al, 2013;Hussey et al, 2014;Olin et al, 2013), suggests that the deep-sea sharks sampled in this study span approximately two trophic levels. Experimental studies done on a limited number of elasmobranch species revealed δ 15 N discrimination factors ranging from 2.3‰ to 3.7‰ using lipid and urea extracted samples Kim et al, 2012aKim et al, , 2012b) and 1.1-1.8‰ using bulk muscle tissue Kim et al, 2012a;MalpicaCruz et al, 2012), but none of these experiments were conducted on deep-sea taxa.…”

Sympatric elasmobranchs and fecal samples provide insight into the trophic ecology of the smalltooth sawfish

“…The SEA c was used to calculate the degree of isotopic niche overlap, with associated 95% confidence intervals, representing a quantitative measure of dietary similarity between size classes. Ranges represent the range in δ To compare inter-and intra-specific resource use of the size classes of the 3 elasmobranchs, SEA c was calculated using their δ 13 C and δ 15 N data adjusted with the diet-tissue discrimination factors developed for elasmobranch fin tissues (Caut et al 2013; 1.06 ‰ Δ 13 C and 0.49 ‰ for Δ 15 N). The adjusted SEA c ellipses were graphed over the mean (± SD) stable isotope data of the representative consumer species pooled by 4 functional groups (i.e.…”