First Record of Bramble Sharks, Echinorhinus brucus (Echinorhiniformes, Echinorhinidae), in the United Arab Emirates

Abstract

The first record of bramble sharks (Echinorhinus brucus) in the United Arab Emirates is presented. In situ observations of multiple bramble sharks were made at depths between 460 and 720 m from two piloted submersibles and a remotely operated vessel, representing the first known observations of this species in its native deep-water habitat in Arabia and the Indian Ocean. Notably, this research expands on the documented regional distribution of E. brucus for the Gulf of Oman/Arabian Sea and extends this species’ regional records to deeper mesophotic zones. These findings underscore the need for further research to understand the ecology and distribution of this cryptic shark species, particularly given its global endangered status and the limited knowledge of its regional population dynamics.

The Gulf of Oman is an area of critical geopolitical importance as the seaway that terminates at the Strait of Hormuz provides the entrance to the Persian Gulf, where 20% of global oil tanker traffic and 35% of seaborne trade transit annually [1]. Despite its socio-economic importance, compared to many other regions, this area remains relatively understudied in terms of its biodiversity, and the limited research that has been conducted was focused on shallow coastal marine systems (e.g., [2,3]). In the Gulf of Oman, only a narrow coastal fringe exists in the shallow photic zone, with depths reaching more than 3600 m, and the vast majority of the area is at depths where sunlight cannot penetrate. While the oceanography and biogeochemistry of these deep-water environments have been reasonably studied [4], very limited information is known about the biodiversity in the mesophotic zone of the Gulf of Oman, because research has largely focused on species targeted by commercial fisheries.

Sharks remain one of the least studied faunal groups in eastern Arabia [5], and it is only in the past decade that there has been considerable expansion in research on the occurrence, distribution, and diversity of shark assemblages in this region [6,7,8,9]; much of this research, unfortunately, originates from fisheries landings data. Of the 47 shark species known to exist in Oman’s coastal waters, for example, 44 have been confirmed in commercial landings, although 8 species make up the majority of the landings (Rhizoprionodon acutus, Iago omanensis, Carcharhinus sorrah, Loxodon macrorhinus, C. macloti, C. limbatus, Sphyrna lewini, and C. falciformis) [10]. Since the late 1990s [11], fishing pressure has been estimated to have reduced shark populations by 80% in this region, and it is likely to further impair our understanding of the ecology of this understudied group. Currently, it is estimated that there are 68 shark species around the Arabian Peninsula, of which 47 are thought to exist in the Gulf of Oman [8,10], although this is likely an underestimate due to taxonomic uncertainties [12]. Of the bordering nations, the United Arab Emirates (UAE) has been confirmed to host 31 shark species [5], but many of these records originate from fisheries landings on its extensive Arabian/Persian Gulf coast [6], and knowledge of the shark communities on the UAE’s Gulf of Oman coast remains underdeveloped.

In December 2023, a multidisciplinary team of researchers embarked on a two-week mission aboard the research vessel, OceanXplorer, operated by OceanX, to explore the understudied marine environment and ecosystems of the UAE’s east coast in the waters off the Emirate of Fujairah. Research areas included the assessment of mesophotic coral reefs and aphotic coral communities, oceanography and water quality, marine geology, marine megafauna, and others. As part of the research program, surveys conducted by human-operated submersibles and a remotely operated vehicle (ROV) were performed at various locations and depths off of the UAE’s Gulf of Oman coast, with the human-operated submarines capable of diving to depths of 1000 m, and the ROV to depths of 6000 m, permitting the first ever in situ research activities in the UAE’s deep-water environs. Although not intended as part of the research mission, these deep-water surveys provided opportunities for multiple observations of a shark species not previously known to exist in this nation.

