Evolution of the Critically Endangered Green Sawfish Pristis zijsron (Rhinopristiformes, Pristidae), Inferred from the Whole Mitochondrial Genome.

The green sawfish Pristis zijsron (Bleeker, 1851), a species of sawfish in the family Pristidae (Rhinopristiformes), mainly inhabits the Indo-West Pacific region. In this study, the complete mitochondrial genome of the critically endangered green sawfish is first described. The length of the genome is 16,804 bp, with a nucleotide composition of 32.0% A, 24.8% C, 13.1% G, and 30.0% T. It contains 37 genes in the typical gene order of fish. Two start (GTG and ATG) and two stop (TAG and TAA/T-) codons are found in the thirteen protein-coding genes. The 22 tRNA genes range from 67 bp (tRNA-Ser) to 75 bp (tRNA-Leu). The ratio of nonsynonymous substitution (Ka) and synonymous substitution (Ks) indicates that the family Pristidae are suffering a purifying selection. The reconstruction of Bayesian inference and the maximum likelihood phylogenetic tree show the same topological structure, and the family Pristidae is a monophyletic group with strong posterior probability. Pristis zijsron and P. pectinata form a sister group in the terminal clade. And the divergence time of Rhinopristiformes show that P. zijsron and P. pectinata diverged as two separate species in about Paleogene 31.53 Mya. Complete mitochondrial genomes of all five sawfishes have been published and phylogenetic relationships have been analyzed. The results of our study will provide base molecular information for subsequent research (e.g., distribution, conservation, phylogenetics, etc.) on this endangered group.

Emergent community architecture despite distinct diversity in the global whale shark (Rhincodon typus) epidermal microbiome.

Microbiomes confer beneficial physiological traits to their host, but microbial diversity is inherently variable, challenging the relationship between microbes and their contribution to host health. Here, we compare the diversity and architectural complexity of the epidermal microbiome from 74 individual whale sharks (Rhincodon typus) across five aggregations globally to determine if network properties may be more indicative of the microbiome-host relationship. On the premise that microbes are expected to exhibit biogeographic patterns globally and that distantly related microbial groups can perform similar functions, we hypothesized that microbiome co-occurrence patterns would occur independently of diversity trends and that keystone microbes would vary across locations. We found that whale shark aggregation was the most important factor in discriminating taxonomic diversity patterns. Further, microbiome network architecture was similar across all aggregations, with degree distributions matching Erdos-Renyi-type networks. The microbiome-derived networks, however, display modularity indicating a definitive microbiome structure on the epidermis of whale sharks. In addition, whale sharks hosted 35 high-quality metagenome assembled genomes (MAGs) of which 25 were present from all sample locations, termed the abundant 'core'. Two main MAG groups formed, defined here as Ecogroup 1 and 2, based on the number of genes present in metabolic pathways, suggesting there are at least two important metabolic niches within the whale shark microbiome. Therefore, while variability in microbiome diversity is high, network structure and core taxa are inherent characteristics of the epidermal microbiome in whale sharks. We suggest the host-microbiome and microbe-microbe interactions that drive the self-assembly of the microbiome help support a functionally redundant abundant core and that network characteristics should be considered when linking microbiomes with host health.

Scaling of Activity Space in Marine Organisms across Latitudinal Gradients.

AbstractUnifying models have shown that the amount of space used by animals (e.g., activity space, home range) scales allometrically with body mass for terrestrial taxa; however, such relationships are far less clear for marine species. We compiled movement data from 1,596 individuals across 79 taxa collected using a continental passive acoustic telemetry network of acoustic receivers to assess allometric scaling of activity space. We found that ectothermic marine taxa do exhibit allometric scaling for activity space, with an overall scaling exponent of 0.64. However, body mass alone explained only 35% of the variation, with the remaining variation best explained by trophic position for teleosts and latitude for sharks, rays, and marine reptiles. Taxon-specific allometric relationships highlighted weaker scaling exponents among teleost fish species (0.07) than sharks (0.96), rays (0.55), and marine reptiles (0.57). The allometric scaling relationship and scaling exponents for the marine taxonomic groups examined were lower than those reported from studies that had collated both marine and terrestrial species data derived using various tracking methods. We propose that these disparities arise because previous work integrated summarized data across many studies that used differing methods for collecting and quantifying activity space, introducing considerable uncertainty into slope estimates. Our findings highlight the benefit of using large-scale, coordinated animal biotelemetry networks to address cross-taxa evolutionary and ecological questions.

