Galeorhinus Research Articles

Beneath the surface: DNA barcoding of shark fins in Singapore.

The global decline of shark populations, largely driven by overfishing to supply the shark fin trade, poses a significant threat to marine ecosystems. Southeast Asia, and particularly Singapore, is a key hub for the transit and trade of shark fins that contribute to the exploitation of these apex predators. Through the use of DNA barcoding techniques, this study aimed to determine what species of shark are involved in the Singapore shark fin trade. Fins were collected from markets, dried goods shops and traditional Chinese medicine halls throughout Singapore. In total, DNA was extracted from 684 fins collected in January 2024 and PCR amplification targeted a fragment of the mitochondrial COI gene for species identification. Results revealed fins from 24 species across 16 genera, with 19 species listed on CITES Appendices II, and 16 listed as threatened on the IUCN Red List (critically endangered = 2, endangered = 4, vulnerable = 10). The top five most frequently identified species were Carcharhinus falciformis, Galeorhinus galeus, Rhizoprionodon oligolinx, Sphyrna lewini and Rhizoprionodon acutus. Of these, four are listed on CITES Appendix II and four are listed as threatened on the IUCN Red List.

Observation of the critically endangered soupfin shark (Galeorhinus galeus) in the Changing Salish Sea

Observation of the critically endangered soupfin shark (Galeorhinus galeus) in the Changing Salish Sea

Sharkaeology: Expanding Understandings of Historical Chinese Diaspora Shark Fisheries in Monterey Bay, California, through the Genetic Species Identification of Archaeological Chondrichthyes Remains

Prior to burning down in 1906 CE, Point Alones in the Monterey Bay region of Central California was home to one of the largest Chinese fishing communities in the United States of America. Both historical records and the recovery of numerous cartilaginous fish (Chondrichthyes) vertebrae during archaeological excavations of the village indicate sharks were among the taxonomic groups being regularly harvested by its inhabitants. However, as shark vertebrae are difficult to identify past the family-level using conventional morphology-based approaches, our understanding of the Point Alones shark fishery remains incomplete. In this study, we address this issue by using ancient DNA analysis to assign species-level identifications to a sample of 54 shark vertebrae from the site. We successfully amplified a 173 bp fragment of the mitochondrial cytochrome c oxidase I gene from 47 of the 54 analyzed specimens (87.03%). Our results indicate that Tope Shark (Galeorhinus galeus; n = 39) was the primary focus of the site’s shark fishery, with Brown Smooth-Hound (Mustelus henlei; n = 7) and Leopard Shark (Triakis semifasciata; n = 1) also harvested to a lesser extent. All three of these species are found locally in the waters overlying the continental shelf, suggesting Chinese fishers were harvesting sharks from these coastal environments. While some of the sharks caught by fishers from Point Alones were likely being consumed at the village, historical records suggest a significant number of fins from harvested Tope Sharks were also likely being exported to China and other diaspora communities.

High levels of mislabelling of shark flesh in Australian fish markets and seafood shops

High levels of mislabelling of shark flesh in Australian fish markets and seafood shops

Filling the gap: The unique isotopic niche of medium-sized and large sharks in the northwestern Mediterranean Sea

The Mediterranean Sea is considered a biodiversity hotspot and hosts a broad diversity of shark species. Unfortunately, many shark populations have suffered a long story of overexploitation and their abundance has strongly declined over the last decades. A few studies have addressed the trophic role of sharks in the northwestern Mediterranean Sea, yet little is known about their isotopic niches. In this study, the isotopic niches of six shark species (Alopias vulpinus, Galeorhinus galeus, Hexanchus griseus, Isurus oxyrinchus, Prionace glauca and Squalus acanthias) from the northwestern Mediterranean Sea were determined and compared with those of sympatric predatory teleosts and marine mammals. Overall, δ13C and δ15N values were similar amongst all the shark species except for the three individuals of G. galeus, whose high δ15N values indicate recent immigration from a distinct area with a different isotopic baseline and hence were excluded from further analysis. The mixing models identified cephalopods as the dominant prey of the other five shark species and revealed that the medium-sized and large shark species studied here filled unique isotopic niches. Their trophic positions were similar to those of predatory teleosts and small dolphins but lower than those of larger dolphins, pilot whales and beaked whales. The high contribution of cephalopods to the diet of sharks in the northwestern Mediterranean Sea might be indicative of fish scarcity and deserves further research in the context of potential ontogenetic diet changes.

