Calcitonin-like immunoreactivity in serum and tissues of the bonnethead shark,Sphyrna tiburo

Calcitonin is a 32-amino acid peptide hormone that is best known for its actions in maintaining skeletal integrity and calcium homeostasis in mammals. Calcitonin also appears to function in regulating certain aspects of animal reproduction, but the nature of this role remains unclear, particularly in nonmammalian vertebrates. The present study investigated the relationship between calcitonin and reproduction in the bonnethead shark (Sphyrna tiburo), a well-studied member of the oldest living vertebrate group (i.e. elasmobranchs) known to possess a calcitonin-producing organ. Serum calcitonin concentrations were measured in 28 reproductively mature female S. tiburo using a heterologous enzyme-linked immunosorbent assay (ELISA) system. Sites of calcitonin immunoreactivity were detected in tissues of mature female and embryonic S. tiburo using immunocytochemistry. Significant increases in serum calcitonin concentrations of mature female S. tiburo occurred during early stages of gestation, a period characterized by yolk-dependency of developing embryos. Immunoreactive calcitonin was detected in the duodenum and pancreas of embryonic S. tiburo sampled during the same period. The results from this study suggest that calcitonin obtained from endogenous and/or maternal sources may function in regulating yolk digestion in embryonic S. tiburo. Therefore, the association between calcitonin and reproduction in elasmobranchs may reflect an important role for this hormone in embryonic development.     

Placentation and associated aspects of gestation in the bonnethead shark, Sphyrna tiburo

The left ovary of the bonnethead shark, Sphyrna tiburo, is rudimentary, and the right ovary supplies both oviducts which share a common ostium situated in the falciform ligament. Preceding ovulation the nidamental gland of each oviduct hypertrophies and the caudal two-thirds of each oviduct is modified to form a uterus. In the Florida-Caribbean area Sphyrna tiburo probably mates in March and 3–7 eggs are fertilized in the vicinity of the nidamental gland of each oviduct. The developing embryo is nourished during the first 3–4 months of gestation by yolk stored in its extensive yolk sac. Approximately three and one-half months after fertilization, the distal portion of the yolk sac becomes convoluted and interdigitates with deep folds in the uterine wall to form a yolk-sac placenta. As the placenta develops, the maternal uterine epithelium is reduced from columnar cells to squamous cells, and the foetal yolk-sac epithelium is reduced from columnar and cuboidal cells to squamous cells. Exchange between the maternal and foetal blood systems takes place through maternal endothelium, reduced maternal epithelium, egg-case membrane, reduced foetal epithelium, and foetal endothelium.     

Geographic variation in reproduction between two populations of the bonnethead shark,Sphyrna tiburo

Synopsis A study of two populations of the bonnethead shark,Sphyrna tiburo was conducted in Florida Bay and Tampa Bay, Florida from September 1982 to December 1986. The maintenance of sharks in captivity at the Marine Science and Conservation Center in the Florida Keys, and the collection of sharks from widely separated geographical areas allowed the examination of latitudinal variation in reproduction. Several reproductive parameters were found to differ: (1) size at maturation, (2) age at maturation, (3) time of fertilization, (4) rate of embryonic development, (5) size at birth, (6) the energetic investment in producing offspring, (7) gestation period, and (8) the incidence of infertility. Average litter size and maximum age of females was not different between the two populations. These contrasting life history parameters are not easily explained. Food limitation and seasonal differences between the two areas are considered as factors controlling reproduction in these populations. It is noteworthy that the average size of adult females in Tampa Bay is significantly greater than that of Florida Keys sharks. This size difference may be important in explaining the observed differences in reproduction.     

The sexually dimorphic cephalofoil of bonnethead sharks, Sphyrna tiburo

Sexually dimorphic head shape is common in vertebrates from teleosts to mammals. Herein we document that cephalic sexual dimorphism is also found in the cartilaginous fishes (Chondrichthyes). Male bonnethead sharks develop a prominent bulge along the anterior margin of the cephalofoil at the onset of sexual maturity. This contrasts with the uniformly rounded anterior margin of adult females and juveniles and embryos of both sexes. The anterior cephalic bulge is produced by elongation of the rod-like rostral cartilages, and its appearance corresponds temporally with the elongation of the rod-like cartilages of the male intromittent organs (claspers). We propose that the rostral cartilage elongation is a byproduct of endocrinological changes at the onset of sexual maturity that stimulate growth of the clasper cartilages. The basal location of the chondrichthyan fishes within the vertebrate clade extends the earliest appearance of cephalic sexual dimorphism among the vertebrates.     