From rivers to ocean basins: The role of ocean barriers and philopatry in the genetic structuring of a cosmopolitan coastal predator.

The Bull Shark (Carcharhinus leucas) faces varying levels of exploitation around the world due to its coastal distribution. Information regarding population connectivity is crucial to evaluate its conservation status and local fishing impacts. In this study, we sampled 922 putative Bull Sharks from 19 locations in the first global assessment of population structure of this cosmopolitan species. Using a recently developed DNA-capture approach (DArTcap), samples were genotyped for 3400 nuclear markers. Additionally, full mitochondrial genomes of 384 Indo-Pacific samples were sequenced. Reproductive isolation was found between and across ocean basins (eastern Pacific, western Atlantic, eastern Atlantic, Indo-West Pacific) with distinct island populations in Japan and Fiji. Bull Sharks appear to maintain gene flow using shallow coastal waters as dispersal corridors, whereas large oceanic distances and historical land-bridges act as barriers. Females tend to return to the same area for reproduction, making them more susceptible to local threats and an important focus for management actions. Given these behaviors, the exploitation of Bull Sharks from insular populations, such as Japan and Fiji, may instigate local decline that cannot readily be replenished by immigration, which can in turn affect ecosystem dynamics and functions. These data also supported the development of a genetic panel to ascertain the population of origin, which will be useful in monitoring the trade of fisheries products and assessing population-level impacts of this harvest.

Rapid assessment of adult abundance and demographic connectivity from juvenile kin pairs in a critically endangered species.

The viability of spatially structured populations depends on the abundance and connectivity between subpopulations of breeding adults. Yet, for many species, both are extremely difficult to assess. The speartooth shark is a critically endangered elasmobranch inhabiting tropical rivers with only three adults ever recorded in Australia. Close-kin mark-recapture models, informed by sibling pairs among 226 juveniles, were developed to estimate adult abundance and connectivity in two Australian river systems. Sixty-eight sibling pairs were found, and adult abundance was estimated at 892 for the Adelaide River and 1128 for the Alligator Rivers. We found strong evidence for female philopatry, with most females returning to the same river to pup. Adelaide River males appear largely philopatric, whereas Alligator Rivers males are highly connected to the Adelaide River. From only 4 years of sampling, our results demonstrate that juvenile-only kin pairs can inform simultaneous estimates of abundance and connectivity in a rare and threatened species.

Zone specific trends in coral cover, genera and growth-forms in the World-Heritage listed Ningaloo Reef.

On coral reefs, changes in the cover and relative abundance of hard coral taxa often follow disturbance. Although the ecological responses of common coral taxa have been well documented, little is known about the ecological responses of uncommon coral taxa or of coral morphological groups across multiple adjacent reef zones. We used Multivariate Auto-Regressive State-Space modelling to assess the rate and direction of change of hard coral cover across a variety of coral genera, growth forms, and susceptibility to bleaching and physical damage covering multiple reef zones at northern Ningaloo Reef in Western Australia. Trends were assessed between 2007 and 2016, during which multiple episodic disturbances occurred including cyclones and a heatwave. We provide evidence of zone specific trends, not only in total hard coral cover, but also in taxonomic and morphological groups of corals at Ningaloo Reef. Declines in total coral cover on the reef flat corresponded with declines in fast growing corals, particularly Acropora. In contrast, total coral cover on the reef slope and inshore (lagoon) did not undergo significant change, despite divergent trajectories of individual genera. Importantly, we also show that changes in the composition of coral assemblages can be detected using a morphological based approach when changes are not evident using a taxonomic approach. Therefore, we recommend that future assessments of coral reef trends incorporate not just standard metrics such as total coral cover, but also metrics that provide for detailed descriptions of trends in common and uncommon taxa and morphological groups across multiple reef zones.

One panel to rule them all: DArTcap genotyping for population structure, historical demography, and kinship analyses, and its application to a threatened shark.

