Gastric evacuation in the sandbar shark, Carcharhinus plumbeus

Sandbar sharks maintained in an enclosure in the natural environment were fed meals of soft blue crab, Callinectes sapidus , or menhaden, Brevoortia lyrannus , and were sacrificed at intervals after feeding to measure rates of gastric emptying. Regression analysis was used to evaluate the adequacy of various models in describing the decrease in stomach contents with time after feeding. A Gompertz growth curve provided the best fit to the data for both food types. This model suggests that gastric emptying is characterized by an initial lag phase during which evacuation rate increases, a period of maximal evacuation, and a decreasing evacuation rate during the later stages of digestion. Passage of food from the stomach was slow relative to other species of fish studied. The time required to evacuate 98% of a meal was 92–3 and 70–7 h for sharks fed menhaden and blue crab respectively. Evacuation rates did not differ between day and night periods.     

Isolation and characterization of five dinucleotide microsatellite loci in the sandbar shark,Carcharhinus plumbeus

Five dinucleotide markers were isolated and optimized from a microsatellite‐enriched genomic library obtained from the sandbar shark, Carcharhinus plumbeus. Genotypic distributions of all markers were found to be in conformance with the expectations of Hardy–Weinberg equilibrium with four to 39 alleles present per locus. We amplified these loci in two female sharks and their litters. A maternal allele was recovered at each locus in all progeny indicating reliable amplification. More than two paternal alleles were recovered across both litters indicating genetic polyandry. Additionally, these markers were amplified across 10 carcharhiniform species to examine their utility in other studies.     

Rapid and Simultaneous Identification of Body Parts from the Morphologically Similar Sharks Carcharhinus obscurus and Carcharhinus plumbeus (Carcharhinidae) Using Multiplex PCR

Many commercially exploited carcharhinid sharks are difficult to identify to species owing to extensive morphological similarities. This problem is severely exacerbated when it comes to identifying detached shark fins, and the finless and headless shark carasses typically sold in markets. To assist in the acquisition of urgently needed conservation and management data on shark catch and trade, we have developed a highly streamlined approach based on multiplex polymerase chain reaction (PCR) that uses species-specific primers derived from nuclear ribosomal ITS2 sequences to achieve rapid species identification of shark body parts. Here we demonstrate the utility of this approach for identifying fins and flesh from two globally distributed, morphologically very similar carcharhinid sharks (Carcharhinus obscurus and Carcharhinus plumbeus) intensively targeted in fisheries worldwide, and often confused for each other even as whole animals. The assay is conducted in a 4-primer multiplex format that is structured to simultaneously achieve the following efficiency and cost-reduction objectives: it requires only a single-tube amplification reaction for species diagnosis, it incorporates an internal positive control to allow detection of false-negative results, and it is novel in that it allows species identification even when DNAs from two species are combined in the same tube during the PCR reaction. The latter innovation reduces the required effort for screening a set of unknown samples by 50%. The streamlined approach illustrated here should be amenable for use in a shark conservation and management context where large numbers of samples typically need to be screened; the approach shown may also provide a model for a rapid diagnostic method applicable to species identification in general. Received September 15, 2000; accepted December 15, 2000     

World phylogeography and male‐mediated gene flow in the sandbar shark,Carcharhinus plumbeus

The sandbar shark, Carcharhinus plumbeus, is a large, cosmopolitan, coastal species. Females are thought to show philopatry to nursery grounds while males potentially migrate long distances, creating an opportunity for male‐mediated gene flow that may lead to discordance in patterns revealed by mitochondrial DNA (mtDNA) and nuclear markers. While this dynamic has been investigated in elasmobranchs over small spatial scales, it has not been examined at a global level. We examined patterns of historical phylogeography and contemporary gene flow by genotyping 329 individuals from nine locations throughout the species’ range at eight nuclear microsatellite markers and sequencing the complete mtDNA control region. Pairwise comparisons often resulted in fixation indices and divergence estimates of greater magnitude using mtDNA sequence data than microsatellite data. In addition, multiple methods of estimation suggested fewer populations based on microsatellite loci than on mtDNA sequence data. Coalescent analyses suggest divergence and restricted migration among Hawaii, Taiwan, eastern and western Australia using mtDNA sequence data and no divergence and high migration rates, between Taiwan and both Australian sites using microsatellite data. Evidence of secondary contact was detected between several localities and appears to be discreet in time rather than continuous. Collectively, these data suggest complex spatial/temporal relationships between shark populations that may feature pulses of female dispersal and more continuous male‐mediated gene flow.     

