Adult blacktip sharks (Carcharhinus limbatus) use shallow water as a refuge from great hammerheads (Sphyrna mokarran)

A refuge can be any space that keeps an organism safe from danger. Prey usually seek protection in the closest refuge available to minimize cost while maximizing survival. Aerial drone footage of blacktip sharks, Carcharhinus limbatus, along the coast of southeast Florida, USA, shows adult blacktips fleeing to the shallow water adjacent to the beach when confronted with or chased by a predatory great hammerhead shark, Sphyrna mokarran. To authors’ knowledge, this is the first evidence of adult C. limbatus using shallow waters as a refuge.     

Metabolic rates and the energetic cost of external tag attachment in juvenile blacktip sharks Carcharhinus limbatus

This study reports on the metabolic rate of the blacktip shark Carcharhinus limbatus and the energetic costs of external tag attachment. Metabolic rates, swimming speed and tail‐beat (B_T) frequency were measured in a static respirometer with untagged animals and animals equipped with a small data logger. Tagged sharks showed significantly higher routine oxygen consumption and lower swimming speeds than untagged animals, indicating that tagging significantly affected the swimming efficiency and energetic requirements in these small sharks, and that these effects must be accounted for when interpreting telemetry data from free‐ranging individuals.     

Natural variability and effects of cleaning and storage procedures on vertebral chemistry of the blacktip shark Carcharhinus limbatus

Key methodological assumptions regarding the degree of natural variability and influence of sample handling and storage of elasmobranch vertebral chemistry were assessed using laser‐ablation inductively coupled plasma mass spectrometry. Vertebral chemistry of juvenile blacktip sharks Carcharhinus limbatus was examined to identify whether differences existed among different regions of the vertebral column, between thoracic vertebrae of individual fish or within individual vertebrae. Additionally, the effects of bleach exposure and storage in ethanol on vertebral chemistry were compared. No significant variation in vertebral chemistry was found among different regions of the vertebral column or between thoracic vertebrae, but significant differences among transect locations within individual vertebrae were observed. The variation at all three levels appears comparable with published data on sagittal otoliths of bilaterally symmetrical teleost fishes. The experimental assessment of potential treatment effects indicated vertebral chemistry was not significantly affected by bleach or exposure to ethanol. Taken together, these results support the idea that vertebrae taken from the same region of the vertebral column can be treated as equivalent and at least certain elements remain robust to exposure to bleach and ethanol.     

Genetic and reproductive evidence for two species of ornate wobbegong shark Orectolobus spp. on the Australian east coast

This study reports on evidence for reproductive isolation among Orectolobus ornatus and Orectolobus halei, two previously cryptic and recently redescribed species of wobbegong shark (Orectolobiformes: Orectolobidae) from the east coast of Australia. The evidence is based on disparity in size at sexual maturity, diagnostic nuclear and mitochondrial DNA variants, and marked phylogenetic divergence. Plots of total length (L_T) and maturity for the two species were non‐overlapping and illustrative of statistically significant size dimorphism. Genetic analyses and phylogenetic reconstruction did not provide indication of hybridization between O. ornatus and O. halei. In fact, sequence divergence between them was higher than in comparisons with another congeneric and largely co‐distributed wobbegong species (Orectolobus maculatus). The assumption of a molecular clock revealed that the two species have evolved in isolation for c. 3·9 million years. These results challenge a paradigm often mentioned in the biodiversity literature that most cryptic species are the product of recent speciation events and will contribute to the development of effective management strategies for wobbegong sharks.     

Dionchus postoncomiracidia (Monogenea: Dionchidae) from the skin of blacktip sharks, Carcharhinus limbatus (Carcharhinidae)

Postoncomiracidia of Dionchus sp. are described from specimens collected from the skin of 2 blacktip sharks Carcharhinus limbatus, captured in the northern Gulf of Mexico. The parasites resemble nonciliated oncomiracidia hatched from eggs laid by Dionchus sp. on gills of a cobia Rachycentron canadum and adults of Dionchus remorae that were collected from gills of a common sharksucker Echeneis naucrates, captured in association with a third blacktip shark. The hamuli of the postoncomiracidia were morphologically similar to those of adult D. remorae. This is the first report of dionchids from an elasmobranch and from a location other than the gills. These findings support the idea that some dionchid oncomiracidia colonize the skin of sharks or other aquatic vertebrates that sponsor remoras, prior to transferring to other remoras and maturing.     