With recent advances in sequencing technology, genomic data are changing how important conservation management decisions are made. Applications such as Close-Kin Mark-Recapture demand large amounts of data to estimate population size and structure, and their full potential can only be realised through ongoing improvements in genotyping strategies. Here we introduce DArTcap, a cost-efficient method that combines DArTseq and sequence capture, and illustrate its use in a high resolution population analysis of Glyphis garricki, a rare, poorly known and threatened euryhaline shark. Clustering analyses and spatial distribution of kin pairs from four different regions across northern Australia and one in Papua New Guinea, representing its entire known range, revealed that each region hosts at least one distinct population. Further structuring is likely within Van Diemen Gulf, the region that included the most rivers sampled, suggesting additional population structuring would be found if other rivers were sampled. Coalescent analyses and spatially explicit modelling suggest that G. garricki experienced a recent range expansion during the opening of the Gulf of Carpentaria following the conclusion of the Last Glacial Maximum. The low migration rates between neighbouring populations of a species that is found only in restricted coastal and riverine habitats show the importance of managing each population separately, including careful monitoring of local and remote anthropogenic activities that may affect their environments. Overall we demonstrated how a carefully chosen SNP panel combined with DArTcap can provide highly accurate kinship inference and also support population structure and historical demography analyses, therefore maximising cost-effectiveness.

Comparisons of stable isotope composition among tissues of green turtles.

Ecologists often need to make choices about what body parts (tissues or organs) of an animal to sample. The decision is typically guided by the need to treat animals as humanely as possible, as well as the information that different body parts can provide. When using stable isotopes, decisions are also influenced by whether specimens would require preservation, and whether they have properties (such as high lipid concentrations) that would influence measurements. Sometimes we cannot use a preferred tissue (for example, because of ethical or logistical constraints), and in such cases an ability to reliably predict stable isotope composition for one tissue from data yielded by another would be useful. METHODS: In this study we analysed multiple tissues (skin, whole blood, red blood cells, plasma and nail) from green turtles (Chelonia mydas) to evaluate variation in C:N ratios, and test hypotheses about the intercept and slope of regressions of stable carbon and nitrogen isotope compositions among tissues. RESULTS: Regression models revealed that linear relationships were present for most comparisons, except those involving the δ13 C of skin, and the slopes (ß1 ) of most regressions were different from unity. The C:N ratios of skin were significantly higher and more variable than those of other tissues. The δ13 C and δ15 N of nail were highly correlated with those of the whole blood, red blood cells and plasma. Nail and red blood cells showed low variation in C:N. CONCLUSIONS: The patterns in slopes of regressions indicate that comparisons of measurements yielded by different tissues of wild animals are complicated by the fact that the tissues are unlikely to be in isotopic equilibrium with their diet. Of the tissues used in this study, nail is simple to collect, requires minimal disturbance to the animal and no special preservation; these traits should make it attractive to turtle ecologists, but more information is needed on aspects such as growth rates.

Declining abundance of coral reef fish in a World-Heritage-listed marine park.

One of the most robust metrics for assessing the effectiveness of protected areas is the temporal trend in the abundance of the species they are designed to protect. We surveyed coral-reef fish and living hard coral in and adjacent to a sanctuary zone (SZ: where all forms of fishing are prohibited) in the World Heritage-listed Ningaloo Marine Park during a 10-year period. There were generally more individuals and greater biomass of many fish taxa (especially emperors and parrotfish) in the SZ than the adjacent recreation zone (RZ: where recreational fishing is allowed) - so log response ratios of abundance were usually positive in each year. However, despite this, there was an overall decrease in both SZ and RZ in absolute abundance of some taxa by up to 22% per year, including taxa that are explicitly targeted (emperors) by fishers and taxa that are neither targeted nor frequently captured (most wrasses and butterflyfish). A concomitant decline in the abundance (measured as percentage cover) of living hard coral of 1-7% per year is a plausible explanation for the declining abundance of butterflyfish, but declines in emperors might be more plausibly due to fishing. Our study highlights that information on temporal trends in absolute abundance is needed to assess whether the goals of protected areas are being met: in our study, patterns in absolute abundance across ten years of surveys revealed trends that simple ratios of abundance did not.

Using individual-based movement information to identify spatial conservation priorities for mobile species.