Trophic ecology and coastal niche partitioning of two sympatric shark species in the Azores (mid-Atlantic)

The exploitation of marine resources has caused drastic declines of many large predatory fishes. Amongst these, sharks are of major conservation concern due to their high vulnerability to overfishing and their ecological role as top predators. The 2 protected and endangered shark species tope Galeorhinus galeus and smooth hammerhead Sphyrna zygaena use overlapping coastal areas around the globe as essential fish habitats, but data to assess their trophic ecology and niche partitioning are scarce. We provide the first comparative assessment of the trophic ecology, ontogenetic shifts, and niche partitioning of the co-occurring tope and juvenile smooth hammerhead around the Azores Islands, mid-north Atlantic, based on δ13C, δ15N, and δ34S (CNS) stable isotope analysis of muscle tissue of the sharks and their putative prey species. Overall, isotopic niches of both species indicated a reliance on similar resources throughout the sampled sizes (tope: 35-190; smooth hammerhead 54-159 cm total length), with significant ontogenetic shifts. Topes displayed a gradual shift to higher trophic levels and a more generalist diet with increasing size (increasing δ15N values and isotopic niche volumes, respectively), whereas smooth hammerhead diet shifted towards prey with lower δ34S at a constant trophic level and a more specialized diet than tope of comparable body size (decreasing δ34S and constant δ15N and δ13C values, respectively). Our results indicate contrasting ontogenetic shifts in δ13C and δ34S along with pronounced differences between niche overlap of life stages pointing to intra- and interspecific niche partitioning of habitat and prey.

DNA barcoding continues to identify endangered species of shark sold as food in a globally significant shark fin trade hub.

Shark fins are a delicacy consumed throughout Southeast Asia. The life history characteristics of sharks and the challenges associated with regulating fisheries and the fin trade make sharks particularly susceptible to overfishing. Here, we used DNA barcoding techniques to investigate the composition of the shark fin trade in Singapore, a globally significant trade hub. We collected 505 shark fin samples from 25 different local seafood and Traditional Chinese Medicine shops. From this, we identified 27 species of shark, three species are listed as Critically Endangered, four as Endangered and ten as Vulnerable by the International Union for Conservation of Nature (IUCN). Six species are listed on CITES Appendix II, meaning that trade must be controlled in order to avoid utilization incompatible with their survival. All dried fins collected in this study were sold under the generic term "shark fin"; this vague labelling prevents accurate monitoring of the species involved in the trade, the effective implementation of policy and conservation strategy, and could unwittingly expose consumers to unsafe concentrations of toxic metals. The top five most frequently encountered species in this study are Rhizoprionodon acutus, Carcharhinus falciformis, Galeorhinus galeus, Sphyrna lewini and Sphyrna zygaena. Accurate labelling that indicates the species of shark that a fin came from, along with details of where it was caught, allows consumers to make an informed choice on the products they are consuming. Doing this could facilitate the avoidance of species that are endangered, and similarly the consumer can choose not to purchase species that are documented to contain elevated concentrations of toxic metals.