Residency and movement patterns of bonnethead sharks, Sphyrna tiburo, in a large Florida estuary

The use of a coastal estuary by bonnethead sharks, Sphyrna tiburo, was examined by acoustic monitoring, gillnet sampling and tag- recapture studies. Acoustic monitoring data were used to define the residency and movement patterns of sharks within Pine Island Sound, Charlotte Harbor, Florida. Sharks were monitored for periods of 1–173 days with individuals regularly moving in and out of the detection range of the acoustic system. Patterns of movement could not be correlated with tidal level or time of day. Home range sizes within the Pine Island Sound population were typically small with individuals using core areas on a daily basis. However, core areas shifted within the study site over time resulting in eventual usage of most of the available habitat. Gillnet sampling revealed that S. tiburo were abundant in shallow water near seagrass beds, but that presence of individuals at specific sites was variable. Tag-recapture data showed that most individuals remained within the Pine Island Sound region over time and did not appear to undergo long coastal migrations. The movement and residence patterns of S. tiburo suggest that individuals are resident within the estuary, but do not show site fidelity to specific areas within the estuary.     

Serum steroid hormones including 11-ketotestosterone, 11-ketoandrostenedione, and dihydroprogesterone in juvenile and adult bonnethead sharks, Sphyrna tiburo.

Previous studies in the placental viviparous bonnethead shark, Sphyrna tiburo, have correlated 17 beta-estradiol, progesterone, testosterone, and dihydrotestosterone with reproductive events in both males and females. However, several key reproductive events, including implantation, maintenance of pregnancy, and parturition, did not correlate with these four steroid hormones. Therefore, the present study investigated three steroid hormones, 11-ketotestosterone, 11-ketoandrostenedione, and dihydroprogesterone, which have demonstrably important roles in the reproductive cycles of teleosts. It was hypothesized that one or more of these three hormones would correlate with specific reproductive events in S. tiburo. Concurrently, developmental (growth and/or maturation) analyses of these three steroids plus 17 beta-estradiol, progesterone, testosterone, and dihydrotestosterone were investigated in juvenile bonnethead sharks. Serum dihydroprogesterone concentrations were highest in mature females and 11-ketotestosterone concentrations were highest in mature males. In mature females, 11-ketoandrostenedione levels were elevated from the time of mating, through six months of sperm storage and another four months of gestation. At parturition concentrations became significantly lower and remained lower until mating occurred again in another two to three months. Serum 11-ketotestosterone concentrations were the highest at implantation though not significant. In mature males, significantly elevated serum levels of dihydroprogesterone occurred in April and May, near the start of annual testicular development. During growth in males, testosterone and dihydrotestosterone increased progressively and in females, testosterone increased progressively. At maturity in males, significant increases occurred in testosterone and 11-ketotestosterone concentrations while, in females, dihydroprogesterone, 11-ketotestosterone, 17 beta-estradiol, progesterone, testosterone, and dihydrotestosterone concentrations increased. This study shows that although testosterone may be the primary androgen in the bonnethead shark, other derived androgens may have important functions in growth, maturation, and reproduction. J. Exp. Zool. 284:595-603, 1999.     

Gill lesions associated with Erpocotyle tiburonis (Monogenea: Hexabothriidae) on wild and aquarium-held bonnethead sharks (Sphyrna tiburo).

Gill lesions associated with infections of Erpocotyle tiburonis (Brooks, 1934) (Monogenea: Hexabothriidae) on wild bonnethead sharks (Sphyrna tiburo (L., 1758) (Carcharhiniformes: Sphyrinidae)) were compared with those on aquarium-held ones using light and scanning electron microscopy. Uninfected gill filaments had slender, triangular, smooth-surfaced lamellae and interlamellar water channels that were approximately equal in size. Four wild sharks were each infected by 3-11 widely separated adult E. tiburonis, and 1 of these sharks hosted a juvenile specimen. Lamellae flanking or touching adult E. tiburonis were pushed aside or bent, but were otherwise identical to those of uninfected filaments. Two aquarium-held sharks were each infected by hundreds of juvenile and adult E. tiburonis. In these sharks, lamellae near juveniles were pushed apart or bent, but were otherwise normal, whereas a thick, ragged-surfaced layer of hyperplastic epithelium both filled interlamellar water channels and partially or completely covered lamellae near adults. Results of this study suggest that the intense infections of E. tiburonis were facilitated by captivity and caused severe hyperplastic lesions that ultimately led to the death of the sharks by reducing or blocking the respiratory water flow over lamellae and thus reducing the exchange of gases and ions across the lamellar epithelium. In contrast, the wild sharks were infected by fewer worms and exhibited relatively minor lesions.     