The optimal design of reserve networks and fisheries closures depends on species occurrence information and knowledge of how anthropogenic impacts interact with the species concerned. However, challenges in surveying mobile and cryptic species over adequate spatial and temporal scales can mask the importance of particular habitats, leading to uncertainty about which areas to protect to optimize conservation efforts. We investigated how telemetry-derived locations can help guide the scale and timing of fisheries closures with the aim of reducing threatened species bycatch. Forty juvenile speartooth sharks (Glyphis glyphis) were monitored over 22 months with implanted acoustic transmitters and an array of hydrophone receivers. Using the decision-support tool Marxan, we formulated a permanent fisheries closure that prioritized areas used more frequently by tagged sharks and considered areas perceived as having high value to fisheries. To explore how the size of the permanent closure compared with an alternative set of time-area closures (i.e., where different areas were closed to fishing at different times of year), we used a cluster analysis to group months that had similar arrangements of selected planning units (informed by shark movements during that month) into 2 time-area closures. Sharks were consistent in their timing and direction of migratory movements, but the number of tagged sharks made a big difference in the placement of the permanent closure; 30 individuals were needed to capture behavioral heterogeneity. The dry-season (May-January) and wet-season (February-April) time-area closures opened 20% and 25% more planning units to fishing, respectively, compared with the permanent closure with boundaries fixed in space and time. Our results show that telemetry has the potential to inform and improve spatial management of mobile species and that the temporal component of tracking data can be incorporated into prioritizations to reduce possible impacts of spatial closures on established fisheries.

Uso de Información de Movimiento Basada en Individuos para Identificar las Prioridades de Conservación Espacial para las Especies Móviles Resumen El diseño óptimo de redes de reservas y los cierres de pesquerías depende de la información sobre la presencia de especies y del conocimiento sobre cómo los impactos antropogénicos interactúan con las especies afectadas. Sin embargo, las dificultades que existen al monitorear especies móviles y crípticas en escalas espaciales y temporales adecuadas pueden enmascarar la importancia de los hábitats particulares, lo que resulta en incertidumbre con respecto a cuáles áreas proteger para optimizar los esfuerzos de conservación. Investigamos cómo las ubicaciones derivadas de la telemetría pueden ayudar a guiar la escala y el momento justo del cierre de las pesquerías con el objetivo de reducir la captura accesoria de especies amenazadas. Se monitorearon 40 tiburones lanza juveniles (Glyphis glyphis) durante 22 meses con transmisores acústicos implantados y una selección de receptores hidrofónicos. Con la herramienta de apoyo para la toma de decisiones Marxan, formulamos un cierre de pesquerías permanente que priorizó las áreas usadas con frecuencia por los tiburones marcados y que consideraba a las áreas percibidas como altamente valiosas para las pesquerías. Para explorar cómo el tamaño del cierre permanente se comparaba con un conjunto de cierres con áreas y tiempos alternativos (es decir, donde las áreas se cerraron a la pesca en diferentes momentos del año) usamos un análisis de clúster para agrupar los meses que tuvieron arreglos similares a las unidades de planeación seleccionadas (informadas por el movimiento de los tiburones durante ese mes) en dos cierres de tiempo-área. Los tiburones fueron consistentes en el tiempo y dirección de sus movimientos migratorios, pero el número de tiburones marcados generó una gran diferencia en la ubicación del cierre permanente; se necesitaron 30 individuos para capturar la heterogeneidad del comportamiento. Los cierres de tiempo-área de la temporada de secas (mayo - enero) y la de lluvias (febrero - abril) abrieron a la pesca un 20% y 25% más de unidades de planeación, respectivamente, en comparación con el cierre permanente con barreras fijas en el tiempo y el espacio. Nuestros resultados muestran que la telemetría tiene el potencial para informar y mejorar el manejo espacial de las especies móviles y que el componente temporal de los datos de rastreo puede ser incorporado a las priorizaciones para reducir los posibles impactos del manejo sobre las pesquerías establecidas.

Inferring contemporary and historical genetic connectivity from juveniles.