On 16 December 2023, three parallel transects were surveyed by two manned submersibles and an ROV as part of a biodiversity and geological assessment, beginning on a gently sloping deep-water plateau before climbing a submarine valley wall to the edge of the more gently sloping continental shelf (maximum survey depth was 780 m and minimum survey depth at the top of the valley wall was 460 m). The transects were conducted approximately 66 km off of the Fujairah coastline and covered a distance of approximately 1600 m each. Observations along these transects showed that the seabed was mainly comprised of fine-silt muds typically overlain with a matrix of cyanobacterial mats, with some limited evidence of bioturbation of the sediments. At these depths, well beyond the photic zone, no sunlight was present, and visibility in the submersibles and the ROV was approximately 12–15 m using the multiple auto-adjusted light sources; the complete lack of light prevented observations outside of this range.

The first observation of a bramble shark (Echinorhinus brucus, Bonnaterre, 1788) [13] was made by the ROV Chimera, which was stationed in a deeper portion of the submarine canyon than the manned submersibles. This individual was observed at a depth of 780 m and was recorded within four minutes of the vessel beginning its operations (Figure 1), possibly because the light, noise, or electrical field of the vessel attracted the individual to the area. The individual swam slowly towards the vessel, glided across the substrate under the vessel, and disappeared from view after 50 s. A video of this encounter is available as an online supplement (Supplementary Video S1).

Similarly, E. brucus sightings also occurred on the manned submersible vessels shortly after the onset of operations on the seabed, albeit at slightly shallower depths than the ROV due to safety constraints. The first observation was made at a depth of 572.4 m, with the same individual being observed by crew members of both the Neptune and Nadir submersibles (Figure 2), which were running parallel transects approximately 20 m apart. A similar encounter was recorded approximately 45 min later at a depth of 483 m and is available as an online supplement (Supplementary Video S2).

In total, 17 records of bramble shark observations were made across all vessels throughout the mission. However, based on the time and location stamps, it is inferred that these observations include several re-sightings of the same individuals who disappeared from view and returned, or those that meandered between adjacent vessels and were double counted. Based on the distance and timing of the records, it is believed that two sharks were observed by the ROV between a depth of 669 m and 780 m, while four distinct individuals were observed between the two manned submersibles, with observations here occurring between a depth of 478 m and 573 m (Figure 3). A table listing the time (in GMT), location, and depth of all observations made in this study is provided in Supplementary Table S1.

To assess the size of sharks, paired lasers calibrated to 10 cm in distance were placed on the body of one individual observed by the submersible Neptune (Figure 4). Post hoc calculations provided an estimate regarding the total length for this individual of 170.96 cm; its sex was indeterminable. Size at maturity is unknown for this species, but the smallest known adult males were reported at 150 cm, and females at 213 cm [14], with individuals growing to a maximum of 310 cm in total length [15]. Most individuals observed during this study were in approximately the same size range as the measured individual, although they were not directly measured.

These observations serve as the first record of bramble sharks (E. brucus) in the UAE, and to our knowledge, the first ever in situ observations of this species in Arabia and the wider Indian Ocean. While an earlier in situ observation was made by an ROV in the Sea of Mamara, Turkey, in 2002, that record was based on a single individual [16]. Here, we report on observations of multiple individuals in their natural habitat, including both still and video footage records, provide georeferenced coordinates and depth data to support a deeper understanding of their ecology, and offer opportunities for future research on this regional population.

E. brucus is an uncommon but widely distributed circum-global species that exists in tropical and temperate seas, with observations primarily reported in the western Atlantic (Argentina to North Carolina), eastern Atlantic (Europe, Africa), the Mediterranean, and the Arabian Sea, with some isolated reports that go as far west as Japan to New Zealand [17], although the latter records may be E. cookie misidentifications [18].

Earlier studies have reported the existence of E. brucus in the southern Gulf of Oman/Arabian Sea region of Oman from fisheries landings [10], as well as from the northern Gulf of Oman waters in Iran [19], so the existence of this species within the intervening waters of the UAE is not surprising, albeit important in further broadening their regional distribution records. Earlier records of the Gulf of Oman were based off of four individuals collected during an experimental longline program conducted across much of the 3000 km coast of Oman, suggesting that E. brucus is relatively rare in this area [10].

