Tra le 14 specie di squali presenti nel Mar di Marmara, le specie batidemersali che trascorrono la maggior parte del giorno in acque profonde su pendii continentali o in trincee, costituiscono circa ...il 43% (6 specie) degli squali della regione. Queste specie sono: Hexanchus griseus, Echinorhinus brucus, Oxynotus centrina, Centrophorus cf. uyato, Dalatias licha e Galeus melastomus. Negli ultimi 30 anni, le quantita di ossigeno disciolto negli strati profondi del Mar di Marmara sono state inferiori ai livelli necessari alla sopravvivenza della vita marina. Sembra che gli squali di acque profonde stiano occupando sempre piu le nicchie di altre specie che vivono sulla piattaforma continentale del Mar di Marmara. L'impatto di questa situazione sulla pesca e le possibili risposte dei pescatori commerciali a questa incertezza ecologica non possono essere al momento previste. Pertanto, sia i ricercatori scientifici che i responsabili politici hanno bisogno di identificare le misure specifiche per una protezione efficace degli squali nel Mar di Marmara, dando priorita alle specie piu minacciate.
The sixgill sharks of the genus
Hexanchus
(Hexanchiformes, Hexanchidae) are large, rarely encountered deep-sea sharks, thought to comprise just two species: the bluntnose sixgill
Hexanchus griseus
...(Bonaterre, 1788) and the bigeye sixgill
Hexanchus nakamurai
(Teng, 1962). Their distribution is putatively worldwide in tropical and temperate waters, but many verified records for these species are lacking, and misidentification is common. Taxonomic uncertainty has long surrounded
H. nakamurai
in particular, with debate as to whether individuals from the Atlantic constitute a separate species. Using 1,310 base pairs of two mitochondrial genes,
COI
and
ND2
, we confirm that bigeye sixgill sharks from the Atlantic Ocean (Belize, Gulf of Mexico, and Bahamas) diverge from those in the Pacific and Indian Oceans (Japan, La Reunion, and Madagascar) with 7.037% sequence divergence. This difference is similar to the genetic distance between both Atlantic and Indo-Pacific bigeye sixgill sharks and the bluntnose sixgill shark (7.965% and 8.200%, respectively), and between the entire genus
Hexanchus
and its sister genus
Heptranchias
(8.308%). Such variation far exceeds previous measures of species-level genetic divergence in elasmobranchs, even among slowly-evolving deep-water taxa. Given the high degree of morphological similarity within
Hexanchus
, and the fact that cryptic diversity is common even among frequently observed shark species, we conclude that these results support the resurrection of the name
Hexanchus vitulus
Springer and Waller, 1969 for bigeye sixgill sharks in the northwest Atlantic Ocean. We propose the common name “Atlantic sixgill shark” for
H. vitulus
, and provide new locality records from Belize, as well as comments on its overall distribution.
Baited lander represents a low impact technique, an alternative to the traditional trawl sampling for collecting data on fish diversity and abundance, especially for threatened species such as ...Chondrichthyes living in sensitive habitats. In this study, distribution and abundance of cartilaginous fish were compared between two geographic areas, the southern Adriatic Sea and the north-western Ionian Sea, with two low impact sampling gears, an experimental bottom longline and a baited lander. Species diversity was evaluated by applying ecological indices and difference in mean abundances were tested using multivariate analysis. A total of 13 species of cartilaginous fish were collected. Significant differences in the assemblage recorded in the same area using different sampling tools were detected and no significant differences were detected among different areas explored with the same method. Using longline, the most abundant species collected in both areas was Galeus melastomus, while using lander, the most observed species were Dalatias licha in the southern Adriatic Sea and Hexanchus griseus in the north-western Ionian Sea. According to IUCN classification, of the 13 species collected, 2 are near threatened and 5 are threatened. A better governance of sensitive habitats coinciding with the essential fish habitat for these species would ensure them a better conservation status.
The Strait of Messina is located at the centre of the Mediterranean Sea and is considered a biodiversity hotspot and an obligatory seasonal passage for different pelagic species such as sharks, ...marine mammals, and billfishes. For the first time, in the Strait of Messina, our research group tagged a Mediterranean spearfish (
Tetrapturus belone
) using a pop-up satellite archival tag (PSAT). The observation of abiotic parameters (depth, light, and temperature) recorded by the PSAT confirmed that the tagged specimen was predated after about nine hours. The tag was then regurgitated 14 days after the tag deployment date. The analysis of collected data seems to indicate that the predator may be an ectothermic shark, most likely the bluntnose sixgill shark (
Hexanchus griseus
).
The conservation of threatened sharks requires defining the occurrence and distribution range of the species and key habitats for their survival. During surveys of mesophotic and deeper benthic ...habitats (50 to 340 m depth) off Rapa Nui (Easter Island, Chile, southeastern Pacific), using a remotely operated vehicle, a male bluntnose sixgill shark, Hexanchus griseus (Hexanchidae), was sighted. The estimated total length of this immature specimen was ~146 cm. This is the first documented record of the species off Rapa Nui. The in situ evidence of this threatened deep-water shark highlights the urgency to build management strategies to protect vulnerable apex predators of coastal and deep-waters ecosystems of the recently created Rapa Nui Marine Protected Area for Multiple Uses, above all from the effects of fisheries and other human activities (e.g. bycatch, marine litter).