New insights into the trophic ecology of blacktip sharks (Carcharhinus limbatus) from a subtropical estuary in the western Gulf of Mexico

As environmental change persists, understanding resource use patterns is of value to predict the consequences of shifting trophic structures. While many sharks are opportunistic predators, some exhibit prey selectivity, putting them at higher risk compared to species with greater trophic plasticity. In the Gulf of Mexico (GOM), Clupeids and Sciaenids comprise 69% of blacktip shark (Carcharhinus limbatus) diets, which is consequential considering potential responses of these prey groups to disturbance and over harvesting. We assessed if blacktips exhibit selectivity for Clupeids and Sciaenids in the western GOM based on stomach contents from sharks in coastal Texas. Clupeids comprised <2% of diets, while striped mullet (Mugil cephalus) and red drum (Sciaenops ocellatus) comprised >70% of identifiable prey. Ontogenetic shifts from smaller (Clupeids, small Sciaenids) to larger, higher trophic level (Ariidae, Elasmobranchii) prey fits our understanding of foraging among coastal sharks, and suggests our regional understanding of blacktip trophic ecology may be limited by the sizes of sampled sharks. Observed increases in blacktip densities coupled with declines in prey (Mugilids, Sciaenids) is concerning if blacktips have limited diet plasticity. Yet GOM blacktips may be more generalized than previously thought, which is promising for conservation and management.     

A novel field method to distinguish between cryptic carcharhinid sharks,Australian blacktip shark Carcharhinus tilstoni and common blacktip shark C. limbatus,despite the presence of hybrids

Multivariate and machine‐learning methods were used to develop field identification techniques for two species of cryptic blacktip shark. From 112 specimens, precaudal vertebrae (PCV) counts and molecular analysis identified 95 Australian blacktip sharks Carcharhinus tilstoni and 17 common blacktip sharks Carcharhinus limbatus. Molecular analysis also revealed 27 of the 112 were C. tilstoni × C. limbatus hybrids, of which 23 had C. tilstoni PCV counts and four had C. limbatus PCV counts. In the absence of further information about hybrid phenotypes, hybrids were assigned as either C. limbatus or C. tilstoni based on PCV counts. Discriminant analysis achieved 80% successful identification, but machine‐learning models were better, achieving 100% successful identification, using six key measurements (fork length, caudal‐fin peduncle height, interdorsal space, second dorsal‐fin height, pelvic‐fin length and pelvic‐fin midpoint to first dorsal‐fin insertion). Furthermore, pelvic‐fin markings could be used for identification: C. limbatus has a distinct black mark >3% of the total pelvic‐fin area, while C. tilstoni has markings with diffuse edges, or has smaller or no markings. Machine learning and pelvic‐fin marking identification methods were field tested achieving 87 and 90% successful identification, respectively. With further refinement, the techniques developed here will form an important part of a multi‐faceted approach to identification of C. tilstoni and C. limbatus and have a clear management and conservation application to these commercially important sharks. The methods developed here are broadly applicable and can be used to resolve species identities in many fisheries where cryptic species exist.     

Taxonomic assessment of sharks,rays and guitarfishes (Chondrichthyes: Elasmobranchii) from south‐eastern Arabia,using the NADH dehydrogenase subunit 2 (NADH2) gene

An 829‐bp fragment of the NADH dehydrogenase subunit 2 (NADH2) gene was used to assess the taxonomic status of 1487 elasmobranch specimens, representing 52 putative species. Strong evidence was found for the existence of an undescribed Echinorhinus species and for cryptic speciation within Rhynchobatus djiddensis. The results also provide strong molecular support for the existence of two previously reported, but undescribed, guitarfish species. Potential, but less conclusive, cryptic lineage diversification was also noted in Carcharhinus leucas, Loxodon macrorhinus, Iago omanensis and Gymnura poecilura. A complex situation was found in the genus Himantura, with potentially three distinct lineages evident, one of which is probably an undescribed species, in the H. gerrardi complex. One dasyatid specimen could not be identified, but appears to be closely related to Dasyatis ushiei, while Himantura leoparda and Carcharhinus longimanus are reported from Oman for the first time. The results of the present study also reinforce previously reported geographical divisions within certain putative species, which has important implications for fishery management and conservation. © 2015 The Linnean Society of London     