Food habits and ontogenetic changes in the diet of the sandbar shark, Carcharhinus plumbeus, in Hawaii

The sandbar shark, Carcharhinus plumbeus, is a wide-ranging coastal species in tropical and temperate regions, and it is the most common species of shark in Hawaii, as in many locations where it occurs. Information on the diet and feeding habits of this species in the Pacific Ocean are extremely limited. For this study we quantified the diet of sandbar sharks in Hawaii based on records collected during the Hawaii Cooperative Shark Research and Control Program from 1967 to 1969. During this program a total of 565 stomachs were examined, of which 265 contained food. Sharks ranged in size from 59 to 190 cm total length. Teleosts were the most common prey group, but both cephalopods and crustaceans also occurred frequently. Ontogenetic changes in diet of sandbar sharks were apparent, with crustaceans forming a greater proportion of the diet of smaller sharks. Both cephalopods and elasmobranchs increased in importance with increasing shark size. Prey diversity also increased with size, with large, mobile, and reef prey species found more commonly in the diet of larger sharks. Mature male and female sharks appeared to segregate by depth, though major differences in the diet between the sexes were not apparent. However, there was some evidence of dietary differences between sharks caught in different depths and seasons. The results of this study suggest that sandbar sharks in Hawaii and throughout the world, are primarily piscivores, but also consume a variety of invertebrate prey, and that their diet varies with geographical location and stage of development.     

Clearance of a dermal Huffmanela sp. in a sandbar shark (Carcharhinus plumbeus) using levamisole

A wild-caught captive sandbar shark Carcharhinus plumbeus developed a contiguous network of darkly pigmented linear tracks that progressed from the snout to the ventral cervical region. Microscopic examination of a skin scraping revealed nematode eggs of the genus Huffmanela, a group of histozoic nematodes that is known to parasitize requiem sharks and marine and freshwater teleosts. The fresh eggs were darkly pigmented with bipolar plugs, contained a larva, and measured 73.3 to 86.4 by 39.0 to 47.4 microm (n = 10). Formalin-fixed and paraffin-embedded eggs were significantly smaller (Wilcoxon rank sums test, p < 0.005), measuring 70.5 to 78.9 by 33.6 to 41.3 microm (n = 13). These measurements do not correlate with previously reported species of Huffmanela. Serial treatment with levamisole (10 mg kg(-1), intramuscular [i.m.]) cleared the egg tracks within 21 d, with no recurrence or apparent complications.     

Age and growth of the sandbar shark, Carcharhinus plumbeus, in Hawaiian waters through vertebral analysis

Age and growth estimates were determined for the sandbar shark, Carcharhinus plumbeus, from Oahu, Hawaii in the central Pacific Ocean. Age estimates were obtained through vertebral centra analysis of 187 sharks. We verified our age estimates through marginal increment analysis of centra and oxytetracycline marking methods of at liberty sandbar sharks. Sizes of sampled sharks ranged from 46 to 147 cm pre-caudal length. Four growth models were fitted to length-at-age data; two forms of the von Bertalanffy growth model, the Gompertz growth model, and a logistic growth model. Males and females exhibited statistically significant differences in growth, indicating that females grow slower and attain larger sizes than males. Growth parameter estimates revealed slower growth rates than previously estimated (based on captive specimens) for Hawaiian sandbar sharks. The von Bertalanffy growth model using empirical length-at-birth provided the best biological and statistical fit to the data. This model gave parameter estimates of L _∞ = 138.5 cm PCL and k = 0.12 year⁻¹ for males and L _∞ = 152.8 cm PCL, k = 0.10 year⁻¹ for females. Male and female sandbar sharks mature at approximately 8 and 10 years of age, respectively.     