The broadnose sevengill shark, Notorynchus cepedianus, a common coastal species in the eastern North Pacific, was sampled during routine capture and tagging operations conducted from 2005-2012. One ...hundred and thirty three biopsy samples were taken during these research operations in Willapa Bay, Washington and in San Francisco Bay, California. Genotypic data from seven polymorphic microsatellites (derived from the related sixgill shark, Hexanchus griseus) were used to describe N. cepedianus genetic diversity, population structure and relatedness. Diversity within N. cepedianus was found to be low to moderate with an average observed heterozygosity of 0.41, expected heterozygosity of 0.53, and an average of 5.1 alleles per microsatellite locus. There was no evidence of a recent population bottleneck based on genetic data. Analyses of genetic differences between the two sampled estuaries suggest two distinct populations with some genetic mixing of sharks sampled during 2005-2006. Relatedness within sampled populations was high, with percent relatedness among sharks caught in the same area indicating 42.30% first-order relative relationships (full or half siblings). Estuary-specific familial relationships suggest that management of N. cepedianus on the U.S. West Coast should incorporate stock-specific management goals to conserve this ecologically important predator.
This study investigates the inorganic elements and the persistent organic pollutants (POPs) accumulated in liver of a sexually mature Hexanchus griseus living in the Mediterranean Sea. The casual ...finding of a specimen in the Strait of Messina (April 2018) allowed us to carefully analyse its liver which can be considered a very important biological indicator. The determination of inorganic elements was carried out by ICP-MS technique. Quantitative determination of polycyclic aromatic hydrocarbons (PAH) and poly-chloro-biphenyl compounds (PCB), distinguished in dioxine-like (DL) and not-dioxine-like (NDL) derivates, has been carried out by HRGC/MS and HRGC-MS/MS respectively. Worrying levels of PCB-DL (TEQ = Σ(PCB-DL)*TEF = 5.96 ng g
−1
), PCB-NDL (Σ(PCB-NDL) = 1390.4 ng g
−1
) together with the presence of pesticides and plasticizers were found in the shark liver oil. The levels of heavy metals were below the legal limits.
The bluntnose sixgill shark (
Hexanchus griseus
Bonnaterre 1788) is a large, deepwater apex predator that, in a few unique locations such as the Strait of Georgia, Canada, occupies shallow, inshore ...waters as juveniles. The occurrence of pregnant females in the Strait of Georgia suggests that this is an important area for parturition and juvenile rearing. Bluntnose sixgill sharks are listed under the Canadian
Species at Risk Act
, and an understanding of their behaviour in habitat selection during important life stages is an important component for conservation measures. We employed satellite tags to identify the daily and seasonal depth and thermal habitats of juvenile bluntnose sixgill sharks in the Strait of Georgia. Tag release and depth information suggests that juvenile bluntnose sixgill sharks within the Strait of Georgia remain there until they mature and migrate to offshore deepwaters. The dominant depth habitat utilized by the juvenile sharks in our study was deepwater (>200 m), with only occasional occupation of depths <100 m, indicating that even within coastal areas, juveniles still prefer deepwater. Overall the sharks occupied shallower depths at night than during the day with increased vertical activity in both the day and night compared to crepuscular periods. Seasonally, the sharks occupied shallower depths in the summer than in winter, but inconsistency in the temperatures with which those depths were associated suggests that their vertical behaviour is influenced by local foraging opportunities and not by thermoregulation.
Understanding movement patterns is fundamental to population and conservation biology. The way an animal moves through its environment influences the dynamics of local populations and will determine ...how susceptible it is to natural or anthropogenic perturbations. It is of particular interest to understand the patterns of movement for species which are susceptible to human activities (e.g. fishing), or that exert a large influence on community structure, such as sharks.
We monitored the patterns of movement of 34 sixgill sharks Hexanchus griseus using two large-scale acoustic arrays inside and outside Puget Sound, Washington, USA. Sixgill sharks were residents in Puget Sound for up to at least four years before making large movements out of the estuary. Within Puget Sound, sixgills inhabited sites for several weeks at a time and returned to the same sites annually. Across four years, sixgills had consistent seasonal movements in which they moved to the north from winter to spring and moved to the south from summer to fall. Just prior to leaving Puget Sound, sixgills altered their behavior and moved twice as fast among sites. Nineteen of the thirty-four sixgills were detected leaving Puget Sound for the outer coast. Three of these sharks returned to Puget Sound.
For most large marine predators, we have a limited understanding of how they move through their environment, and this clouds our ability to successfully manage their populations and their communities. With detailed movement information, such as that being uncovered with acoustic monitoring, we can begin to quantify the spatial and temporal impacts of large predators within the framework of their ecosystems.
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