Genetic divergence and reproductive barriers among morphologically heterogeneous sympatric clones of Eunotia bilunaris sensu lato (Bacillariophyta)

The study of reproductive isolation between populations, combined with estimates of genetic divergence, provides important insights into mechanisms of speciation. In this study, sixteen morphologically heterogeneous sympatric clones of Eunotia bilunaris sensu lato (Bacillariophyta) were brought into culture to study their phylogenetic relationships and pre- and postzygotic reproductive barriers. An ITS rDNA phylogeny was congruent with morphology and divided the clones into three groups ('slender', 'robust' and 'labile'), pointing to the presence of several species in E. bilunaris. Whereas most strains had a heterothallic mating system, four 'labile' clones displayed apomictic behaviour. A further 'labile' clone had a heterothallic mating behaviour, however, suggesting a very recent origin for apomixis. Despite high sequence divergence, hybridization occurred between clones belonging to different groups, but was 20-400 times less frequent than in intra-group matings. F1 hybrids had an intermediate morphology and were almost completely sterile; gamete formation was generally arrested in the early stages of meiosis I. The ITS divergence of 11.5-12.3% between the 'robust' and 'slender' clones seems to represent an upper limit of divergence in which cell pairing, gamete formation and auxosporulation are still possible but heavily reduced, and where hybrid sterility has already evolved.     

Cryptic species in Diacyclops bicuspidatus (Copepoda:Cyclopoida): evidence from crossbreeding studies

The problem of cryptic species in Diacyclops bicuspidatus was examined using interpopulation crosses of four populations collected from a: (1) permanent flood lake in Kiev, Ukraine, (2) temporary pool in Kiev, (3) permanent pond in St. Petersburg, Russia (1200 km to north from Kiev) and (4) lake in Crimea (1100 km south of Kiev). The only interpopulation crosses to exhibit fertility were those between the St. Petersburg population and each of the two Kiev populations. The crosses between the Kiev and Crimea populations, between the St. Petersburg and Crimea populations, and between the two Kiev populations were sterile, as evidenced by either nonviable eggs, empty egg membranes or incomplete copulations. The F₁ hybrids resulting from the St. Petersburg permanent pond X Kiev flood lake cross were fertile and produced mature F₂ offspring. Some data on development times of parental and hybrid lines are presented. The St. Petersburg parental line showed development times almost twice as long as those of the Kiev flood lake population when reared at 10 °C and 20 °C in the laboratory. The F₁ offspring of the cross between St. Petersburg females and Kiev floodlake males showed similar development times to females of the St. Petersburg parental lines at both temperatures. The F₂ hybrids also showed development times that approximated those of the St. Petersburg parental line. These crossbreeding studies suggest the presence of cryptic species in the D. bicuspidatus inhabiting ecologically different populations in many parts of its large holarctic range.     

Marphysa (Eunicidae,polychaete, Annelida) species of the Sanguinea group from Australia,with comments on pseudo‐cryptic species

The genus Marphysa is widely collected for bait by recreational fishermen in Australia, and yet the number of species remains unresolved. A new species is described together with additional information on a previously described species. This study reveals that while species of Marphysa superficially resemble one another, detailed studies on the distribution of different types of chaetae along the body, together with molecular studies, uncover species which may co‐occur. Correct identification is critical for management of these largely intertidal species. This study also highlights the increasing awareness of the presence of pseudo‐cryptic species in polychaetes.     

