Discovery of an unusual rectal gland in the Atlantic sixgill shark Hexanchus vitulus led us to examine the rectal glands of 31 species of sharks to study diversity in rectal‐gland morphology. ...Twenty‐four of 31 species of sharks had digitiform glands (mean width–length ratio ± SD = 0.17 ± 0.04) previously assumed to be characteristic of all elasmobranchs regardless of habitat depth or phylogenetic age. Rectal glands from the family Somniosidae were kidney bean‐shaped (mean width: length ± SD = 0.46 ± 0.05); whereas those from families Echinorhinidae and Hexanchidae were lobulate (mean width: length ± SD = 0.55 ± 0.06). Rectal gland width: length were different among species with digitiform morphology and lobulate morphology (ANOVA; R2 = 0.9; df = 15, 386; 401, F = 219.24; P < 0.001). Histological and morphological characteristics of the digitiform morphology from deep‐sea sharks were similar to those from shallow‐water sharks. Histology of lobulate rectal glands from hexanchids were characterised by tubule bundles separated by smooth muscle around a central lumen. Additionally, we examined plasma chemistry of four species of sharks with digitiform rectal glands and two species with lobulate rectal‐gland morphology to see if there were differences between morphologies. Plasma chemistry analysis showed that urea and trimethylamine N‐oxide (TMAO) followed the piezolyte hypothesis, with TMAO being highest and urea being lowest in deep‐sea sharks. Among electrolytes, Na+ was highest in species with lobulate rectal glands. Hexanchids and echinorhinids both have lobulate rectal glands similar to those of holocephalans, despite the more than 400 million years separating these two groups. The morphological similarities between the lobulate rectal‐gland anatomy of primitive sharks and the secretory morphology of holocephalans may represent an intermediate state between Holocephali and derived shark species.
This is the first in-situ study of feeding behaviors exhibited by bluntnose sixgill sharks. Bait was placed beneath the Seattle Aquarium pier situated on the waterfront in Elliott Bay, Puget Sound, ...Washington at 20m of water depth. Cameras and lights were placed around the bait box to record sixgill shark presence and behavior while feeding. Analysis of feeding behavior revealed that sixgills utilize a bite comparable to many other elasmobranchs and aquatic vertebrates, have the ability to protrude their upper jaw, change their feeding behavior based on the situation, and employ sawing and lateral tearing during manipulation. The versatility of their feeding mechanism and the ability of sixgills to change their capture and food manipulation behaviors may have contributed to the species' worldwide distribution and evolutionary success.
Identifying feeding patterns of large-bodied predators is necessary for predicting their potential effects on food web dynamics. However, diet information from stomach contents can be impractical to ...obtain because required sample sizes can be prohibitively large. In contrast, diet estimates obtained using Bayesian stable isotope mixing models require less sampling effort and can also reveal both population- and individual-level variation in diet. Here, we used an extensive stable isotope data set to evaluate the trophic role of bluntnose sixgill shark (
Hexanchus griseus
), a globally distributed species and among the largest sharks in the North Pacific. In total, 43 subadult sixgill sharks were sampled from Puget Sound, Washington, USA. Mixing model results indicated that the population feeds primarily on benthic fish and invertebrates (estimated median diet percentages: 33 and 35%, respectively). Further, the model indicated low individual variation in diets and that the feeding behavior of both individuals and the population as a whole tended towards generalism. Specifically, sixgill sharks appear to feed on prey groups approximately in proportion to their average biomass densities in the Puget Sound food web. As generalists, sixgill sharks are less likely to be affected by changes in the abundance of any single prey resource, and our results suggest they are unlikely to be important predators to at least some species of management concern. In addition, stable isotope data obtained opportunistically from an adult sixgill shark supports previously suggested ontogenetic movement patterns, whereby some adults make brief migrations into Puget Sound from outer coastal habitats, likely to birth, and pups feed, grow, and remain resident in Puget Sound for several years. Our findings provide insights into the trophic role of this important but understudied species and demonstrate how stable isotope analyses can further understanding of shark ecology.
