In this study, we present a new depth record for Carcharhinus altimus in the eastern Red Sea ( Figure 1 ). On March 31, 2022, one male C. altimus was observed at 886 meters during an ROV dive ( Figure 2 ) (22.16512N, 38.70729E). From the analysis of photographs and videos of this animal coupled with laser measurements, its size was estimated at 1.90 m TL. In this section we also provide images gathered during RSDE ROV and submersible dives, that support the accurate identification of this species and can be used as basis for future studies ( Figure 3 , Supplementary Material Videos 1 - 3 ).

Through ROV and submersible dives in the eastern Red Sea, we gathered two direct observations which were both below the maximum reported depth record of R. punctifer: one observation at 318 m from the RSDE (22.48269N, 38.86660E) ( Supplementary Material Video 4 ) and another at 486 m from the RSDBE (27.59992N, 35.33483E) ( Figure 4 ; previously published without depth information in Garzon et al., 2022). These observations in the Red Sea support a much wider depth niche for this species, in contrast with the more neritic habitat of many other guitarfish known in other areas of the ocean ( Figure 1 ) (e.g., Ebert and Stehmann, 2013).

We present a new range extension for Iago omanensis. Our recent findings during a 2020 cruise in the Northern Red Sea (previously published without depth records in Garzon et al., 2022) reveal a new upper limit record of 92 m for this species ( Figure 1 ) (7.59441N, 35.30155E). Furthermore, the RSDE provided us with an opportunity to observe the behavior of I. omanensis in the wild, including feeding and potential interactions with other sharks.

The feeding behavior of I. omanensis has been inferred previously only by indirect methods, as direct observations of this species in the wild are almost inexistent. Here, we provide footage of two possibly different feeding strategies, recorded during ROV and submersible dives. The first two videos ( Supplementary Material Videos 5 , 6 ) provide evidence of a suction feeding behavior, typically used to target prey buried in the sand. Other records included lateral head shaking to cut pieces of bait that were deployed to attract sharks (baited dive, Supplementary Material Videos 7 - 9 ). During an ROV dive, we also recorded a possible species interaction in which a small I. omanensis was followed immediately after by a C. altimus individual ( Supplementary Material Video 10 ).

In the various footage sequences from RSDE ROV and submersible dives, multiple images for different individuals of I. omanensis were obtained, some of which supported robust identification ( Figure 5 ). We found evident differences in the colors of the pectoral and caudal fin margins, presenting clear white edges in some of the animals recorded ( Figure 6 ). The quality of the footage was often suboptimal due to poor lighting at the depth and distance from our vehicles to the sharks. In one of the recordings the individual is small in size (~30 cm), however other individuals with average in size around 50 cm, also carried the same markings ( Supplementary Material Videos 11 - 14 ).

The bignose shark, C. altimus, has previously been recorded at a maximum depth of 810 m in Northern Australia (Stevens, 1991). During the RSDE we recorded this species at 886 m, extending the known depth record. C.altimus has been described as a deep-benthic shark (Compagno, 2001; Smith and Heemstra, 1986), possibly undergoing diel vertical migrations rising toward the surface at night, following prey on related biomass movements (Brierley, 2014; Behrenfeld et al., 2019). Reliable data on the specific depth range of this species or its effective distribution is scarce, often due to misidentification with other related species of the Family Carcharhinidae, like the Sandbar shark (C. plumbeus) or the Dusky shark (C. obscuros) (Duffy and Struthers, 2017; Ayas et al., 2020). This can possibly be ascribed to the almost neutral features this species possesses, preventing an easy morphological identification, particularly given many observations are fleeting. Accurate identification of C. altimus individuals remains troublesome, even though some characters can be used for identification, such as the position and size of the fins. Moreover, it is difficult to have a confident basis for their identification in the absence of accurate images of these animals in their natural environment. In our study, the correct species was identified based on distinctive characteristics observed. These include a heavy cylindrical body shape, a large and elongated snout with long nasal flaps, a prominent interdorsal ridge. The pectoral and dorsal fins are large and straight. The first dorsal fin begins in the middle of the pectoral fin and exhibits an angular or slightly rounded apex. These peculiar traits allowed for accurate species identification (Compagno et al., 1989; Grace, 2001; Serena, 2005).

