Oliver N. Shipley

A global perspective on the trophic geography of sharks

Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.Carbon isotopic analysis reveals global biogeographic traits in shark trophic interactions, and sheds light on the diverse foraging behaviour of sharks.

Horizontal and vertical movements of Caribbean reef sharks (Carcharhinus perezi): conservation implications of limited migration in a marine sanctuary

Despite the ecological and economic importance of the Caribbean reef shark (Carcharhinus perezi), little data exist regarding the movements and habitat use of this predator across its range. We deployed 11 pop-up satellite archival tags on Caribbean reef sharks captured in the northeast Exuma Sound, The Bahamas, to assess their horizontal and vertical movements throughout the water column. Sharks showed high site fidelity to The Bahamas suggesting Bahamian subpopulations remain protected within the Bahamian Shark Sanctuary. Depth data indicate that Caribbean reef sharks spent a significant proportion (72–91%) of their time above 50 m in narrow vertical depth bands, which varied considerably on an individual basis. This may be indicative of high site fidelity to specific bathymetric features. Animals exhibited three broadly categorized sporadic off-bank excursions (more than 50 m excursions) down to a depth of 436.1 m, which were more frequent during the night. These deeper excursions during night may be indicative of foraging in relation to prey on mesophotic reefs, as well as diel-vertically migrating prey from the deeper meso- and bathypelagic zones. These vertical movements suggest that Caribbean reef sharks can be significant vectors of ecosystem connectivity further warranting holistic multi-system management and conservation approaches.

Stable isotope analysis in deep-sea chondrichthyans: recent challenges, ecological insights, and future directions

Deep-sea chondrichthyans are cryptic species subject to increasing anthropogenic exploitation. Defining their role in deep-water ecosystems is therefore crucial for predicting the ecosystem-wide effects of their removal. Stable isotope analyses (SIA) of carbon and nitrogen have been increasingly used in chondrichthyan studies as a non-lethal method to investigate aspects of their ecology. In recent years these methods have been applied to deep-sea chondricthyans to investigate their trophic structure, niche width, and describe energy flow in the deep sea. Despite the increasing popularity of SIA in deep-sea chondrichthyan studies, methods rely on a multitude of assumptions, such as the need to determine accurate trophic discrimination and tissue turnover rates, which are currently lacking for most species. These uncertainties may preclude the reliability of isotope-based approaches, and as a result inferences from isotopic data must be viewed with relative caution. Due to the growing use of isotopic-based approaches in deep-sea chondrichthyans, we review the literature in the context of current methodological challenges and ecological inferences. We provide recommendations and novel approaches that may help develop and refine a rapidly growing field of study.

Trophic niche dynamics of three nearshore benthic predators in The Bahamas

Carbon and nitrogen stable isotopes (δ13C and δ15N) were used to examine trophic niche dynamics of three co-occurring predators in The Bahamas. Variable estimates of core trophic niche width and total trophic niche overlap were observed between nurse sharks (Ginglymostoma cirratum), southern stingrays (Hypanus americanus), and Atlantic chupare stingrays (Styracura schmardae). Nurse sharks exhibited little trophic overlap with either species of stingray and exhibited higher δ15N values, suggesting limited potential resource competition. Southern and Atlantic chupare stingrays exhibited higher total trophic niche overlap, highlighting higher levels of resource competition. Nurse sharks exhibited the broadest δ13C range, implying utilization of multiple resource pools to facilitate energetic requirements, compared with southern stingrays and Atlantic chupare stingrays. Southern stingrays exhibited the smallest core trophic niche width, highlighting reliance upon a narrower range of resource pools, likely supported by a single habitat type. We postulate that the persistence of the three sub-sampled predator populations is supported by potential resource partitioning and utilization of multiple trophic resource pools. Further information regarding trophic niche dynamics and patterns of resource use is required for data-poor species to identify how populations and communities may respond to the adverse effects of exploitation, such as fisheries impacts, habitat alteration, and pollution.

Validation of a portable, waterproof blood pH analyser for elasmobranchs

Analysis of blood chemistry, including whole blood pH, can provide a valuable metric of physiological disturbance in fishes exposed to anthropogenic stress. Here, we demonstrate the utility of a cost-effective, portable, waterproof pH meter in measuring lemon shark and Cuban dogfish whole blood pH.

