Introduction
The Red Sea is a semi-enclosed marine basin. Its connection to other bodies of water is limited to two shallow, narrow passages: the Bab el-Mandeb Strait and the Suez Canal, which connect to the Indian Ocean and Mediterranean Sea, respectively. This partial physical isolation corresponds to marked biogeographic patterns among marine organisms in the Red Sea (e.g. endemism and reduced gene flow) and the basin is widely considered a distinct ecological unit (Spalding et al., Reference Spalding, Fox, Allen, Davidson, Ferdaña, Finlayson, Halpern, Jorge, Lombana, Lourie, Martin, McManus, Molnar, Recchia and Robertson2007; DiBattista et al., Reference DiBattista, Roberts, Bouwmeester, Bowen, Coker, Lozano-Cortés, Howard Choat, Gaither, Hobbs, Khalil, Kochzius, Myers, Paulay, Robitzch, Saenz-Agudelo, Salas, Sinclair-Taylor, Toonen, Westneat, Williams and Berumen2016, Reference DiBattista, Saenz-Agudelo, Piatek, Cagua, Bowen, Choat, Rocha, Gaither, Hobbs, Sinclair-Taylor, McIlwain, Priest, Braun, Hussey, Kessel and Berumen2020; Bogorodsky & Randall, Reference Bogorodsky, Randall, Rasul and Stewart2019). It is therefore critical to understand the composition of biological assemblages within the Red Sea when developing localized frameworks for management and ecological functioning. Here, we contribute to the current knowledge of pelagic ichthyofauna in the Red Sea by providing evidence of range extensions for wahoo (Acanthocybium solandri (Cuvier, 1832)) and striped marlin (Kajikia audax (Philippi, 1887)) into the basin.
Methods and Results
Three specimens (two A. solandri and one K. audax) were captured in the Red Sea using conventional hook-and-line gear and then photographed (Figures 1–3). One A. solandri specimen was captured near an offshore reef north-west of Thuwal, Saudi Arabia (22.55389203N 38.915995E) on 19 March 2022. Another A. solandri and one K. audax specimen were captured off the Jordanian coast of the Gulf of Aqaba, an embayment at the northern extent of the Red Sea, during August 2015 and August 2021, respectively. Exact coordinates of Jordanian collections are unavailable, although the territorial waters of Jordan constitute a relatively small area (20 × 5 km). No tissues were available for genetic analysis, and species were identified based on distinct morphological characteristics.
Fig. 1. Documented locations of wahoo (Acanthocybium solandri) and striped marlin (Kajikia audax) from the north-western Indian Ocean derived from the Ocean Biodiversity Information System (OBIS; 30 March 2022) and records described in the current study.
Fig. 2. Photographs of wahoo (Acanthocybium solandri) specimens captured in the Red Sea. The specimen in panel (A) was collected near Thuwal, Saudi Arabia, while panels (B, C) correspond to a collection in the Gulf of Aqaba, Jordan.
Fig. 3. Photographs of a striped marlin (Kajikia audax) captured in the Red Sea (Gulf of Aqaba, Jordan).
The A. solandri specimens documented from the Red Sea in this study were identified by their elongated fusiform body, sharply pointed head, a very long dorsal fin with XXIII–XXVII spines and 12–16 rays, 8–9 finlets dorsally and ventrally along the caudal peduncle, vertically aligned second dorsal and anal fins, and dark vertical bars along the body (Figure 2) (Fischer & Bianchi, Reference Fischer and Bianchi1984b; Collette & Graves, Reference Collette and Graves2019). Specimens were distinguished from the morphologically similar narrow-barred Spanish mackerel (Scomberomorus commerson (Lacepède, 1800)) by a longer first dorsal fin (XXIII–XXVII versus XV–XVIII spines), lateral line curving abruptly downward below middle of the first dorsal fin (versus lateral line distinctly sloping ventrally below second dorsal fin in S. commerson), and a pre-orbital length (i.e. distance from the tip of the snout to the anterior edge of the eye) approximately the same length as the rest of the head (i.e. distance from the anterior edge of the eye to the posterior edge of the opercle), whereas the snout is shorter than the rest of the head in S. commerson (Figure 2) (Fischer & Bianchi, Reference Fischer and Bianchi1984b; Collette & Graves, Reference Collette and Graves2019).