Trophic ecology of juvenile bull sharks (Carcharhinus leucas) in the Coyote estuary,Costa Rica

Bull shark (Carcharhinus leucas) is a near-threatened elasmobranch species capable of moving between the fresh and salty waters of tropical and subtropical coastal areas, for which we still lack important ecological information. During their first years of life, bull sharks use estuarine systems as nursery areas, making them highly susceptible to environmental and anthropogenic pressures. We studied the trophic ecology of juveniles found in the Coyote estuary, a potential nursery area in Costa Rica, to understand the potential impact of further bull shark declines and gain knowledge that could aid in their conservation. We analysed the trophic ecology of juvenile bull sharks [81–103 cm total length (TL)] in the Coyote estuary, Costa Rica, using stable isotopes of δ¹⁵N and δ¹³C. Since one problem using this technique in juveniles is the confounding effect of the maternal signature, we sampled different tissues (muscle and plasma), verified the status of the shark's umbilical scar and identified the size at which the isotope signature is a result of the animal's current diet. The isotopic values of the muscle tissue reflected the maternal isotopic signature. In contrast, plasma values reflected the diet of juvenile bull sharks >95 cm TL and with a closed umbilical scar. Juvenile bull sharks fed primarily on teleost fishes of the order Anguilliformes and Siluriformes, and have a high trophic position (≥4.0) in the Coyote estuary. Our findings suggest that this estuary is an important feeding site for juvenile bull sharks of the Pacific of Costa Rica. Thus, the protection of essential habitats such as the Coyote estuary will benefit not only bull shark conservation, but also the conservation of an array of fish species that also use this habitat as a rookery, many of which are of commercial interest.     

Worldwide phylogeography of the blacktip shark (Carcharhinus limbatus) inferred from mitochondrial DNA reveals isolation of western Atlantic populations coupled with recent Pacific dispersal

Although many coastal shark species have widespread distributions, the genetic relatedness of worldwide populations has been examined for few species. The blacktip shark, (Carcharhinus limbatus), inhabits tropical and subtropical coastal waters throughout the world. In this study, we examined the genetic relationships of blacktip shark populations (n = 364 sharks) throughout the majority of the species' range using the entire mitochondrial control region (1067-1070 nucleotides). Two geographically distinct maternal lineages (western Atlantic, Gulf of Mexico, and Caribbean Sea clades, and eastern Atlantic, Indian, and Pacific Ocean clades) were identified and shallow population structure was detected throughout their geographic ranges. These findings indicate that a major population subdivision exists across the Atlantic Ocean, but not the Pacific Ocean. The historical dispersal of this widespread, coastal species may have been interrupted by the rise of the Isthmus of Panama. This scenario implies historical dispersal across the Pacific Ocean (supported by the recovery of the same common haplotype from the Philippines, Hawaii, and the Gulf of California reflecting recent/contemporary dispersal abilities) and an oceanic barrier to recent migration across the Atlantic. Genetic structure within the eastern Atlantic/Indo-Pacific (Phi(ST) = 0.612, P < 0.001) supports maternal philopatry throughout this area, expanding previous western Atlantic findings. Eastern Atlantic/Indo-Pacific C. limbatus control region haplotypes were paraphyletic to Carcharhinus tilstoni haplotypes in our maximum-parsimony analysis. The greater divergence of western Atlantic C. limbatus than C. tilstoni from eastern Atlantic/Indo-Pacific C. limbatus reflects the taxonomic uncertainty of western Atlantic C. limbatus.     

Evidence of a reproductive barrier between two forms of the marine periwinkle Littorina fabalis (Gastropoda)

Studies of allozyme variation may reveal unexpected patterns of genetic variation which challenge earlier conclusions of species delimitations based on morphological data. However, allozyme variation alone may not be sufficient to resolve this kind of problem. For example, populations of the marine intertidal snail Littorina fabalis (=Littorina mariae) from wave exposed parts end from protected parts of the same shores are distinguished by different alleles of arginine kinase (Ark) while indifferent, or very nearly so, in another 29 loci. Intermediate populations have large deficiencies of exposed/sheltered heterozygote classes of Ark and we have earlier suggested habitat-related selection in this locus as the explanation. In this study we estimated growth rate of individual snails of different Ark-genotypes in three different habitats (exposed, sheltered and intermediate). In all habitats the snails homozygous for alleles of ‘exposed’ type grew faster and matured at a larger size than did snails homozygous for alleles of ‘sheltered’ types. This relationship was indirectly confirmed in three additional sites of intermediate exposure where exposed AA-genotypes dominated among large (>8 mm) snails while the sheltered genotypes dominated among small (<5 mm) snails of truly sympatric samples. We furthermore found small differences in allele frequencies of two other loci (Pgi and Pgm-2) and in shell colour frequencies, comparing sympatric snails of exposed and sheltered Ark-homozygotes. Although we found no signs of habitat-related selection among snails of different Ark-genotype, or selection against heterozygotes, we cannot reject selection in Ark, as our experiments only covered one island, one season and grown-up snails. The coupling between allozyme and phenotypic characters in strictly sympatric samples of snails suggests the presence of two gene pools. Perhaps the large and small forms of L. fabalis represent very closely related cryptic taxa. However, introgression between them seems a possible explanation for the striking similarities in the vast majority of morphological and allozyme characters.     