Was my science project eaten? A novel approach to validate consumption of marine biologging instruments

AbstractBackground Biologging and tracking instruments provide valuable, remote surveillance on otherwise unobservable marine animals. Instruments can be consumed (ingested) by predators while collecting data, and if not identified, the retrieved dataset could be assigned to the incorrect individual and/or species. Consumption events of instruments, such as pop-up satellite archival tags and data loggers that record ambient light, are typically identified by negligible light levels and visual assessment of data records. However, when light-level data are not available (e.g., environments below the euphotic zone, instrument model), instrument consumption is not easily discernible. Instruments that record concurrent, time-series temperature and depth data provide detailed information on the ambient temperature in the water column. However, if the instrument is consumed, the temperature profile may dissociate from the depth profile, providing evidence and a means for detecting consumption.ResultsTo quantify the dissociation between time-series depth and temperature profiles, we applied the cross-correlation function to evaluate the time delay and uncoupling between time-series depth and temperature profiles, suggestive of instrument consumption. Given that instruments may be consumed midway through the deployment duration, we extended the cross-correlation function to systematically slide across time-series profiles, sequentially considering subsets of data, to infer time of consumption. This method was applied to datasets from both deep-water (disphotic and aphotic) and epipelagic (euphotic) environments to evaluate instrument consumption. Results were dependent on ambient environment, data sampling rate, predator physiology, and function parameters.ConclusionsUtilization of the cross-correlation function objectively indicates potential instrument consumption events without the bias induced by subjective methods such as visual assessment of tag-recorded data, and does not require the simultaneous collection of light-level data. This methodology aids in the appropriate biological interpretation of tag-recorded data, ensures that data are not attributed to the incorrect species, and can be used to authenticate data during the validation process. Additionally, it is particularly useful for contrasting datasets from comparable studies (i.e., same location and species) and is applicable across taxa and electronic biologging instrument variations.

Phylotranscriptomic insights into the diversification of endothermic Thunnus tunas

Abstract Birds, mammals, and certain fishes, including tunas, opahs and lamnid sharks, are endothermic, conserving internally generated, metabolic heat to maintain body or tissue temperatures above that of the environment. Bluefin tunas are commercially important fishes worldwide, and some populations are threatened. They are renowned for their endothermy, maintaining elevated temperatures of the oxidative locomotor muscle, viscera, brain and eyes, and occupying cold, productive high-latitude waters. Less cold-tolerant tunas, such as yellowfin tuna, by contrast, remain in warm-temperate to tropical waters year-round, reproducing more rapidly than most temperate bluefin tuna populations, providing resiliency in the face of large-scale industrial fisheries. Despite the importance of these traits to not only fisheries but also habitat utilization and responses to climate change, little is known of the genetic processes underlying the diversification of tunas. In collecting and analyzing sequence data across 29,556 genes, we found that parallel selection on standing genetic variation is associated with the evolution of endothermy in bluefin tunas. This includes two shared substitutions in genes encoding glycerol-3 phosphate dehydrogenase, an enzyme that contributes to thermogenesis in bumblebees and mammals, as well as four genes involved in the Krebs cycle, oxidative phosphorylation, β-oxidation, and superoxide removal. Using phylogenetic techniques, we further illustrate that the eight Thunnus species are genetically distinct, but found evidence of mitochondrial genome introgression across two species. Phylogeny-based metrics highlight conservation needs for some of these species.

Stable isotope fractionation between maternal and embryo tissues in the Bonnethead shark ( Sphyrna tiburo )

Evaluating tissue fractionation between mothers and their offspring is fundamental for informing our interpretation of stable isotope values in young individuals and can provide insight into the dynamics of maternal provisioning. The objectives of this study were to investigate the isotopic relationships between maternal reproductive (i.e., yolk, yolk-sac placenta) and somatic tissues (i.e., muscle and liver) relative to embryos in the Bonnethead Shark Sphyrna tiburo, to evaluate the fractionation of stable carbon (δ13C) and nitrogen (δ15N) isotopes between these tissues. Additionally, we examined intra-uterine variability in the isotopic relationships to ascertain whether this species may exhibit variable nutrient allocation. Embryos showed similar magnitudes of enrichment in 13C (i.e., Δδ13C, difference between adult and embryo) relative to adult tissues (Δδ13C = ~1.0‰). However, embryos were depleted in 15N relative to adult muscle tissues (Δδ15N = −1.0‰), a finding that contrasts Δδ15N values reported for other placentotrophic sharks. Embryo-muscle Δδ15N was correlated with length, supporting the contention that the magnitude of enrichment between embryonic and maternal tissues results from the shift from yolk to placental feeding. Embryo δ15N and Δδ15N values showed significant intra-uterine variability; a result not observed for δ13C and Δδ13C values. The contrasting patterns in fractionation among placentotrophic sharks highlight the importance of evaluating these relationships across elasmobranch taxa with consideration for different tissues, reproductive strategies and stages of gestation. The divergent findings support future evaluation of stable isotope relationships between mothers and offspring for purposes of estimating inherent isotopic variability and how this variability may inform physiological and dietary mechanisms.