A comprehensive phylogenomic study unveils evolutionary patterns and challenges in the mitochondrial genomes of Carcharhiniformes: A focus on Triakidae

The complex evolutionary patterns in the mitochondrial genome (mitogenome) of the most species-rich shark order, the Carcharhiniformes (ground sharks) has led to challenges in the phylogenomic reconstruction of the families and genera belonging to the order, particularly the family Triakidae (houndsharks). The current state of Triakidae phylogeny remains controversial, with arguments for both monophyly and paraphyly within the family. We hypothesize that this variability is triggered by the selection of different a priori partitioning schemes to account for site and gene heterogeneity within the mitogenome. Here we used an extensive statistical framework to select the a priori partitioning scheme for inference of the mitochondrial phylogenomic relationships within Carcharhiniformes, tested site heterogeneous CAT + GTR + G4 models and incorporated the multi-species coalescent model (MSCM) into our analyses to account for the influence of gene tree discordance on species tree inference. We included five newly assembled houndshark mitogenomes to increase resolution of Triakidae. During the assembly procedure, we uncovered a 714 bp-duplication in the mitogenome of Galeorhinus galeus. Phylogenetic reconstruction confirmed monophyly within Triakidae and the existence of two distinct clades of the expanded Mustelus genus. The latter alludes to potential evolutionary reversal of reproductive mode from placental to aplacental, suggesting that reproductive mode has played a role in the trajectory of adaptive divergence. These new sequences have the potential to contribute to population genomic investigations, species phylogeography delineation, environmental DNA metabarcoding databases and, ultimately, improved conservation strategies for these ecologically and economically important species.

How many species of genus Lernaeopoda Blainville, 1822 (Siphonostomatoida: Lernaeopodidae) are there in the southwestern Atlantic?

The family Lernaeopodidae includes 14 genera parasitizing elasmobranchs. Fourteen species of this family have been cited from Argentina, four of which were found on chondrichthyans. Schroederichthys bivius Mller and Henle and Galeorhinus galeus (Linnaeus) from Argentina harbored parasitic copepods of the genus Lernaeopoda. The objective of this study was to identify the species using an integrative approach. The morphology was examined by Optical Microscopy and Scanning Electron Microscopy and the molecular analysis was based on partial sequences of the COI mtDNA gene. Despite differences in the antenna, antennule, mandible, maxilliped and maxillae among the specimens, the morphological and molecular analyses revealed that they belonged to Lernaeopoda bivia Leigh-Sharpe, 1930. The species so far reported for Argentina are L. bivia and L. galei Kryer, 1837, which are distinguished by the size (less and greater than 7 mm, respectively). Here, we report partial sequences of the COI mtDNA gene of L. bivia for the first time, obtained from eleven specimens attached to the mouth, fins, anal slit and claspers of the two shark hosts. The COI mtDNA gene tree shows that the Lernaeopoda group forms a sister clade with Pseudocharopinus bicaudatus (Kryer, 1837), while the genus Pseudocharopinus does not appear to be a natural group. We propose that the material described from Argentinean waters as L. galei was misidentified and actually belongs to L. bivia. The wide variability within the specimens of L. bivia emphasizes the importance of using an integrative approach to revise the taxonomy of the Lernaeopoda species from all over the world.

Comparison of acoustic particle acceleration detection capabilities in three species of sharks.

Behavioural studies have shown that sharks are capable of directional orientation to sound. However, only one previous experiment addressed the physiological mechanisms of directional hearing in sharks. In this study we used a directional shaker table in combination with the auditory evoked potential (AEP) technique to understand the broadscale directional hearing capabilities in the New Zealand carpet shark Cephaloscyllium isabellum, rig shark Mustelus lenticulatus, and school shark Galeorhinus galeus. The aim of this experiment was to test if sharks are more sensitive to vertical (z-axis) or head-to-tail (x-axis) accelerations, and whether there are any differences between species. Our results support previous findings, suggesting that shark ears can receive sounds from all directions. Acceleration detection bandwidth was narrowest for the carpet shark (40-200Hz), and broader for rig and school sharks (40-800Hz). Greatest sensitivity bands were 40-80Hz for the carpet shark, 100-200Hz for the rig, and 80-100Hz for the school shark. Further, our results indicate that there may be differences in directional hearing abilities among sharks. The bottom-dwelling carpet shark is equally sensitive to vertical and head-to-tail particle accelerations. In contrast, both benthopelagic rig and school sharks appeared to be more sensitive to vertical accelerations at frequencies up to 200 Hz. This is the first study to provide physiological evidence that sharks may differ in their directional hearing and sound localisation abilities. Further comparative physiological and behavioural studies in more species with different lifestyles, habitats, and feeding-strategies are needed to further explore the drivers for increased sensitivity to vertical accelerations among elasmobranchs.