The early stages of mildew colony development on susceptible oats

The chronology of primary infection by Erysiphe graminis f. sp. avenae on detached leaves of a susceptible host, and growth patterns of the primary haustorium and secondary hyphae, proved similar to those of wheat and barley mildew found by other workers. The timing of formation and subsequent growth of the primary haustorium was not affected by the light regime, but the formation of subsequent haustoria was highly synchronous under an alternating dark/light cycle, and far less so under continuous illumination. Five days after inoculation almost twice as many secondary and tertiary haustoria were formed per colony under the dark/light treatment than under continuous light. Because of the synchronous formation of haustoria subsequent to the primary, haustoria selected at random from leaves of susceptible host cultivars showed a bimodal distribution in length, the less well developed tertiary haustoria being distinguished from earlier formed primary and secondary haustoria. There was also a significant positive correlation between length and the number of digitate processes/haustorium. The energy required to produce one secondary haustorium was calculated to be equivalent to that required to produce approximately 4–7 hyphal cells.     

Short-term Movements of Juvenile and Neonate Sandbar Sharks, Carcharhinus plumbeus, on their Nursery Grounds in Delaware Bay

We investigated short-term movements of neonate and juvenile sandbar sharks, Carcharhinus plumbeus, on their nursery grounds in Delaware Bay. The majority of sharks tracked limited their movements to water less than 5m deep, remained within 5km of the coastline, and occupied oblong activity spaces along the coast. In addition to site-attached coastal movements observed, several sharks moved entirely across Delaware Bay or spent considerable time in deeper portions of the central bay. Sharks tracked on the New Jersey side of the bay tended to spend more time in deeper water, farther from shore than sharks tracked on the Delaware side. Observation-area curves estimated that optimal tracking time for sandbar sharks in Delaware Bay was 41h. Indices of site attachment showed that movement patterns of tracked sandbar sharks varied from nomadic to home ranging. There was no significant difference in rate of movement for day/night, crepuscular periods, or between juveniles and neonates. In general, young sandbar sharks patrolled the coast and appeared to be site attached to some extent, but were capable of making longer excursions, including movement entirely across Delaware Bay.     

Purification of a novel heterodimer from shark (Carcharhinus plumbeus) serum by gel-immobilized metal chromatography.

1. Resolution of the fraction of sandbar shark (Carcharhinus plumbeus) serum that was soluble in 50% saturated ammonium sulfate by gel-immobilized metal-affinity chromatography allowed the isolation of a novel disulfide-bonded heterodimer of intact mass 70 kDa. 2. Following reduction, the molecule could be resolved into two chains of apparent mass 36 and 24 kDa. 3. The molecules were glycoproteins as determined by an observed reduction in molecular weight following enzymatic glycosylation. 4. The two separate chains were related to one another on the basis of amino-acid composition analysis and by comparison of the N-terminal amino acids (seven out of 10 identities). 5. The exact relationship of this molecule to characterized heterodimers of higher vertebrates is unknown. 6. Cross-linked agarose-acetate was synthesized and proved to be an efficient concentrating agent and also a hydrophobic interaction adsorbant.     

Genetic control of early stages of leaf development

The results of studies of genetic regulation of the early leaf morphogenesis, demarcation of the future primordium and transition of cells to determination, have been reviewed. The genetic systems of control of these developmental stages were shown to be conservative and hypotheses of possible mechanisms underlying the evolution of leaf morphology on their basis have been considered.     

The notch ligand delta-like 4 regulates multiple stages of early hemato-vascular development

Background

In mouse embryos, homozygous or heterozygous deletions of the gene encoding the Notch ligand Dll4 result in early embryonic death due to major defects in endothelial remodeling in the yolk sac and embryo. Considering the close developmental relationship between endothelial and hematopoietic cell lineages, which share a common mesoderm-derived precursor, the hemangioblast, and many key regulatory molecules, we investigated whether Dll4 is also involved in the regulation of early embryonic hematopoiesis.

