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Macrochelys temminckii (alligator snapping turtle) is an aquatic turtle endemic to the southeastern United States that was proposed for listing under the Endangered Species Act in 2021. In the present study we analyzed total mercury (THg) concentrations in skeletal muscle, tail clips, and nail tissue of 93 M. temminckii sampled from 14 waterbodies in eastern Texas (USA). Our objectives were to assess (1) the degree of correlation between internal tissue (skeletal muscle and tail clip samples) and keratin (nail samples), (2) the influence of ecological factors (turtle size and waterbody/sampling site) on THg concentrations, and (3) whether THg concentrations were high enough to pose a risk to human consumers. The mean (±SE) THg concentrations of muscle and nail were 1.16 ± 0.08 μg/g dry weight and 4.21 ± 0.24 μg/g dry weight, respectively, and THg concentrations were highly dependent on the sampling site. The THg concentrations of nails were correlated with muscle concentrations (R 2  = 0.56, p < 0.001). The effect of body size on THg concentrations varied by sampling site, indicating that size is not a good predictor of Hg concentration across sites. Finally, THg concentrations in M. temminckii of eastern Texas were high enough to pose a potential risk to human health based on US Environmental Protection Agency dietary guidelines. Environ Toxicol Chem 2024;43:1903-1913. © 2024 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA., (© 2024 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

Biodiversity collections are experiencing a renaissance fueled by the intersection of informatics, emerging technologies, and the extended use and interpretation of specimens and archived databases. In this article, we explore the potential for transformative research in ecology integrating biodiversity collections, stable isotope analysis (SIA), and environmental informatics. Like genomic DNA, SIA provides a common currency interpreted in the context of biogeochemical principles. Integration of SIA data across collections allows for evaluation of long-term ecological change at local to continental scales. Challenges including the analysis of sparse samples, a lack of information about baseline isotopic composition, and the effects of preservation remain, but none of these challenges is insurmountable. The proposed research framework interfaces with existing databases and observatories to provide benchmarks for retrospective studies and ecological forecasting. Collections and SIA add historical context to fundamental questions in freshwater ecological research, reference points for ecosystem monitoring, and a means of quantitative assessment for ecosystem restoration.

Humans are altering nutrient dynamics through myriad pathways globally. Concurrent with the addition of nutrients via municipal, industrial, and agricultural sources, widespread consumer exploitation is changing consumer-mediated nutrient dynamics drastically. Thus, altered nutrient dynamics can occur through changes in the supply of multiple nutrients, as well as through changes in the sources of these nutrients. Seagrass ecosystems are heavily impacted by human activities, with highly altered nutrient dynamics from multiple causes. We simulate scenarios of altered nutrient supply and ratios, nitrogen:phosphorus (N:P), from two nutrient sources in seagrass ecosystems: anthropogenic fertilizer and fish excretion. In doing so we tested expectations rooted in ecological theory that suggest the importance of resource dynamics for predicting primary producer dynamics. Ecosystem functions were strongly altered by artificial fertilizer (e.g., seagrass growth increased by as much as 140%), whereas plant/algae community structure was most affected by fish-mediated nutrients or the interaction of both treatments (e.g., evenness increased by ~140% under conditions of low fish nutrients and high anthropogenic nutrients). Interactions between the nutrient sources were found for only two of six response variables, and the ratio of nutrient supply was the best predictor for only one response. These findings show that seagrass structure and function are well predicted by supply of a single nutrient (either N or P). Importantly, no single nutrient best explained the majority of responses-measures of community structure were best explained by the primary limiting nutrient to this system (P), whereas measures of growth and density of the dominant producer in the system were best explained by N. Thus, while our findings support aspects of theoretical expectations, the complexity of producer community responses belies broad generalities, underscoring the need to manage for multiple simultaneous nutrients in these imperiled coastal ecosystems., (© 2018 by the Ecological Society of America.)

The isotopic (δ 13 C and δ 15 N) and stoichiometric (C:N:P) compositions of four fish species (Family Centrarchidae: Lepomis auritus, Lepomis cyanellus; Family Cyprinidae: Nocomis leptocephalus, Semotilus atromaculatus) were examined across four North Carolina Piedmont streams arrayed along an urbanization gradient. Both isotopic and stoichiometric composition of fishes appeared to track changes occurring in basal resource availability. Values of δ 13 C of basal resources and consumers were more enriched at the most urbanized streams. Similarly, basal resources and consumers were δ 15 N-enriched at more urbanized streams. Basal resource stoichiometry varied across streams, with periphyton being the most variable. Primary consumers stoichiometry also differed across streams. Intraspecific variation in fish stoichiometry correlated with the degree of urbanization, as the two cyprinids had higher N content and L. cyanellus had higher P content in more urbanized streams, probably due to enrichment of basal resources. Intrinsic factors, specifically species identity and body size also affected stoichiometric variation. Phosphorus (P) content increased significantly with body size in centrarchids, but not in cyprinids. These results suggest that although species identity and body size are important predictors of elemental stoichiometry, the complex nature of altered urban streams may yield imbalances in the elemental composition of consumers via their food resources., (© 2018 The Fisheries Society of the British Isles.)