The four individuals collected in Oman were captured between a depth of 220 and 250 m, while those in Iran were collected at a depth of 240 m [10,19]; these long-line catches reflect the mesopelagic nature of the sharks observed in this study. However, the sightings reported here in the UAE (which ranged from 478.8 m to 780.0 m) were much deeper than those in earlier studies, suggesting that further research at a greater range of depths is needed to better understand the distribution and ecology of bramble sharks in this region (it is also possible that they also exist at shallower depths in the UAE, but the dive survey was terminated at a depth of 450 m). Globally, bramble sharks typically exist between a depth of 350 m and 900 m [20], in line with the observations made in the current study, suggesting that commercial or experimental long-line fisheries data (which are much shallower, typically <250 m) are likely to underestimate regional species distribution and abundance; further research focusing on depths >500 m is warranted.

Bramble sharks are sufficiently uncommon throughout their global distribution, so it has been suggested that most records warrant publication [10,21], and with this being the first observations for the United Arab Emirates, and they are recorded at deeper depths than reported in the region earlier, we hope to encourage research and conservation efforts for this species throughout the Gulf of Oman. In addition, it was learned post mission that all three transects surveyed in this study occur within a ca. 30 km diameter of an ‘explosive dumping ground’, as laid out on marine navigation maps. While we saw no evidence of explosives ordinance during the surveys, a large crater-like depression was noted at the terminus of the submersible surveys, suggesting that past detonations may have occurred in this area. These observations suggest that research should be undertaken immediately to better understand the distribution and abundance of bramble sharks across the UAE’s deep and offshore waters so that conservation and management efforts may be put in place where warranted.

Interestingly, an earlier population genetic study (using the marker, NADH2) on five individuals from Oman has demonstrated that the regional species being labeled as E. brucus are genetically distinct from the other global populations and may represent an undescribed and distinct Echinorhinus species or subspecies [22]. This thesis was supported by more recent research utilizing a wider array of genetic markers on a larger number of samples, which showed strong divergence between the Oman E. brucus populations and those in the Atlantic/Mediterranean [18]. Together, these findings suggest that all previous records of E. brucus from the Gulf of Oman and Arabian Sea may need to be reassessed, and that further sampling for validation of morphological and genetic taxonomy is warranted. While the observed individuals in this study were morphologically typical of E. brucus, we were unable to collect tissue samples, and until such time as genetic confirmation of the local populations and taxonomic revision for this region are validated, it remains to be determined whether they most accurately fall under E. brucus or should be reclassified as a new subspecies/congener. These discrepancies support the call for further research on this cryptic and understudied group of sharks in the Gulf of Oman.

A significant proportion of deep-water shark species are highly susceptible to overexploitation due to their low productivity [23,24]. The bramble shark is not an exception, and this species is globally classified as endangered by the IUCN [25], with several populations showing significant decline due to overfishing and bycatch [21,26,27,28]. Overall, the global population of bramble sharks is estimated to have decreased by 50–79% over the past half-century based on abundance data and exploitation levels [25]. Although this species has a global distribution, distinct populations are present in different regions, including the Arabian Sea and the Gulf of Oman. Populations in the adjacent waters of India, near the eastern Arabian Sea, have collapsed, with catches declining from 132 t in 2008 to 2.6 t in 2018 [25]. Kyne and Simpfendorfer [24] highlighted the extremely low productivity of many deep-water shark species, making them highly vulnerable to fisheries-related pressure, while the expansion of global fisheries into deep waters has raised concern about overexploitation [29]. The limited availability of population data beyond India in the wider Arabian Sea suggests that survey data are necessary to understand the status and trends of bramble shark populations across this region, and that the conservation of this understudied species is warranted.