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Trophic ecology and resource use are challenging to discern in migratory marine species, including sharks. However, effective management and conservation strategies depend on understanding these life history details. Here we investigate whether dental enameloid zinc isotope (δ⁶⁶Zn_en) values can be used to infer intrapopulation differences in foraging ecology by comparing δ⁶⁶Zn_enwith same-tooth collagen carbon and nitrogen (δ¹³C_coll, δ¹⁵N_coll) values from critically endangered sand tiger sharks (Carcharias taurus) from Delaware Bay (USA). We document ontogeny and sex-related isotopic differences indicating distinct diet and habitat use at the time of tooth formation. Adult females have the most distinct isotopic niche, likely feeding on higher trophic level prey in a distinct habitat. This multi-proxy approach characterises an animal’s isotopic niche in greater detail than traditional isotope analysis alone and shows that δ⁶⁶Zn_enanalysis can highlight intrapopulation dietary variability thereby informing conservation management and, due to good δ⁶⁶Zn_enfossil tooth preservation, palaeoecological reconstructions.

 

Shark teeth are one of the most abundant vertebrate fossils, and because tooth size generally correlates with body size, their accumulations document the size structure of populations. Understanding how ecological and environmental processes influence size structure, and how this extends to influence these dental distributions, may offer a window into the ecological and environmental dynamics of past and present shark populations. Here, we examine the dental distributions of sand tigers, including extant Carcharias taurus and extinct Striatolamia macrota , to reconstruct the size structure for a contemporary locality and four Eocene localities. We compare empirical distributions against expectations from a population simulation to gain insight into potential governing ecological processes. Specifically, we investigate the influence of dispersal flexibility to and from protected nurseries. We show that changing the flexibility of initial dispersal of juveniles from the nursery and annual migration of adults to the nursery explains a large amount of dental distribution variability. Our framework predicts dispersal strategies of an extant sand tiger population, and supports nurseries as important components of sand tiger life history in both extant and Eocene populations. These results suggest nursery protection may be vital for shark conservation with increasing anthropogenic impacts and climate change. 

Abstract Diet is a crucial trait of an animal’s lifestyle and ecology. The trophic level of an organism indicates its functional position within an ecosystem and holds significance for its ecology and evolution. Here, we demonstrate the use of zinc isotopes (δ 66 Zn) to geochemically assess the trophic level in diverse extant and extinct sharks, including the Neogene megatooth shark ( Otodus megalodon ) and the great white shark ( Carcharodon carcharias ). We reveal that dietary δ 66 Zn signatures are preserved in fossil shark tooth enameloid over deep geologic time and are robust recorders of each species’ trophic level. We observe significant δ 66 Zn differences among the Otodus and Carcharodon populations implying dietary shifts throughout the Neogene in both genera. Notably, Early Pliocene sympatric C. carcharias and O. megalodon appear to have occupied a similar mean trophic level, a finding that may hold clues to the extinction of the gigantic Neogene megatooth shark. 

Nitrogen isotope ratios in fossil teeth place extinct megatooth sharks at the top of the marine food web. 

Niche differentiation and intraguild predation (IGP) can allow ecologically similar species to coexist, although it is unclear which coexistence mechanism predominates in consumer communities. Until now, a limited ability to quantify diets from metabarcoding data has precluded the use of sequencing data to determine the relative importance of these mechanisms.

Here, we pair a recent metabarcoding quantification approach with stable isotope analysis to examine diet composition in a wolf spider community.

We compare the prevalence of resource partitioning and IGP in these spiders and test whether factors that influence foraging performance, including individual identity, morphology, prey community and environmental conditions, can explain variation in diet composition and IGP.

Extensive IGP is likely the primary coexistence mechanism in this community, and other factors to which foraging variation is often attributed do not explain diet composition and IGP here. Rather, IGP increases as prey diversity decreases.

Foragers are driven to IGP where resource niches are limited. We highlight the need to examine how drivers of predator–prey interaction strengths translate into foraging in natural systems.

 

Many explanations for Eocene climate change focus on the Southern Ocean—where tectonics influenced oceanic gateways, ocean circulation reduced heat transport, and greenhouse gas declines prompted glaciation. To date, few studies focus on marine vertebrates at high latitudes to discern paleoecological and paleoenvironmental impacts of this climate transition. The Tertiary Eocene La Meseta (TELM) Formation has a rich fossil assemblage to characterize these impacts;Striatolamia macrota, an extinct (†) sand tiger shark, is abundant throughout the La Meseta Formation. Body size is often tracked to characterize and integrate across multiple ecological dimensions. †S. macrotabody size distributions indicate limited changes during TELMs 2–5 based on anterior tooth crown height (n = 450, mean = 19.6 ± 6.4 mm). Similarly, environmental conditions remained stable through this period based on δ¹⁸O_PO4values from tooth enameloid (n = 42; 21.5 ± 1.6‰), which corresponds to a mean temperature of 22.0 ± 4.0°C. Our preliminaryε_Nd(n = 4) results indicate an early Drake Passage opening with Pacific inputs during TELM 2–3 (45–43 Ma) based on single unit variation with an overall radiogenic trend. Two possible hypotheses to explain these observations are (1) †S. macrotamodified its migration behavior to ameliorate environmental changes related to the Drake Passage opening, or (2) the local climate change was small and gateway opening had little impact. While we cannot rule out an ecological explanation, a comparison with climate model results suggests that increased CO₂produces warm conditions that also parsimoniously explain the observations.