The determinants of sexual segregation in the scalloped hammerhead shark,Sphyrna lewini

Synopsis Female scalloped hammerhead sharks move offshore at a smaller size than do males to form schools composed primarily of intermediate size female sharks. This movement results in smaller females feeding more on pelagic prey than do males and with greater predatory success. It is contended that this change in habitat causes females to grow more rapidly to reproductive size. Intermediate size females grow at a more rapid rate than males. Female scalloped hammerhead sharks mature at a size larger than males. For many elasmobranch species, females: (1) occupy a different habitat, (2) grow more rapidly prior to maturity and continue growth following maturation, (3) feed on different prey with increased feeding success, and (4) reproduce at a size larger than males. It is suggested that female segregation increases fitness, resulting in more rapid growth for the former sex. The females reach maturity at the larger size necessary to support embryonic young, yet similar age to males, matching the female reproductive lifetime to that of males.     

New microsatellite loci for the endangered scalloped hammerhead shark, Sphyrna lewini

We isolated 15 microsatellite markers for the scalloped hammerhead shark, Sphyrna lewini. Loci were tested on 80 specimens of S. lewini from four Eastern Pacific samples. The number of alleles per locus ranged from 6 to 31 (mean = 14). Observed and expected levels of heterozygosity per locus ranged from 0.39 to 0.91 (mean = 0.70) and from 0.54 to 0.90 (mean = 0.76), respectively. No pairs of loci were in gametic disequilibrium after Bonferroni correction of α. One locus showed significantly lower heterozygosity than expected under Hardy–Weinberg proportions in two populations, possibly caused by null alleles.     

Assessment of acid-base derangements among bonnethead (Sphyrna tiburo), bull (Carcharhinus leucas), and lemon (Negaprion brevirostris) sharks from gillnet and longline capture and handling methods

Blood gasses of wild bonnethead, bull, and lemon sharks were measured with the i-STAT clinical analyzer with the CG4+ cartridge immediately after capture; and again immediately prior to release after tagging, handling and morphometric measurements were taken. Relative reference ranges of post-capture status were established. Among species, stress response to capture was similar for all parameters; however, pH declined and lactate concentrations rose over time, indicating continued insult from capture and/or response to additional handling stress. pCO(2) rose faster for S. tiburo than for C. leucas, and lactate concentrations rose faster for S. tiburo than for N. brevirostris. All species caught in gillnets experienced lower pH and higher lactate concentrations than on longlines. Discriminant analysis justified the use of blood gas analysis to assess physiological stress induced by different capture methods. From these results, we recommend 1) that gear be monitored closely and sharks be removed immediately, or suboptimally, that gear is deployed for the shortest soak time possible; 2) longline over gillnet gear; and 3) extra caution with sensitive species (e.g., S. tiburo), which may include the administration of blood buffers and other therapeutics if a shark is beyond the limits of relative reference ranges reported here.     

Baseline health parameters and species comparisons among free-ranging Atlantic sharpnose (Rhizoprionodon terraenovae), bonnethead (Sphyrna tiburo), and spiny dogfish (Squalus acanthias) sharks in Georgia, Florida, and Washington, USA

Sharks are of commercial, research, conservation, and exhibition importance but we know little regarding health parameters and population status for many species. Here we present health indicators and species comparisons for adults of three common wild-caught species: 30 Atlantic sharpnose sharks (Rhizoprionodon terraenovae) and 31 bonnethead sharks (Sphyrna tiburo) from the western Atlantic, and 30 spiny dogfish sharks (Squalus acanthias) from the eastern Pacific. All animals were captured during June-July 2009 and 2010. Median values and preliminary reference intervals were calculated for hematology, plasma biochemistry, trace nutrients, and vitamin A, E, and D concentrations. Significant differences, attributable to physiologic differences among the species, were found in the basic hematologic and plasma biochemistry variables. Significant species differences in arsenic and selenium plasma concentrations were found and appear to coincide with diet and habitat variability among these three species. Vitamin E was significantly higher in the bonnethead shark, again related to the foraging ecology and ingestion of plant material by this species. The Atlantic sharpnose had significantly higher vitamin A concentrations, supported by the higher proportion of teleosts in the diet. Vitamin D was below the limit of quantification in all three species. These preliminary reference intervals for health variables can be used to assess and monitor the population health and serve as indicators of nutritional status in these populations of wild elasmobranchs.     