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.
Innovative telemetry and biologging technology has increased the amount of available movement data on aquatic species. However, real-time information on the environmental factors influencing animal ...movements can be logistically challenging to obtain, particularly in habitats where tides and currents vary locally. Hydrodynamic models are capable of simulating complex tidal flow, and may thus offer an alternative method of contextualizing animal movement in coastal habitats. Here we use this tool to examine the influence of tide on the movement of broadnose sevengill sharks (
Notorynchus cepedianus
) in the San Francisco Bay estuary. Three sharks were actively tracked using acoustic transmitters for 3 to 4 days. We then generated a hydrodynamic model of the estuary and calculated current vectors along each track. We hypothesized that the sharks would adjust their swimming speed and direction depending on current strength when passing through the channel underneath the Golden Gate Bridge. Our results indicate that sharks did tend to follow the current flow in the channel, but their overall displacement did not significantly correlate with tidal amplitude. We conclude that the sharks may respond to environmental factors other than tidal flow, altering their movement at a finer scale than initially considered. Overall, this suggests that hydrodynamic simulation models can be used to visualize and quantify environmental factors that may affect movement patterns in aquatic organisms. We recommend future studies combine these models with other biologging techniques to measure energy expenditure at a finer spatial scale.
The first results of the investigation of the Late Jurassic (Kimmeridgian–early Tithonian) fish fauna from the fossil-rich Pálihálás Limestone Formation (“Long trench”, Eperkés-hegy, Olaszfalu, ...Hungary) are detailed here. The present study provides the first systematic faunal data of a Jurassic marine fish community from the Transdanubian Mountains. The low-diversity neoselachian fauna includes
Notidanodon
sp.,
Sphenodus
sp., and indeterminate synechodontiform, possible indeterminate scyliorhinids, and further, yet indeterminate forms. Actinopterygians are represented by
Caturus
sp. and indeterminate actinopterygian teeth and scales. The Olaszfalu hexanchid is the most similar to
Notidanodon lanceolatus
; however, specific determination is not possible. The Olaszfalu record is the second Jurassic (and also the earliest) report of the genus
Notidanodon
, which re-dates the earliest occurrence of Hexanchidae back into the boundary of the Kimmeridgian–early Tithonian.
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).
•Heptranchias perlo has an olfactory organ with a roundish raphe.•Its connective capsule probably can expand along with water influx.•The raphe has a mechanical role and does not contain any nerve ...fibers and blood vessels.•Supporting cells in the sensory epithelium have also a function in mucus secretion.•Olfactory receptor neurons are immunoreactive to Gαo and Gαi3, but not to Gαolf.
Sharks belonging to the family Hexanchidae have six or seven gill slits, unlike all other elasmobranchs, which have five gill slits. Their olfactory organs have a round shape, which is common for holocephalans, but not for elasmobranchs. Thus, the shape of the olfactory organ represents a further, less striking, peculiarity of this family among elasmobranchs. Despite that, the microscopic anatomy and histology of the olfactory organ have not yet been studied in any species of this family. Here, an anatomical and histological description of the olfactory organ of the sharpnose sevengill shark Heptranchias perlo is given. The organ is a rosette, with a central raphe and 31–34 primary lamellae, which bear secondary lamellae with a more or less branched shape. The elastic connective capsule which envelops the olfactory rosette possibly changes its shape along with water influx. In the olfactory epithelium, the supporting cells also have a secretory function, while no specialized mucous cells are visible; regarding this feature the olfactory epithelium of H. perlo differs from that of other chondrichthyan species. The immunohistochemical investigation of the sensory epithelium shows the absence of immunoreactivity for Gαolf in receptor neurons, which confirms previous observations in Chondrichthyes.
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.
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