Measuring population connectivity is a critical task in conservation biology. While genetic markers can provide reliable long-term historical estimates of population connectivity, scientists are still limited in their ability to determine contemporary patterns of gene flow, the most practical time frame for management. Here, we tackled this issue by developing a new approach that only requires juvenile sampling at a single time period. To demonstrate the usefulness of our method, we used the Speartooth shark (Glyphis glyphis), a critically endangered species of river shark found only in tropical northern Australia and southern Papua New Guinea. Contemporary adult and juvenile shark movements, estimated with the spatial distribution of kin pairs across and within three river systems, was contrasted with historical long-term connectivity patterns, estimated from mitogenomes and genome-wide SNP data. We found strong support for river fidelity in juveniles with the within-cohort relationship analysis. Male breeding movements were highlighted with the cross-cohort relationship analysis, and female reproductive philopatry to the river systems was revealed by the mitogenomic analysis. We show that accounting for juvenile river fidelity and female philopatry is important in population structure analysis and that targeted sampling in nurseries and juvenile aggregations should be included in the genomic toolbox of threatened species management.

Complete mitochondrial genome of the Endangered Narrow Sawfish Anoxypristis cuspidata (Rajiformes: Pristidae).

In this study, we describe the first complete mitochondrial sequence for the Endangered Narrow Sawfish Anoxypristis cuspidata. It is 17,243 bp in length and contains 13 protein-coding genes, two rRNA genes, 22 tRNA genes, and a control region with the common vertebrate mitogenomic organization. A total of 30 bp overlaps and 28 bp short intergenic spaces are located between all genes. The overall base composition is 32.7% A, 25.7% C, 12.9% G, and 28.6% T. Two start codons (ATG and GTG) and two stop codons (TAG and TAA/T) were used in all protein-coding genes. The origin of L-strand replication (OL) sequence (38 bp) formed a hairpin structure (13 bp stem and 12 bp loop) to initiate the replication of L-strand.

Complete mitogenome of the Graceful Shark Carcharhinus amblyrhynchoides (Carcharhiniformes: Carcharhinidae).

In this manuscript we describe the first complete mitochondrial sequence for the Near Threatened Graceful Shark Carcharhinus amblyrhynchoides. It is 16,705 bp in length, consists of two rRNA genes, 22 tRNA genes, 13 protein-coding genes and one control region with the typical gene arrangement pattern and translate orientation in vertebrates. The overall base composition is 31.4% A, 25.1% C, 13.2% G and 30.3% T. The shortest tRNA-Ser2 cannot fold into a clover-leaf secondary structure due to the lack of the dihydrouridine arm.

Mitogenomics of the Speartooth Shark challenges ten years of control region sequencing.

BACKGROUND: Mitochondrial DNA markers have long been used to identify population boundaries and are now a standard tool in conservation biology. In elasmobranchs, evolutionary rates of mitochondrial genes are low and variation between distinct populations can be hard to detect with commonly used control region sequencing or other single gene approaches. In this study we sequenced the whole mitogenome of 93 Critically Endangered Speartooth Shark Glyphis glyphis from the last three river drainages they inhabit in northern Australia. RESULTS: Genetic diversity was extremely low (π =0.00019) but sufficient to demonstrate the existence of barriers to gene flow among river drainages (AMOVA Φ ST =0.28283, P <0.00001). Surprisingly, the comparison with single gene sub-datasets revealed that ND5 and 12S were the only ones carrying enough information to detect similar levels of genetic structure. The control region exhibited only one mutation, which was not sufficient to detect any structure among river drainages. CONCLUSIONS: This study strongly supports the use of single river drainages as discrete management units for the conservation of G. glyphis. Furthermore when genetic diversity is low, as is often the case in elasmobranchs, our results demonstrate a clear advantage of using the whole mitogenome to inform population structure compared to single gene approaches. More specifically, this study questions the extensive use of the control region as the preferential marker for elasmobranch population genetic studies and whole mitogenome sequencing will probably uncover a large amount of cryptic population structure in future studies.

Multi year observations reveal variability in residence of a tropical Demersal Fish, Lethrinus nebulosus: implications for spatial management.