Comparison of auditory evoked potential thresholds in three shark species.

Auditory sensitivity measurements have been published for only 12 of the more than 1150 extant species of elasmobranchs (sharks, skates and rays). Thus, there is a need to further understand sound perception in more species from different ecological niches. In this study, the auditory evoked potential (AEP) technique was used to compare hearing abilities of the bottom-dwelling New Zealand carpet shark (Cephaloscyllium isabellum) and two benthopelagic houndsharks (Triakidae), the rig (Mustelus lenticulatus) and the school shark (Galeorhinus galeus). AEPs were measured in response to tone bursts (frequencies: 80, 100, 150, 200, 300, 450, 600, 800 and 1200 Hz) from an underwater speaker positioned 55 cm in front of the shark in an experimental tank. AEP detection thresholds were derived visually and statistically, with statistical measures slightly more sensitive (∼4 dB) than visual methodology. Hearing abilities differed between species, mainly with respect to bandwidth rather than sensitivity. Hearing was least developed in the benthic C. isabellum [upper limit: 300 Hz, highest sensitivity: 100 Hz (82.3±1.5 dB re. 1 µms-2)] and had a wider range in the benthopelagic rig and school sharks [upper limit: 800 Hz; highest sensitivity: 100 Hz (79.2±1.6 dB re. 1 µms-2) for G. galeus and 150 Hz (74.8±1.8 dB re. 1 µms-2) for M. lenticulatus]. The data are consistent with those known for 'hearing non-specialist' teleost fishes that detect only particle motion, not pressure. Furthermore, our results provide evidence that benthopelagic sharks exploit higher frequencies (max. 800 Hz) than some of the bottom-dwelling sharks (max. 300 Hz). Further behavioural and morphological studies are needed to identify what ecological factors drive differences in upper frequency limits of hearing in elasmobranchs.

Estimating the effects of recreational fisheries on sharks in the English Channel and adjacent seas using social networks

Recreational shark fishing in the English Channel and adjacent seas was analysed through the analysis of videos posted on social networks, which, for the first time, allowed us to assess which species in the area are the most affected by fishing techniques, the hotspots and the biological data of the species caught. Using this novel methodology, we found that recreational anglers target spotted and common smooth-hound sharks (Mustelus asterias and M. mustelus), particularly in French waters. Anglers also target the blue shark (Prionace glauca) and tope (Galeorhinus galeus), mainly in the waters of the United Kingdom (UK). Results show that recreational fishing has implications for the conservation of all these vulnerable species, and particularly for blue shark and tope since, respectively, 49% and 36% of the individuals of these two species caught were juveniles. Based on the results, we propose some recommendations to reduce fishing pressure on these species which should be implemented on a collaborative basis between France and UK for this shared resource.

Species of Hexabothriidae (Monogenea) may have extensive distribution ranges reflecting multiple host species: evidence from three new South African records

Many hexabothriid genera require renewed taxonomic attention to provide additional detailed comparative data from new material to resolve existing species quandaries. Our current study describes and provides additional museum material for three hexabothriid species from three genera from South Africa: Erpocotyle catenulata (Guberlet), which is confirmed as a valid species, Heteronchocotyle gymnurae Neifar, Euzet and Ben Hassine, and Hypanocotyle bullardi Chero, Cruces, Sáez, Carolina, Camargo, Portes Santos and Luque. Evidence of these three hexabothriid species from South Africa suggests that hexabothriids have more globally widespread distributions, and that they are less host-specific than previously considered. A detailed summary of hexabothriids reported from the triakid hosts, Galeorhinus galeus and Mustelus mustelus is provided, and the host-specificity of hexabothriids is discussed.