The K. audax specimen captured in the Red Sea possessed several morphological characteristics consistent with this species that differentiate it from other Indo-Pacific istiophorids (Fischer & Bianchi, Reference Fischer and Bianchi1984a; Collette & Graves, Reference Collette and Graves2019), including a long, round bill (distinguishing it from shortbill spearfish [Tetrapturus angustirostris Tanaka, 1915]), sharply pointed, elongate pectoral fins that fold against the body and are non-sickle shaped (distinguishing it from black marlin [Istiompax indica (Cuvier, 1832)]), a pointed first anal fin, moderately sized eyes, and vertical bars along the body (Figure 3) (Fischer & Bianchi, Reference Fischer and Bianchi1984a). The specimen was further distinguished from the morphologically similar sailfish (Istiophorus platypterus (Shaw, 1792)) by its first dorsal fin which is elevated anteriorly, abruptly decreasing in height backward (the first dorsal fin rays of I. platypterus are all quite elongate with the middle rays longest, about twice the body depth), and from the blue marlin (Makaira nigricans Lacepède, 1802) by the anterior part of its first dorsal fin which is elevated higher than the depth of its comparatively compressed body (fin folded back but visible in Figure 3) (Fischer & Bianchi, Reference Fischer and Bianchi1984a).
Discussion
The captures of two large pelagic species described in this study represent the first confirmed documentation of A. solandri and K. audax in the Red Sea. Comprehensive checklists of Red Sea fish species and the Ocean Biodiversity Information System show no evidence of these species in the basin (Golani & Fricke, Reference Golani and Fricke2018; OBIS, 2022a, 2022b). A school of A. solandri was purportedly caught in the southern Red Sea in 2012 (GBIF, 2022); however, this observation cannot be verified because no photographs or tissues are available. Similarly, the range of K. audax has previously been expected to extend into the southern Red Sea (Nakamura, Reference Nakamura1985). Yet, no data have been presented to support this claim and the model-predicted distribution of K. audax does not include the Red Sea (Thoya et al., Reference Thoya, Kadagi, Wambiji, Williams, Pepperell, Möllmann, Schiele and Maina2022).
There are multiple possible explanations why A. solandri and K. audax have remained undocumented in the Red Sea until now, including species misidentifications, a lack of targeted research and fishery sampling, and low regional abundances. Two common pelagic fish species in the Red Sea, S. commerson and I. platypterus, exhibit similar morphological features to A. solandri and K. audax, respectively. It is possible that misidentifications between these species have resulted in the absence of A. solandri and K. audax from scientific records in the Red Sea. Fisheries in the Red Sea also primarily harvest coastal and reef-associated species (Tesfamichael & Pauly, Reference Tesfamichael and Pauly2016) and, to our knowledge, no systematic surveys of large pelagic fish diversity have been conducted in the basin. Furthermore, sea surface temperatures within the Red Sea are typically above the optimal thermal niche of both A. solandri (18–28°C; Theisen & Baldwin, Reference Theisen and Baldwin2012) and K. audax (21–24°C; Boyce et al., Reference Boyce, Tittensor and Worm2008); in the central and southern portions of the basin, the mean sea surface temperature does not decrease below 28 and 29°C, respectively, except in winter (Shaltout, Reference Shaltout2019). Thus, we expect low abundance (driven by habitat unsuitability), coupled with minimal pelagic fishing and research effort, to decrease the likelihood of detection for A. solandri and K. audax in the Red Sea.
Acanthocybium solandri is distributed circumglobally across tropical and sub-tropical marine environments, whereas the range of K. audax is restricted to the Indo-Pacific region. We can conclude that the K. audax specimen described in this study represents a connection to the Indian Ocean proper. However, we cannot determine whether the A. solandri specimens documented here are derived from Mediterranean or Indian Ocean stocks. In any case, it remains surprising that such large animals have evaded detection in close proximity to populated Red Sea coastlines until now. Our findings demonstrate how little is currently known about pelagic predators in the Red Sea and emphasize the need for biodiversity surveys of pelagic fish assemblages in this under-studied region.
Data
The data supporting this research are fully available within the article.
Acknowledgements
Foremost, a great deal of appreciation is owed to Ibrahem Alkaraki, who captured the K. audax specimen documented in this study and has graciously granted permission for his photographs to be published. The authors also thank Ute Langner for creating the map presented in this publication, Ashlie McIvor for editing the photographs, and Prof. Burton Jones for his insightful discussion regarding our findings. The authors further thank Sergey V. Bogorodsky (SMF) and one anonymous reviewer for their reviews of the manuscript.
Author contributions
M.S. captured the A. solandri specimen in the Gulf of Aqaba and alerted other authors to the capture of K. audax; C.T.W., M.C.A., C.D.B. and P.G. captured the A. solandri specimen near Thuwal, Saudi Arabia; C.T.W. recognized the novelty of these observations and wrote the first draft of the manuscript; C.T.W. and M.C.A. morphologically verified the identity of both species; M.L.B. supervised the research and provided funding resources; all authors contributed to the refinement of the final manuscript.
Financial support
This research was supported by KAUST (baseline research funds to M.L.B.). M.C.A. was supported by the Postdoctoral Scholar Program at Woods Hole Oceanographic Institution with funding provided by the Dr George D. Grice Postdoctoral Scholarship Fund.
Conflict of interest
The authors declare none.
Ethical standards
This research was conducted under KAUST's Institutional Animal Care and Use Committee approval 18IACUC14, regulated by the Saudi National Committee of Bio-Ethics.
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