Carcharinus altimus is threatened both as a target species exploited for its meat and fins and as bycatch from pelagic longline, gillnet, and bottom trawl fisheries (White et al., 2006; Ebert et al., 2013; Fields et al., 2018). As a consequence, it is assessed as Near Threatened with a decreasing population trend in the IUCN Red List. However, existing uncertainties on the correct identification of this shark species render current assessments unreliable. Increasing the amount of data available for this species is necessary to promote its protection and conservation.

Rhinobatos punctifer is a batoid inhabiting neritic waters in subtidal sandy, subtidal sandy-muddy and subtidal muddy habitats. This species was described for the first time in the Red Sea (Compagno, 1987), and subsequently reported in other areas ranging from the northern Red Sea to the Arabian Sea and Arabian Gulf (Golani, 1993; Ebert et al., 2017; Last et al., 2019; Bogorodsky et al., 2021). The species is distinguished by a brown-colored dorsal surface of the body and tail, and small sparse light spots on the dorsal side. There is limited information available for this guitarfish, further complicated by previous misidentification with similar species in the same family (Ebert et al., 2017). The deepest record of this batoid species was 150 m in the Gulf of Aqaba, dating back to 1996 (Goldshmidt et al., 1996) and our new observation extends its depth record down to 486 m. It is considered as Near Threatened on the IUCN Red List, but additional data are needed in order to better define the level of risk to which this species is exposed, due to fisheries activities in the Red Sea and habitat loss (Ebert et al., 2017). Catch data on the Spotted Guitarfish is limited, but it is captured with demersal trawls, inshore set gillnets and set net operations common in the northern Red Sea (Bonfil and Abdallah, 2004). The level of exploitation of habitats considered suitable for this organism in the Arabian Peninsula has been increasing over time, likely impacting on their populations (Valinassab et al., 2006; Bruckner et al., 2011). As above, more data on the ecology and behavior of this species are needed. The novel depth records presented in this publication provide further context into the habitats utilization of this species and offers a valuable reference for more reliable in-water future identifications.

Iago omanensis is a small Triakidae inhabiting relatively deep waters of the Red Sea, Arabian Sea and Arabian Gulf (Compagno, 2001; Randall, 1995). First described by Norman and Murray (1939), it is characterized by dark-gray dorsal and pale ventral color, with dark edges on the dorsal fins and on the upper caudal lobe of the tail. The margins of the fins appear almost transparent (Baranes and Ben-Tuvia, 2013). The recently-reviewed depth distribution of this species indicates the maximum depth at 2,522 m (Pearce et al., 2023), reported in the Suakin Trough (Red Sea). In Figure 1 we present the most recent depth distribution for this shark, including a shallower upper limit, 92 m, than the previously established 110 m (Compagno, 2001). The diet of this animal is composed of small bony fish (mainly mychtophids), remarkably abundant in the mesopelagic zone of the Red Sea (Dalpadado and Gjøsaeter, 1987; Klevjer et al., 2012), along with cephalopods and crustaceans (Waller and Baranes, 1991; Waller and Baranes, 1994; Goldshmidt et al., 1996). It has been postulated that I. omanensis is prey to larger shark species, like C. plumbeus and Galeocerdo cuvier, even though there is no evidence of this species being found in the stomach content of these predators.

We observed the presence of an individual of I. omanensis followed right after by an individual of C. altimus ( Supplementary material Video 10 ). This may be linked to previous suggestions that bigger sharks feed on Iago omanensis in the aphotic zone of the Red Sea (Goldshmidt et al., 1996). However, our observations do not provide evidence of a predation attempt, which was not captured in the footage, and further investigations are needed in order to resolve the trophic interactions among these two species.