Body size is frequently claimed to be a major determinant of animal trophic interactions, yet few studies have explored relationships between body size and trophic interactions in rivers, especially within the tropics. We examined relationships between body size and trophic position (TP) within fish assemblages in four lowland rivers of the Lower Mekong Basin in Cambodia. Stable isotope analysis (based on δ 15 N) was used to estimate TP of common fish species in each river, and species were classified according to occupation of benthic versus pelagic habitats and major feeding guilds. Regression analysis yielded strong correlations between body size and TP among fishes from the Sesan and Sreprok rivers, but not those from the Mekong and Sekong rivers. The Mekong fish assemblage had higher average TP compared with those of other rivers. The relationship between body size and TP was positive and significantly correlated for piscivores and omnivores, but not for detritivores and insectivores. The body size-TP relationship did not differ between pelagic and benthic fishes. Body size significantly predicted TP within the orders Siluriformes and Perciformes, but not for Cypriniformes, the most species-rich and ecologically diverse order in the Lower Mekong River. We conclude that for species-rich, tropical fish assemblages with many detritivores and invertivores, body size would not be an appropriate surrogate for TP in food web models and other ecological applications.

The dry Chaco, a semiarid thorn forest, is experiencing some of the highest deforestation rates globally, coupled with the fact that small-bodied fish are at the highest risk of extinction, the killifish inhabiting this region may be some of the most threatened taxa. Yet, aspects of ecology and life history for Neotropical killifishes in the Bolivian Gran Chaco region are completely lacking, and basic life-history data is of critical importance for the design and implementation of conservation measures. Collections were conducted during the early (January 2011) and late (March-April 2011) rainy season using an area-based sampler and dip net surveys. Fish standard length and body depth were measured as well as the number of oocytes per size class, mean oocyte diameter per size class, and total fecundity for the females of each species. A total of 490 specimens of rivulids were captured; Austrolebias vandenbergi: 85 females, 105 males and 39 juveniles, and N. ornatipinnis: 62 females, 113 males, 86 juveniles. Sexual size dimorphism, absolute fecundity, oocyte developmental stages, oocyte diameter, and population sex ratios were determined for each species. Both species exhibited sexual size dimorphism. Male A. vandenbergi exhibited longer standard length (mean +/- SD; males: 27.07 +/- 3.89mm, females: 23.6 +/- 2.02mm) and body depth (males: 8.9 +/- 1.7mm, females: 7.2 +/- 1.1 mm) as compared to females. Male N. ornatipinnis had a similar pattern for both standard length (males: 26.0 +/- 7.1mm, females: 19.1 +/- 5.83mm) and body depth (males: 5.6 +/- 1.9mm, females: 4.7 +/- 1.0mm). Austrolebias vandenbergi had fewer and smaller oocytes per female (47 +/- 31.6) than N. ornatipinnis (206 +/- 131.2). There was a positive relationship between fecundity and female body size in both species. The presence of multiple developmental stages of oocytes (immature, maturing, and mature) suggest that both species of rivulids exhibit fractional spawning, a reproductive strategy that enhances reproductive success in these extreme habitats. Neofundulus ornatipinnis exhibited a higher mean oocyte diameter for all three developmental stages (immature, maturing, and mature) as compared to A. vandenbergi. Austrolebias vandenbergi exhibited an equal adult sex ratio (males:females, 1:1), but there was a slight female biased ratio for N. ornatipinnis (males:females, 1:1.8). These results provide fundamental and valuable information for ensuring rivulid conservation in tropical regions, and also improve the knowledge on the biology and ecology of these poorly known species. To our knowledge, this study represents the first contribution on the reproductive biology of two Neotropical annual rivulid fishes (Austrolebias vandenbergi and Neofundulus ornatipinnis) inhabiting semi-permanent and ephemeral ponds in the Gran Chaco of Southeastern Bolivia.

Neotropical rivers are home to the largest assemblage of freshwater fishes, but little is known about the phylogeny of these fishes at the species level using multi-locus molecular markers. Here, we present a phylogeny for all known species of the genus Satanoperca, a widespread group of Neotropical cichlid fishes, based on analysis of six unlinked genetic loci. To test nominal and proposed species limits for this group, we surveyed mtDNA sequence variation among 320 individuals representing all know species. Most nominal species were supported by this approach but we determined that populations in the Xingu, Tapajós, and Araguaia+Paraná Rivers are likely undescribed species, while S. jurupari and S. mapiritensis did not show clear genetic distinction. To infer a phylogeny of these putative species, we conducted maximum likelihood and Bayesian non-clock and relaxed clock analyses of concatenated data from three genes (one mitochondrial, two nuclear). We also used a multi-species coalescent model to estimate a species tree from six unlinked loci (one mitochondrial, five nuclear). The topologies obtained were congruent with other results, but showed only minimal to moderate support for some nodes, suggesting that more loci will be needed to satisfactorily estimate the distribution of coalescent histories within Satanoperca. We determined that this variation results from topological discordance among separate gene trees, likely due to differential sorting of ancestral polymorphisms., (Copyright © 2012 Elsevier Inc. All rights reserved.)