Off the Ningaloo coast of North West Western Australia, Spangled Emperor Lethrinus nebulosus are among the most highly targeted recreational fish species. The Ningaloo Reef Marine Park comprises an area of 4,566 km2 of which 34% is protected from fishing by 18 no-take sanctuary zones ranging in size from 0.08-44.8 km2. To better understand Spangled Emperor movements and the adequacy of sanctuary zones within the Ningaloo Reef Marine Park for this species, 84 Spangled Emperor of a broad spectrum of maturity and sex were tagged using internal acoustic tags in a range of lagoon and reef slope habitats both inside and adjacent to the Mangrove Bay Sanctuary zone. Kernel Utilisation Distribution (KUD) was calculated for 39 resident individuals that were detected for more than 30 days. There was no relationship with fish size and movement or site fidelity. Average home range (95% KUD) for residents was 8.5±0.5 km2 compared to average sanctuary zone size of 30 km2. Calculated home range was stable over time resulting in resident animals tagged inside the sanctuary zone spending ∼80% of time within the sanctuary boundaries. The number of fish remaining within the array of receivers declined steadily over time and after one year more than 60% of tagged fish had moved outside the sanctuary zone and also beyond the 28 km2 array of receivers. Long term monitoring identified the importance of shifting home range and was essential for understanding overall residency within protected areas and also for identifying spawning related movements. This study indicates that despite exhibiting stable and small home ranges over periods of one to two years, more than half the population of spangled emperor move at scales greater than average sanctuary size within the Ningaloo Reef Marine Park.

The effects of freshwater to seawater transfer on circulating levels of angiotensin II, C-type natriuretic peptide and arginine vasotocin in the euryhaline elasmobranch, Carcharhinus leucas.

This study examined the effect of transfer to increased environmental salinity on the circulating levels of angiotensin II (ANG II), C-type natriuretic peptide (CNP), and arginine vasotocin (AVT) in the euryhaline elasmobranch, Carcharhinus leucas. Plasma levels of ANG II and CNP were significantly increased in C. leucas chronically acclimated to seawater (SW) in comparison to freshwater (FW) acclimated fish. There was no difference in plasma AVT levels. Acute transfer of FW fish to 75% SW induced an increase in plasma ANG II levels within 12 h, and subsequent transfer from 75 to 100% SW further increased plasma ANG II levels at both 24 and 72 h. No change in plasma CNP was observed during acute transfer to increased salinity. However, a significant increase in plasma AVT levels was observed following 96 h in 75% SW and 24 h in 100% SW. In chronically SW acclimated C. leucas plasma osmolality, sodium, chloride, and urea were all significantly higher than FW acclimated fish but there was no difference in haematocrit. Acute transfer of C. leucas to 75% SW induced a significant increase in plasma osmolality, sodium and urea concentrations within 96 h of transfer. Subsequent transfer from 75 to 100% SW induced a further increase in these variables within 24 h in addition to a significant increase in plasma chloride above control levels. Haematocrit did not differ between the experimental and control groups throughout the acute study. Circulating levels of ANG II were significantly correlated to plasma, sodium, chloride, and urea concentrations during acclimation to SW. Conversely, circulating levels of CNP and AVT did not correlate to plasma osmolytes, however, CNP was significantly correlated to haematocrit during acclimation to seawater.

Hepatic urea biosynthesis in the euryhaline elasmobranch Carcharhinus leucas.

Plasma urea levels and hepatic urea production in the euryhaline bull shark, Carcharhinus leucas, acclimated to freshwater and seawater environments were measured. It was found that plasma urea concentration increased with salinity and that this increase was, in part, the result of a significant increase in hepatic production of urea. This study provides direct evidence that hepatic production of urea plays an important role in the osmoregulatory strategy of C. leucas.

Sequence, circulating levels, and expression of C-type natriuretic peptide in a euryhaline elasmobranch, Carcharhinus leucas.

The present study has examined expression and circulating levels of C-type natriuretic peptide (CNP) in the euryhaline bull shark, Carcharhinus leucas. Complementary DNA and deduced amino acid sequence for CNP in C. leucas were determined by RACE methods. Homology of CNP amino acid sequence in C. leucas was high both for proCNP and for mature CNP when compared with previously identified elasmobranch CNPs. Mature CNP sequence in C. leucas was identical to that in Triakis scyllia and Scyliorhinus canicula. Levels of expression of CNP mRNA were significantly decreased in the atrium but did not change in either the brain or ventricle following acclimation to a SW environment. However, circulating levels of CNP significantly increased from 86.0+/-7.9 fmol ml(-1) in FW to 144.9+/-19.5 fmol ml(-1) in SW. The results presented demonstrate that changes in environmental salinity influences both synthesis of CNP from the heart and also circulating levels in C. leucas. Potential stimulus for release and modes of action are discussed.

