INTRODUCTION
Knowledge of the spatial distribution of any marine organism is essential to understanding its ecology. While the geographic range of many shark species is often (and increasingly) well documented, their vertical distribution can be much less well understood owing to the numerous challenges in collecting accurate data. The geographic distribution of the scalloped hammerhead Sphyrna lewini (Griffith & Smith, 1834) (n.b. notwithstanding unresolved taxonomy, e.g. Zemlak et al., Reference Zemlak, Ward, Connell, Holmes and Hebert2009) is well known, and encompasses a range of habitats from estuaries to the open ocean in tropical and warm temperate waters worldwide (Ebert et al., Reference Ebert, Fowler and Compagno2013). However, the vertical distribution of S. lewini is not as well understood. Compagno et al. (Reference Compagno, Dando and Fowler2005) cite ‘surface to >275 m’, and while studies using tagging technology have reported the species to greater depths, accurate depth recording has often been constrained by the limitations of the tagging technology. Using ultrasonic transmitters on four individuals in the Gulf of California, Klimley (Reference Klimley1993) recorded repeated excursions to a maximum depth of approximately 475 m. Also in the Gulf of California, Jorgensen et al. (Reference Jorgensen, Klimley and Muhlia-Melo2009) recorded a single S. lewini over 74 days diving to depths of at least 980 m with a pop-up satellite archival tag (PSAT). Bessudo et al. (Reference Bessudo, Soler, Klimley, Ketchum, Arauz, Hearn, Guzmán and Calmettes2011) recorded occasional night-time dives to approximately 1000 m by a tagged S. lewini, in the tropical eastern Pacific. Most recently, an individual female S. lewini fitted with a PSAT was recorded as making repeated night-time dives >700 m (with 16 of these >900 m, reaching a maximum depth of 964 m) over a period of 27 days in the Gulf of Mexico (Hoffmayer et al., Reference Hoffmayer, Franks, Driggers and Howey2013). These authors suggested that such diving may be a common behaviour in S. lewini, but noted that more data would be required to verify this. The current paper reports an incidental observation of a S. lewini individual made from a remotely operated vehicle (ROV) that extends the accurately recorded depth range of this species.
METHODS
Footage was collected opportunistically using an Ocean ProHD video camera (1080i) mounted on an Oceaneering International Millennium work class ROV (Mill 113), which was deployed from the Deepsea Metro I drill-ship during routine drill-support operations at BG Group's Jordari hydrocarbon exploration site, approximately 40 km off the coast of southern Tanzania in the Ruvuma Basin. The video was made available because of BG's involvement in the collaborative SERPENT Project (Jones, Reference Jones2009) (www.serpentproject.com), in which ROV footage from the oil and gas industry is made available to marine scientists. Water column parameters (temperature, salinity and depth) were collected during the dive with a datalogger on the ROV. In addition, temperature, salinity and dissolved oxygen at a site 30 km away from the Sphyrna lewini observation were recorded from a datalogger (RBR Model XR-420CTDmTi + pH + DO) fitted to the ROV during a SERPENT offshore visit. The shark was identified as scalloped hammerhead S. lewini based on a cephalic foil with a median and two smaller lateral indentations, and the relative size and shape of the fins (Ebert et al., Reference Ebert, Fowler and Compagno2013).
RESULTS
The video clip (59 s in length) commenced at 00:30 h local time on 27 September 2012, after the ROV had been working in view of the seabed for over 3.5 h. Figure 1 presents still images extracted from video footage (online Supplementary Material 1), and shows an individual Sphyrna lewini swimming just above the seabed at 1043 m depth, making three sharp turns at 5, 20 and 30 s into the clip. On each of these occasions it turned back and re-entered the area of seabed illuminated by the lights of the ROV. After 43 s the individual left the frame, still swimming close to the seabed, and it was not observed after this. Although no claspers were clearly visible, the sex could not be confidently determined, and from the scale of the nearby seabed markers the total length of the shark was estimated at approximately 1.5 m. The water column temperature was 5.9°C and salinity was 35. Based on similar temperature and salinity profiles at both the observation site and the site 30 km distant, it is estimated that dissolved oxygen would also be similar and approximately 1–1.5 ml l−1 (Figure 2). Similar video surveys at other sites near this observation recorded the following biota in low abundance: xenophyophores, sponges, molluscs (cirrate octopods and squid), suprabenthic crustaceans, echinoderms and fishes including grenadiers (Macrouridae), cusk eels and relatives (Ophidiiformes) and cutthroat eels (Synaphobranchiidae).
Stills from the video of Sphyrna lewini. (A) Cropped image of the shark as it passed close to the ROV, (B) full screen view as the shark swims out of shot close to one of the marker buoys at the seabed.