Fine-scale variation in the proximity of baited remote underwater video stations (BRUVS) to rocky reefs reveals changes in the structure of temperate fish assemblages

We investigated changes in the structure of coastal fish assemblages in Northern Patagonia, Southwestern Atlantic, by using baited remote underwater video stations (BRUVS) deployed at increasing distances from rocky reefs: 0–5 m, 15–20 m and 50–60 m. We estimated species richness and abundance (total and by preferred habitat type) and searched for diagnostic species in each distance range. We recorded 14 taxa across 11 families in 19 areas surveyed. Species richness and abundance were higher on reef ledges and decreased with distance from them, at a finer spatial scale than previously reported. Acanthistius patachonicus and Sebastes oculatus were indicative of reef ledges; they were less abundant at 15–20 m and disappeared at 50–60 m. Callorinchus callorynchus and Odontesthes spp. occurred only at distances >15–20 m from the reefs, while Galeorhinus galeus was distributed homogeneously throughout the surveyed area. Our findings have practical implications for monitoring ecotone demersal habitats with BRUVS.

Quaternary fossil shark (Neoselachii: Galeomorphii and Squalomorphii) diversity from southern Brazil

Sharks are cartilaginous fish that appeared 245 million years ago and became a highly diversified group, occupying several niches successfully. On the coast of Rio Grande do Sul, Brazil, shark fossils are commonly found ex situ associated with accumulations of biodetrital gravels along the current beach line. The present study consisted of a faunistic and paleobiologic survey of sharks from the Quaternary in the southern region of the Rio Grande do Sul Coastal Plain (RSCP), southern Brazil. Based on taxonomy, about 3611 fossilized shark teeth collected on the beaches of Cassino and Hermenegildo were analyzed. A total of 13 taxa were identified, among them cf. Carcharhinus brachyurus, cf. Carcharhinus leucas, cf. Carcharhinus longimanus, Carcharhinus sp., Carcharias taurus, Carcharodon carcharias, Galeocerdo cuvier, Galeorhinus galeus, Isurus oxyrinchus, Notorynchus cepedianus, Rhizoprionodon sp., Sphyrna sp. and Squatina sp. Besides the already known species, here we also report the presence of Carcharhinus brachyurus, Carcharhinus longimanus, Galeorhinus galeus, Rhizoprionodon sp. and Squatina sp. as the first fossil record in the Pleistocene of Brazil for these taxa in the region. The study showed that the fossil taxa identified are similar to those extant taxa that inhabit the coast of Rio Grande do Sul. Evidence of climate oscillations that occurred during the Quaternary can be seen throughout the presence of the species Carcharhinus leucas, Carcharodon carcharias, and Galeocerdo cuvier (when compared to the composition of the modern shark fauna). Since, currently, the occurrence of these species are given as rare in the region. The present study expanded the occurrence of fossil species on the coast of the state of Rio Grande do Sul and confirmed the occurrence of populations of Carcharias taurus, Carcharodon carcharias, Carcharhinus sp., and Carcharhinus leucas during the Quaternary.

Small marine reserves provide conservation benefits for coastal sharks in southern New Zealand

Abstract With the increasingly imperilled status of shark populations, there is a pressing need to evaluate management solutions. Given the threats posed by fishing, marine reserves (MRs) present a promising option. Ata Whenua (Fiordland) in the South Island of Aotearoa/New Zealand is an ideal location to investigate this phenomenon owing to the presence of several shark species in coastal MRs. One‐hundred and sixty‐seven baited remote underwater video deployments were made in five MRs. A multi‐model inference approach using generalized linear modelling was used to assess the combined effect of the MRs on two trophic groups of coastal sharks. Generalized linear modelling was used to assess the effect of protection on, firstly, the presence of broadnose sevengill sharks (Notorynchus cepedianus), while accounting for variations in environmental variables, and secondly, the combined relative abundance of mesopredatory sharks detected (spiny dogfish, Squalus acanthias; school shark, Galeorhinus galeus; and carpet shark, Cephaloscyllium isabellum). Mesopredators had a higher relative abundance in MRs by a factor of 2.5 and there was a strong significant effect of protection, suggesting that MRs have led to an increase in their abundance or a change in distribution. In contrast, there was no effect of protection detected for sevengill sharks. It is concluded that the relatively small (<40 km2) MRs sampled do not provide conservation benefits for a large, mobile shark, but that they are potentially large enough to offer protection for mesopredatory sharks with smaller home ranges. To be effective for shark conservation, we therefore recommend that MRs need to be appropriately sized for the ranging behaviour of the target species for protection.