Global phylogeography of the scalloped hammerhead shark (Sphyrna lewini)

Large marine fishes typically have little population genetic structure. The exceptions are associated with sedentary behaviour, disjunct distributions, or reproductive philopatry. Scalloped hammerhead sharks (Sphyrna lewini) incorporate the contrasting traits of oceanic habitat (usually associated with high dispersal) and possible fidelity to nursery grounds (for reproductive females). To evaluate the expectations of these contrasting behaviours, we examined the global genetic structure of S. lewini based on collections (n = 271 individuals) from 20 nursery areas. A 548-bp fragment of mitochondrial DNA control region revealed 22 polymorphic sites, 24 haplotypes, and three lineages distinguished by 2.56-3.77% sequence divergence. Coalescence analyses based on a provisional molecular clock indicate an origin in the Indo-West Pacific with late Pleistocene radiations into the central Pacific (Hawaii) and eastern Pacific (Central America), as well as recent interchange between oceans via southern Africa. Population subdivisions are strong (overall Phi(ST) = 0.749, P < 0.0001 and among oceans Phi(ST) = 0.598, P < 0.0098). Genetic discontinuity within oceans (Phi(ST) = 0.519, P < 0.0001) is primarily associated with oceanic barriers (migration across oceans M approximately 0), with much less structure along continental margins (M > 10). We conclude that nursery populations linked by continuous coastline have high connectivity, but that oceanic dispersal by females is rare. Although we cannot rule out philopatry to natal nurseries, oceanic barriers appear to have a much stronger influence on the genetic architecture of this species and may indicate a mechanism for recent evolutionary radiations in the genus Sphyrna.     

Demographic processes underlying subtle patterns of population structure in the scalloped hammerhead shark, Sphyrna lewini

Genetic diversity (θ), effective population size (N(e)), and contemporary levels of gene flow are important parameters to estimate for species of conservation concern, such as the globally endangered scalloped hammerhead shark, Sphyrna lewini. Therefore, we have reconstructed the demographic history of S. lewini across its Eastern Pacific (EP) range by applying classical and coalescent population genetic methods to a combination of 15 microsatellite loci and mtDNA control region sequences. In addition to significant population genetic structure and isolation-by-distance among seven coastal sites between central Mexico and Ecuador, the analyses revealed that all populations have experienced a bottleneck and that all current values of θ are at least an order of magnitude smaller than ancestral θ, indicating large decreases in N(e) (θ = 4N(e)μ), where μ is the mutation rate. Application of the isolation-with-migration (IM) model showed modest but significant genetic connectivity between most sampled sites (point estimates of Nm = 0.1-16.7), with divergence times (t) among all populations significantly greater than zero. Using a conservative (i.e., slow) fossil-based taxon-specific phylogenetic calibration for mtDNA mutation rates, posterior probability distributions (PPDs) for the onset of the decline in N(e) predate modern fishing in this region. The cause of decline over the last several thousand years is unknown but is highly atypical as a post-glacial demographic history. Regardless of the cause, our data and analyses suggest that S. lewini was far more abundant throughout the EP in the past than at present.     

Respiratory and hematological responses of the bonnethead shark, Sphyrna tiburo, to acute changes in dissolved oxygen

Behavioral and physiological responses to acute changes in dissolved oxygen were examined in the bonnethead shark, Sphyrna tiburo. In two sets of respirometry experiments, sharks were randomly exposed to seawater in oxygen contents of 6.0, 5.0, 4.0 and 3.0 mg l⁻¹. During exposure, bonnetheads increased mouth gape from 0.8 cm at 6.0 mg l⁻¹ to 2.2 cm at 3.0 mg l⁻¹, while ventilation volume increased from 0.61 to 5.28 l min⁻¹ kg⁻¹. Standard oxygen consumption remained unchanged (163-181 mg O₂ kg⁻¹ h⁻¹) throughout all treatments and was not significantly different. Utilization (%) declined from 52.3% at 6.0 mg l⁻¹ to 21.3% when oxygen levels reached 3.0 mg l⁻¹. Changes in oxygen content of ambient water also caused no significant change in either blood oxygen content or hematocrit. Using cellulose acetate electrophoresis, a single hemoglobin profile was identified at seawater of 6.0 mg l⁻¹ and hypoxic conditions. Results suggest bonnetheads are physiologically able to tolerate moderate levels of hypoxia.     