The Casiquiare River is a unique biogeographic corridor between the Orinoco and Amazonas basins. We investigated the importance of this connection for Neotropical fishes using peacock cichlids (Cichla spp.) as a model system. We tested whether the Casiquiare provides a conduit for gene flow between contemporary populations, and investigated the origin of biogeographic distributions that span the Casiquiare. Using sequences from the mitochondrial control region of three focal species (C. temensis, C. monoculus, and C. orinocensis) whose distributions include the Amazonas, Orinoco, and Casiquiare, we constructed maximum likelihood phylograms of haplotypes and analyzed the populations under an isolation-with-migration coalescent model. Our analyses suggest that populations of all three species have experienced some degree of gene flow via the Casiquiare. We also generated a mitochondrial genealogy of all Cichla species using >2000 bp and performed a dispersal-vicariance analysis (DIVA) to reconstruct the historical biogeography of the genus. This analysis, when combined with the intraspecific results, supports two instances of dispersal from the Amazonas to the Orinoco. Thus, our results support the idea that the Casiquiare connection is important across temporal scales, facilitating both gene flow and the dispersal and range expansion of species.

To investigate forces influencing diversification in Neotropical fishes, the phylogenetic relationships among species and populations of the cichlid genus Cichla were examined. Mitochondrial DNA was sequenced for 454 individuals of the 5 nominal Cichla species and several putative undescribed species. Phylogenetic analyses support the distinction of two major clades of Cichla. Clade A includes C. temensis and two undescribed species from the lower Amazonas and Xingu Rivers. Clade B includes C. orinocensis, C. monoculus, C. ocellaris. C, intermedia, and an undescribed species from the upper Madeira River. Species boundaries were relatively well-circumscribed for clade B, while incomplete lineage sorting was inferred for clade A. Three probable instances of introgression were observed, including a regional population of C. orinocensis from the Negro River that shows a history of introgression. Biogeographic patterns from Cichla are partially congruent with those seen in several other Neotropical fish clades, and the diversification of Cichla species is inferred to result from both vicariance and sympatric divergence.

Within an organism, lipids are depleted in (13)C relative to proteins and carbohydrates (more negative delta(13)C), and variation in lipid content among organisms or among tissue types has the potential to introduce considerable bias into stable isotope analyses that use delta(13)C. Despite the potential for introduced error, there is no consensus on the need to account for lipids in stable isotope analyses. Here we address two questions: (1) If and when is it important to account for the effects of variation in lipid content on delta(13)C? (2) If it is important, which method(s) are reliable and robust for dealing with lipid variation? We evaluated the reliability of direct chemical extraction, which physically removes lipids from samples, and mathematical normalization, which uses the carbon-to-nitrogen (C:N) ratio of a sample to normalize delta(13)C after analysis by measuring the lipid content, the C:N ratio, and the effect of lipid content on delta(13)C (Deltadelta(13)C) of plants and animals with a wide range of lipid contents. For animals, we found strong relationships between C:N and lipid content, between lipid content and Deltadelta(13)C, and between C:N and Deltadelta(13)C. For plants, C:N was not a good predictor of lipid content or Deltadelta(13)C, but we found a strong relationship between carbon content and lipid content, lipid content and Deltadelta(13)C, and between and carbon content and Deltadelta(13)C. Our results indicate that lipid extraction or normalization is most important when lipid content is variable among consumers of interest or between consumers and end members, and when differences in delta(13)C between end members is <10-12 per thousand. The vast majority of studies using natural variation in delta(13)C fall within these criteria. Both direct lipid extraction and mathematical normalization reduce biases in delta(13)C, but mathematical normalization simplifies sample preparation and better preserves the integrity of samples for delta(15)N analysis.

Stable isotope ratios (typically of carbon and nitrogen) provide one representation of an organism's trophic niche and are widely used to examine aspects of food web structure. Yet stable isotopes have not been applied to quantitatively characterize community-wide aspects of trophic structure (i.e., at the level of an entire food web). We propose quantitative metrics that can be used to this end, drawing on similar approaches from ecomorphology research. For example, the convex hull area occupied by species in delta13C-delta15N niche space is a representation of the total extent of trophic diversity within a food web, whereas mean nearest neighbor distance among all species pairs is a measure of species packing within trophic niche space. To facilitate discussion of opportunities and limitations of the metrics, we provide empirical and conceptual examples drawn from Bahamian tidal creek food webs. These examples illustrate how this methodology can be used to quantify trophic diversity and trophic redundancy in food webs, as well as to link individual species to characteristics of the food web in which they are embedded. Building from extensive applications of stable isotope ratios by ecologists, the community-wide metrics may provide a new perspective on food web structure, function, and dynamics.