Methodology/Principal Findings

Using Embryoid Bodies (EBs) derived from embryonic stem cells harboring hetero- or homozygous Dll4 deletions, we observed that EBs from both genotypes exhibit an abnormal endothelial remodeling in the vascular sprouts that arise late during EB differentiation, indicating that this in vitro system recapitulates the angiogenic phenotype of Dll4 mutant embryos. However, analysis of EB development at early time points revealed that the absence of Dll4 delays the emergence of mesoderm and severely reduces the number of blast-colony forming cells (BL-CFCs), the in vitro counterpart of the hemangioblast, and of endothelial cells. Analysis of colony forming units (CFU) in EBs and yolk sacs from Dll4^+/− and Dll4^−/− embryos, showed that primitive erythropoiesis is specifically affected by Dll4 insufficiency. In Dll4 mutant EBs, smooth muscle cells (SMCs) were seemingly unaffected and cardiomyocyte differentiation was increased, indicating that SMC specification is Dll4-independent while a normal dose of this Notch ligand is essential for the quantitative regulation of cardiomyogenesis.

Conclusions/Significance

This study highlights a previously unnoticed role for Dll4 in the quantitative regulation of early hemato-vascular precursors, further indicating that it is also involved on the timely emergence of mesoderm in early embryogenesis.     

The origin and development of individual size variation in early pelagic stages of fish

Size variation among individuals born at the same time in a common environment (within cohorts) is a common phenomenon in natural populations. Still, the mechanisms behind the development of such variation and its consequences for population processes are far from clear. We experimentally investigated the development of early within-cohort size variation in larval perch (Perca fluviatilis). Specifically we tested the influence of initial variation, resulting from variation in egg strand size, and intraspecific density for the development of size variation. Variation in egg strand size translated into variation in initial larval size and time of hatching, which, in turn, had effects on growth and development. Perch from the smallest egg strands performed on average equally well independent of density, whereas larvae originating from larger egg strands performed less well under high densities. We related this difference in density dependence to size asymmetries in competitive abilities leading to higher growth rates of groups consisting of initially small individuals under high resource limitation. In contrast, within a single group of larvae, smaller individuals grew substantially slower under high densities whereas large individuals performed equally well independent of density. As a result, size variation among individuals within groups (i.e. originating from the same clutch) increased under high densities. This result may be explained by social interactions or differential timing of diet shifts and a depressed resource base for the initially smaller individuals. It is concluded that to fully appreciate the effects of density-dependent processes on individual size variation and size-dependent growth, consumer feedbacks on resources need to be considered.     

Genetic polyandry and sexual conflict in the sandbar shark, Carcharhinus plumbeus, in the western North Atlantic and Gulf of Mexico

To investigate patterns of polyandry in the sandbar shark (Carcharhinus plumbeus), 20 pregnant females were sampled from the western North Atlantic and Gulf of Mexico. Five species-specific microsatellite markers were used to genotype each shark and its litter. Of 20 litters, 17 (85%) were shown to have multiple sires. In multiply sired litters, the estimated minimum number of sires ranged from two to five with an average of 2.3 males per litter. Regression analysis did not demonstrate a significant relationship between female reproductive success and female body size or sire number and female body size. There was a high incidence of reproductive skew noted in litters, and two groups of males with significantly different mean reproductive success were observed. Analyses using Bateman's principles suggest that there is less direct benefit for females that acquire multiple mates than for males who bias paternity within litters. In light of past morphological and behavioural studies, these data suggest that patterns of polyandry in elasmobranchs may be determined by coercive mating, and that breeding behaviour has likely evolved in the context of sexual conflict.     