Freshwater to seawater acclimation of juvenile bull sharks (Carcharhinus leucas): plasma osmolytes and Na+/K+-ATPase activity in gill, rectal gland, kidney and intestine.

This study examined the osmoregulatory status of the euryhaline elasmobranch Carcharhinus leucas acclimated to freshwater (FW) and seawater (SW). Juvenile C. leucas captured in FW (3 mOsm l(-1) kg(-1)) were acclimated to SW (980-1,000 mOsm l(-1) kg(-1)) over 16 days. A FW group was maintained in captivity over a similar time period. In FW, bull sharks were hyper-osmotic regulators, having a plasma osmolarity of 595 mOsm l(-1) kg(-1). In SW, bull sharks had significantly higher plasma osmolarities (940 mOsm l(-1) kg(-1)) than FW-acclimated animals and were slightly hypo-osmotic to the environment. Plasma Na(+), Cl(-), K(+), Mg(2+), Ca(2+), urea and trimethylamine oxide (TMAO) concentrations were all significantly higher in bull sharks acclimated to SW, with urea and TMAO showing the greatest increase. Gill, rectal gland, kidney and intestinal tissue were taken from animals acclimated to FW and SW and analysed for maximal Na(+)/K(+)-ATPase activity. Na(+)/K(+)-ATPase activity in the gills and intestine was less than 1 mmol Pi mg(-1) protein h(-1) and there was no difference in activity between FW- and SW-acclimated animals. In contrast Na(+)/K(+)-ATPase activity in the rectal gland and kidney were significantly higher than gill and intestine and showed significant differences between the FW- and SW-acclimated groups. In FW and SW, rectal gland Na(+)/K(+)-ATPase activity was 5.6+/-0.8 and 9.2+/-0.6 mmol Pi mg(-1) protein h(-1), respectively. Na(+)/K(+)-ATPase activity in the kidney of FW and SW acclimated animals was 8.4+/-1.1 and 3.3+/-1.1 Pi mg(-1) protein h(-1), respectively. Thus juvenile bull sharks have the osmoregulatory plasticity to acclimate to SW; their preference for the upper reaches of rivers where salinity is low is therefore likely to be for predator avoidance and/or increased food abundance rather than because of a physiological constraint.

Plasma osmolyte concentrations and rectal gland mass of bull sharks Carcharhinus leucas, captured along a salinity gradient.

Bull sharks (Carcharhinus leucas) were captured across a salinity gradient from freshwater (FW) to seawater (SW). Across all salinities, C. leucas were hyperosmotic to the environment. Plasma osmolarity in FW-captured animals (642 +/- 7 mosM) was significantly reduced compared to SW-captured animals (1067 +/- 21 mosM). In FW animals, sodium, chloride and urea were 208 +/- 3, 203 +/- 3 and 192 +/- 2 mmol l(-1), respectively. Plasma sodium, chloride and urea in SW-captured C. leucas were 289 +/- 3, 296 +/- 6 and 370 +/- 10 mmol l(-1), respectively. The increase in plasma osmolarity between FW and SW was not linear. Between FW (3 mosM) and 24 per thousand SW (676 mosM), plasma osmolarity increased by 22% or 0.92% per 1 per thousand rise in salinity. Between 24 per thousand and 33 per thousand, plasma osmolarity increased by 33% or 4.7% per 1 per thousand rise in salinity, largely due to a sharp increase in plasma urea between 28 per thousand and 33 per thousand. C. leucas moving between FW and SW appear to be faced with three major osmoregulatory challenges, these occur between 0-10 per thousand, 11-20 per thousand and 21-33 per thousand. A comparison between C. leucas captured in FW and estuarine environments (20-28 per thousand ) in the Brisbane River revealed no difference in the mass of rectal glands between these animals. However, a comparison of rectal gland mass between FW animals captured in the Brisbane River and Rio San Juan/Lake Nicaragua showed that animals in the latter system had a significantly smaller rectal gland mass at a given length than animals in the Brisbane River. The physiological challenges and mechanisms required for C. leucas moving between FW and SW, as well as the ecological implications of these data are discussed.