Characteristics of structure, composition, mass spectra, and iron release from the ferritin of shark liver (Sphyrna zygaena)

The ferritin consists of a protein shell constructed of 24 subunits and an iron core. The liver ferritin of Sphyrna zygaena (SZLF) purified by column chromatography is a protein composed of eight ferritins containing varying iron numbers ranging from 400+/-20 Fe3+/SZLF to 1890+/-20 Fe3+/SZLF within the protein shell. Nature SZLF (SZLFN) consisting of holoSZLF and SZLF with unsaturated iron (SZLFUI) to have been purified with polyacrylamide gel electrophoresis (PAGE) exhibited five ferritin bands with different pI values ranging from 4.0 to 7.0 in the gel slab of isoelectric focusing (IEF). HoloSZLF purified by PAGE (SZLFE) not only had 1890+/-20 Fe3+/SZLFE but also showed an identical size of iron core observed by transmission electron microscopy (TEM). Molecular weight of approximately 21 kDa for SZLFE subunit was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Four peaks of molecular ions at mass/charge (m/z) ratios of 10611.07, 21066.52, 41993.16, and 63555.64 that come from the SZLFE were determined by matrix-assisted laser desorption ionization/time-of-flight mass spectrometry (MALDI-TOF MS), which were identified as molecular ions of the ferritin subunit (M+) and its polymers, namely, [M]2+, [M]+, [2M]+, and [3M]+, respectively. Both SZLFE and a crude extract from shark liver of S. zygaena showed similar kinetic characteristics of complete iron release with biphasic behavior. In addition, a combined technique of visible spectrometry and column chromatography was used for studying ratio of phosphate to Fe3+ within the SZLFE core. Interestingly, this ratio maintained invariable even after the iron release, which differed from that of other mammal ferritins.     

New insights into the trophic ecology of the scalloped hammerhead shark,Sphyrna lewini,in the eastern tropical Pacific Ocean

Scalloped hammerhead shark, Sphyrna lewini, is considered a generalist predator, showing individual specializations and different trophic levels. This species inhabits both oceanic islands and coastal nursery areas in the eastern tropical Pacific (ETP) Ocean. Existing trophic ecology studies on S. lewini have been focused on stomach contents. This study provides new insights into its foraging preferences, habitat use, and trophic position, using stable isotope analysis (SIA) of muscle tissue. Stable isotope signatures of δ¹³C and δ¹⁵N were determined in S. lewini muscles (n?=?29) from the Ecuadorian Pacific in 2013. Trophic position (TP), isotopic niches, and overlap of S. lewini were estimated by sex, age, and maturity stages to infer their dietary habits throughout life stages. SIA revealed complex movement patterns related to sex and age classes of S. lewini, highlighting high degrees of dietary plasticity and habitat use, with a stronger relation to coastal regions than previously reported. This study provides crucial information regarding essential areas and the related migration behavior of S. lewini, with important implications for their conservation and management in the ETP.

     

Population genetic divergence as consequence of past range expansion of the smooth hammerhead shark Sphyrna zygaena

Hydrobiologia - We investigated the roles of a benthivorous (Prochilodus brevis, Steindachner 1875) and a planktivorous (Oreochromis niloticus, Linnaeus, 1758) fish in translocating phosphorus from...     

Structure, composition, and mechanical properties of shark teeth

The teeth of two different shark species (Isurus oxyrinchus and Galeocerdo cuvier) and a geological fluoroapatite single crystal were structurally and chemically characterized. In contrast to dentin, enameloid showed sharp diffraction peaks which indicated a high crystallinity of the enameloid. The lattice parameters of enameloid were close to those of the geological fluoroapatite single crystal. The inorganic part of shark teeth consisted of fluoroapatite with a fluoride content in the enameloid of 3.1 wt.%, i.e., close to the fluoride content of the geological fluoroapatite single crystal (3.64 wt.%). Scanning electron micrographs showed that the crystals in enameloid were highly ordered with a special topological orientation (perpendicular towards the outside surface and parallel towards the center). By thermogravimetry, water, organic matrix, and biomineral in dentin and enameloid of both shark species were determined. Dentin had a higher content of water, organic matrix, and carbonate than enameloid but contained less fluoride. Nanoindentation and Vicker's microhardness tests showed that the enameloid of the shark teeth was approximately six times harder than the dentin. The hardness of shark teeth and human teeth was comparable, both for dentin and enamel/enameloid. In contrast, the geological fluoroapatite single crystal was much harder than both kinds of teeth due to the absence of an organic matrix. In summary, the different biological functions of the shark teeth ("tearing" for Isurus and "cutting" for Galeocerdo) are controlled by the different geometry and not by the chemical or crystallographic composition.