Effective size closely approximates the census size in the heavily exploited western Atlantic population of the sandbar shark, Carcharhinus plumbeus

The sandbar shark, Carcharhinus plumbeus, is a long-lived species with low lifetime fecundity that is heavily fished in the western North Atlantic. Inshore nursery grounds increase survivorship of sandbar shark pups and the principal nurseries are in the mid-Atlantic region. We calculated effective number of breeders (N_b) and effective population size (N_e) for adults utilizing the nursery grounds of the Delaware Bay and the Eastern Shore of Virginia by genotyping 902 animals across five cohorts at eight microsatellite loci. Estimates of N_b and N_e were compared to estimates of census size (N_c) of cohorts obtained from Delaware Bay. The estimated N_e/N_c and N_b/N_c ratios were 0.45 or higher whether the Delaware Bay cohorts were considered as distinct year classes or combined. This is in contrast to estimated N_e/N_c ratios in other exploited marine fishes, which are several orders of magnitude smaller. Instead, the N_e/N_c ratio of sandbar sharks is similar to that found in marine and terrestrial mammals.     

The gradual establishment of cell commitments during the early stages of chick development

The early development of the chick (Stages I-XIII E.G&K) can be regarded as an ideal model for epigenetic development and for the study of the forces and factors involved in the establishment of cellular heterogeneity and the imprinting of polarity. It seems that a physical vectorial force is utilized to imprint upon the cleaving radial-symmetric multilayered blastodisc, a postero-anterior metabolic gradient, which is translated into the first morphogenetic phenomenon - a polarized cell shedding, causing the formation of the area pellucida. In a freshly laid egg, the Stage X blastoderm despite its radial appearance, has a concealed bilateral symmetry which is not susceptible any longer to spatial changes, but nevertheless is still labile and can be changed by several other drastic experimental procedures. This means that the individual cells are still pluripotential and not yet specifically committed. During the first 10 h of incubation a second polar morphogenetic process, the formation of Hyp, occurs following the same orientation of the previous cell shedding and thus stressing and enforcing the postero-anterior axis of bilateral symmetry. In the resulting double layered blastula, each layer separately expresses its own polar characteristics along the mutual axis of symmetry. Even at Stage XIII the Ep still remains a totipotential system which can regenerate a normal Hyp, and its polarity is probably in the form of a labile gradient field of competence for PS formation. The Hyp, on the other hand, has sorted out from the single layered Stage X blastoderm and is more specialized, as demonstrated by several developmental, metabolic, and immunologic criteria. Its only developmental potential is the polar ability to induce a PS in a competent Ep. The Hyp was found to contain two different cell populations, of which only one, of marginal zone origin, has the PS-inducing capacity. Even after the PS has started to form, the induction is still uncompleted and has to go on until the PS is more than half its full length. In order to render possible normal PS formation, at least one of the two layers has to retain its polarity. In this respect the polarity of the competent Ep seems to be more crucial than that of the inductive Hyp. Polarity and inductivity of the Hyp can be dissociated from each other, and a disorganized Hyp can still induce a normal PS as long as the Ep retains its polarity.     

Participation of ornithine decarboxylase in early stages of tomato fruit development

The apparent association of ornithine decarboxylase (ODC) with rapid cell proliferation in developing tomato (Lycopersicon esculentum Mill. cv. Pearson ms-35) fruits has been previously described. Further evidence is provided by the use of two ODC inhibitors, α-difluoromethylornithine (α-DFMO) and α-methylornithine (α-MO). Fruit development was inhibited by these inhibitors if applied during the period of intensive cell division. When applied in vitro, the two inhibitors were shown to inhibit the activity of ODC but not that of arginine decarboxylase (ADC). When applied in vivo, α-DFMO, a catalytic irreversible inhibitor, caused 97.1% reduction of ODC activity in the dialyzed extract from the treated ovaries, while it had no effect on ADC. On the other hand, α-MO, a reversible inhibitor, did not reduce the activity of these two enzymes in the dialyzed extracts when applied in vivo. The dialysis procedure probably removed α-MO from the enzyme fraction. Putrescine, the product of both ODC and ADC, alleviated the inhibition of fruit development but did not restore ODC activity to the control level. These results suggest that in the young developing tomato fruit, ODC is the enzyme responsible for the synthesis of putrescine, which is essential for the early stages of fruit development. The reduced activity of ODC elicited by putrescine suggests a mechanism of feedback regulation by enzyme repression or release of an ODC anti-enzyme.