ERRATUM: Population-scale habitat use by school sharks Galeorhinus galeus (Triakidae) in the Southwest Atlantic: insights from temporally explicit niche modelling and habitat associations

ERRATUM: Population-scale habitat use by school sharks Galeorhinus galeus (Triakidae) in the Southwest Atlantic: insights from temporally explicit niche modelling and habitat associations

Ontogeny of the inner ear maculae in school sharks (Galeorhinus galeus)

Studies on the auditory system of fishes can provide fundamental information about the early evolution of vertebrate hearing. While there are limited data available on the auditory system of bony fishes, comparatively far less is known about auditory structures in elasmobranchs, despite their critical basal position within vertebrate evolution. Specifically, while there is a high degree of plasticity in the nervous system, little is known about how the different sensory epithelia within the inner ear vary throughout life in elasmobranchs. Using a combination of immunohistochemistry and fluorescence microscopy, we quantified macular area, number of sensory hair cells, hair cell density, and hair cell orientations in the saccule, lagena, utricle, and macula neglecta of school sharks (Galeorhinus galeus) of varying body size. In all maculae, macular area and the number of hair cells increased significantly throughout ontogeny, while hair cell density displayed a concurrent ontogenetic decrease (excluding the utricle). There were also significant differences in macular area, hair cell number, and hair cell density between the four maculae. However, hair cell orientation patterns did not vary between individuals and did not change with body growth. These findings represent one of the first comprehensive characterisations of the inner ear sensory epithelia in an elasmobranch, and reveal changes in morphology that may have implications for auditory capabilities through ontogeny.

Gillnet selectivity for non‐targeted shark species in temperate Australia

AbstractThe use of size‐selective fishing gear has been fundamental for the successful management of shark populations. For the main Australian shark fisheries, available data sets on gillnet selectivity experiments conducted since the 1990s were used to estimate gillnet selectivity parameters for nine commonly caught non‐target species: broadnose, draughtboard, Port Jackson and school sharks, common and southern sawsharks, smooth hammerhead and spikey and whitespotted dogfish. Size at selection increased with mesh size, but it varied with species, with some species having broader selectivity curves due to their particular morphological features. In particular, smooth hammerhead had much broader selectivity curves and larger length at maximum selection than other species of comparable size such as the broadnose shark. The selectivity curves derived in this study will allow a more accurate interpretation of catch size composition and model‐based assessments of stock status of these species

Extensive oceanic mesopelagic habitat use of a migratory continental shark species

The identification of movement and behaviour patterns, as well as inter- and intra-population connectivity is crucial in order to implement effective and functional management and conservation measures for threatened migratory species such as tope (Galeorhinus galeus). Yet, previous studies struggled to elucidate clear and consistent movement and depth usage patterns of adult tope in the Northeast Atlantic, suggesting a high plasticity in the migration and behaviour. We deployed pop-up satellite archival tags on adult tope during their seasonal summer aggregations in the inner German Bight of the south-eastern North Sea and near a presumed mating site in southwest Scotland. Depth distribution and migration pathways were derived from time series data with location processing. Four individuals followed migration trajectories leaving coastal areas and crossed the European shelf slope into oceanic areas of the Northeast Atlantic, remaining fully pelagic for the rest of the deployment duration. These sharks showed far-ranging migration trajectories and undertook regular and frequent diel vertical migrations, reaching daytime depths of over 700 m. Vertical migration patterns closely overlapped with biological mesopelagic habitat structures and closely tracked the diel migration of organisms from deep scattering layers derived from hydroacoustic recordings. It is hypothesized that adult tope regularly utilize oceanic habitats, foraging on mesopelagic layers in an environment generally considered of low prey density.