Water column temperature and salinity at the observation site (red) and temperature, salinity and oxygen profiles at a nearby site.
DISCUSSION
Although it cannot be assumed that the single individual we observed at depth originated from surface waters (and, therefore, represents deep diving behaviour), our report from 1042 m exceeds the previous accurately recorded depth maximum of 964 m for this species (Hoffmayer et al., Reference Hoffmayer, Franks, Driggers and Howey2013). It also exceeds the depth of ‘at least 980 m’ (and probably not exceeding 1500 m) recorded by Jorgensen et al. (Reference Jorgensen, Klimley and Muhlia-Melo2009), who were not able to report more accurate depths due to limitations of the pressure sensors on the tags used. The current observation is also the first deep-water record for this species in the Indian Ocean. A further SERPENT observation of Sphyrna lewini at a near-bottom depth of around 580 m in the Indian Ocean off Western Australia is also of interest (Jones et al., Reference Jones, Gates, Curry, Thomson, Pile and Benfield2009). These records of Sphyrna lewini, together with those of whale sharks (e.g. Brunnschweiler et al., Reference Brunnschweiler, Baensch, Pierce and Sims2008) and devil rays (Thorrold et al., Reference Thorrold, Afonso, Fontes, Braun, Santos, Skomal and Berumen2014) add weight to the idea that meso- and bathypelagic environments may be of greater importance than previously thought to taxa traditionally considered as epipelagic.
The current observation was made during the hours of darkness. Although the significance of our single incident should not be overstated, it may add further evidence to previous studies of Sphyrna lewini that have recorded deep dives almost exclusively during night-time and/or evening twilight (Bessudo et al., Reference Bessudo, Soler, Klimley, Ketchum, Arauz, Hearn, Guzmán and Calmettes2011; Hoffmayer et al., Reference Hoffmayer, Franks, Driggers and Howey2013; Hoyos-Pallida et al., Reference Hoyos-Pallida, Ketchum, Klimley and Galvan-Magana2014).
Water column profiles show that this Sphyrna lewini individual was recorded in cold (6°C) waters consistent with the classification of ‘hypoxic’ (<5.5 mg l−1, equivalent to approximately 3.85 ml l−1; it should be noted that the entire water column deeper than approximately 75 m would also be hypoxic according to these criteria, a result consistent with other studies reporting low oxygen concentrations in the tropical Indian Ocean e.g. Schlitzer, Reference Schlitzer2000) in experimental work on three shark species, including Sphyrna tiburo (L. 1758), a congener of Sphyrna lewini (Carlson & Parsons, Reference Carlson and Parsons2001). Both factors are likely to present Sphyrna lewini with significant physiological challenges, although experimental work has suggested that Sphyrna tiburo is physiologically able to tolerate moderate levels of hypoxia (Carlson & Parsons, Reference Carlson and Parsons2003). While endothermy, as an adaptation to cold, has been reported in other elasmobranch taxa (notably lamnid sharks and mobulid rays), it has not been for hammerhead sharks (Bernal et al., Reference Bernal, Carlson, Goldman, Lowe, Carrier, Musick and Heithaus2012), and, therefore, time at this depth is likely limited. Nevertheless, tolerance of this environment, even for short times, presumably provides benefits; although, the purpose remains unclear. It has been suggested that diving of Sphyrna lewini into cold and potentially anoxic water could be to exploit deep-water prey less accessible to other pelagic competitors (Jorgensen et al., Reference Jorgensen, Klimley and Muhlia-Melo2009; Hoffmayer et al., Reference Hoffmayer, Franks, Driggers and Howey2013), and video footage from nearby areas to our observation showed the presence of likely Sphyrna lewini prey items (cephalopods and fishes). Most recently, Hoyos-Pallida et al. (Reference Hoyos-Pallida, Ketchum, Klimley and Galvan-Magana2014) suggested that a single Sphyrna lewini juvenile female tagged in the Gulf of California visited deeper waters (up to 250 m) to increase foraging success and as part of an ontogenetic migration from coastal to offshore waters.
Supplementary materials and methods
A supplementary video is available for download at: http://www.serpentproject.com/sphyrnalewini.php
The supplementary material for this article can be found at http://www.journals.cambridge.org/MBD
ACKNOWLEDGEMENTS
The authors are grateful to the BG Group for funding this research as part of a wider SERPENT Project study in the area, in particular L. Werre, J. Moirana and S. Murray. Thanks to the staff and crew of the Deepsea Metro I for assistance at sea, especially Oceaneering International ROV supervisors (F. Lynch, R. Makowichuk and N. Troup) and their crews.