The Bigeye houndshark I. omanensis is assessed as Least Concern on the IUCN Red List, however the ongoing decline of local populations in the Red Sea suggest that this species is at risk, at least locally, despite limited data on its distribution and behavior (Spaet et al., 2012; Dulvy et al., 2021). The feeding behavior of I. omanensis have been postulated in previous studies only by indirect arguments, due to observations of its behavior in the wild being scarce. Our video recordings provide the first direct evidence of feeding ecology for the species, which supports previous assumptions (Nelson et al., 2001). In particular, our videos support the function of the blade-like teeth with a laterally pointed oblique cusp, similar to the characteristics of Squaloid sharks (e.g., Squalus acanthias) (Compagno, 2001; Wilga and Motta, 1998).

As reviewed in previous studies, some specimens of the genus Mustelus (closely related with the genus Iago) can present white edges on the fins in juvenile life stages (e.g., M. canis), while other Mustelus species, like M. andamanensis, M. norrisi and M. albipinnis, can have the white margins on the fins even as adults (Heemstra, 1997; Castro-Aguirre et al., 2005; White et al., 2021). Indeed, the observation of I. omanensis individuals presenting a peculiar white edge on the fins opens the door to various possibilities, like an intra-specific color variation, a possible shift in color markings with age, or the hypothesis of the presence of a not yet identified species or subspecies of Iago in the Eastern Red Sea, co-existing with the one already reported in the literature, as it happens for Mustelus canis canis and Mustelus canis insularis (Heemstra, 1997). Molecular techniques would be useful to investigate this topic further if specimens can be sampled.

The extended global depth range of three elasmobranch species (Carcharhinus altimus, Rhinobatos punctifer, Iago omanensis) presented here can help improve conservation strategies, as different fishing techniques target different zones of the water column. With rising concern on the global decline of elasmobranch populations, an adequate understanding of their habitat use is fundamental to guide conservation strategies (Walker, 1998; Stevens et al., 2000; Baum et al., 2003; Ferretti et al., 2008).

The possible mechanisms enabling Red Sea elasmobranch to occupy deep water habitats also deserve additional attention. We suggest here that the occupation of deep Red Sea waters by these species may be related to the unique thermal regime of this region, in which temperatures remain > 21°C throughout the seafloor deeper than 2,800 m, suggesting that lower thermal thresholds of species may not be a limiting factor of vertical habitat use.

Information on the movements of the three species examined in this study is extremely limited, but future research utilizing animal-borne sensors (e.g., temperature, depth, etc.) on these species in the Red Sea will likely yield further insights into the novel depth records described here. The increasing utilization of drop cameras, submersibles and ROV to monitor marine fauna in the deep will enable additional observations of elasmobranch species at great depths, allowing an improved understanding of their ecology, habitat use and niches extent.

The newly reported depth ranges of these shark species suggest the possibility that the study area hosts depth refugia of conservation importance for Rhinobatos punctifer. This information is timely, as the northernmost section of the study area is within NEOM, a major development region of Saudi Arabia. The NEOM project has committed to protecting 95% of the 26,500 km² region for nature, and planning is underway for a nature reserve that includes the majority of the marine environment of the region, including habitats covering the full range of depths reported in this study. These results provide important information to help ensure that the NEOM nature reserve provides conservation benefits to R. punctifer, inhabiting depths not previously recorded for this species.

Similarly, with the Kingdom’s ambitious plans to protect 30% of its marine areas by 2030 under the aegis of the National Center for Wildlife, this better understanding of depth ranges of important chondrichthyan species will allow for refined Marine Protected Area design that takes into account these three-dimensional data to protect deep water habitats alongside other coastal and marine areas of importance.