Two sympatric Byrsonima species (Malpighiaceae) differ in phenological and reproductive patterns

Sympatric and taxonomically related species may present pre- and/or postzygotic mechanisms for reproductive isolation. We compared the phenology and reproductive biology of Byrsonima intermedia and B. pachyphylla, two sympatric Malpighiaceae species, growing in a remnant of Cerrado in Central Brazil. Both species are evergreens and do not have an overlapping flowering period. In both species, dispersal occurs during the rainy season with low and intermediate overlap in the production of immature and mature fruits, respectively. Both species exhibit self-incompatibility (SI), or, in the case of B. intermedia, facultative SI, and, hence, depend on pollinators to compensate for the lack of spontaneous self-pollination. The flowers of the Byrsonima species were visited by fifteen species of bees that collected pollen and/or oil. Based on their more appropriate intrafloral posture, Epicharis flava and Centris varia were the main pollinators of both species, presenting more frequent visits and/or exhibiting trapline foraging behavior, tending to promote outcrossing. Temporal variation in flowering period and total or partial self-incompatibility seem to function as prezygotic isolation mechanisms that form barriers to gene flow between the studied species and probably avoid competition among pollinators. Both species make oil and pollen available to flower visitors and pollinators almost the entire year, but since Byrsonima intermedia has a long flowering time and is very abundant in the study area, it seems to be more important in maintaining flower-visiting bees.     

New microsatellite loci for Carcharhinid sharks (Carcharhinus tilstoni and C. sorrah) and their cross‐amplification in other shark species

Twelve microsatellite DNA markers were isolated in the spot‐tail shark (Carcharhinus sorrah) and nine were isolated in Australian black‐tip shark (Carcharhinus tilstoni). These loci plus 18 others developed for sharks from the genera Negaprion, Ginglymostoma, Carcharodon and Isurus were tested for amplification success on four species of Carcharhinus (including C. sorrah and C. tilstoni) and four other species representing three diverse families. Cross‐amplification was most common within families. Five loci were subsequently tested for polymorphism on 50 C. sorrah and 60 C. tilstoni. The number of alleles per locus was two to 24 and the average heterozygosity was 0.54 (range 0.16–0.87) for C. sorrah and 0.64 (range 0.44–0.78) for C. tilstoni. These loci may be useful tools for genetic analyses of the Carcharhinidae.     

Feeding ecology and growth of neonate and juvenile blacktip sharks Carcharhinus limbatus in the Timbalier–Terrebone Bay complex, LA, U.S.A.

Stomach contents and vertebrae from neonate and juvenile blacktip sharks Carcharhinus limbatus ( n = 334) were examined to describe their diet, feeding patterns and growth within the Timbalier–Terrebone Bay complex, LA, U.S.A. In the study area, both neonate and juvenile C. limbatus feed primarily on gulf menhaden Brevoortia patronus . However, based on the index of relative importance ( I _RI), gulf menhaden constituted a larger portion of the diet of neonates (84·05 % I _RI) than for juveniles (47·91 % I _RI). An increase in the index of relative fullness between the afternoon and dusk time intervals and a large decrease in the percentage of empty stomachs between the night and early morning time intervals suggested that these fish exhibited a diel feeding pattern with crepuscular periods being the times of highest feeding activity. A higher percentage of empty stomachs (neonates 68% and juveniles 39%) and a significantly lower growth rate (age 0+ year C. limbatus , 0·62 mm day⁻¹; age 1+ year fish, 0·89 mm day⁻¹) could indicate that neonate C. limbatus are less efficient predators than older conspecifics.