Cephalopods were observed in two of the seven lander deployments. The depths and locations where cephalopods were seen were 5760 m (11.2483′ S/114.8800′ E) and 6957 m (11.1200′ S/114.9283′ E). The bottom time of the two positive deployments was 6 h 41 min and 4 h 59 min respectively. The environmental conditions were 1.34 °C and 1.51 °C respectively with a salinity of 34.7. The substrata at both locations were soft sediments.
At the 5760 m site, a single Grimpoteuthis sp. (130 mm ML, 430 mm TL) approached the lander 2 h 25 min after it reached the seafloor (Fig. 2). The octopod spent 23 min in view, generally not interacting with the bait or lander but apparently foraging around on the seafloor. It exited the field of view for 6 min, returned for 7 min and exited. It was observed in close proximity to a cusk eel (Bassozetus sp.) and a decapod crustacean (Plesiopeneaus sp,). At 6957 m, a single Grimpoteuthis sp. (110 mm ML, 350 mm TL) approached the lander 7 min after the lander set down. There it spent 5 min, approaching the lander, then generally not interacting with the bait or lander but rather “foraging” on the seafloor around the surrounding area similarly to the observation described above. It did, however, pick up a munnopsid isopod (~ 3 cm body length), held it for a few seconds, and dropped it back onto the seafloor before exiting the field of view. It was out of view for 21 min before being observed passing by the lander, several metres away.
We assume here that the two sequential views of cirrates in both observations represent two views of the same animal rather than additional cirrates in the vicinity of the lander. No other cephalopods were observed. As both individuals approached from the horizon of the camera field of view, and in both cases left and returned to the lander, it suggests that artificial illumination was not a deterrent. Furthermore, given both individuals had short residence times relative to the duration of the illumination suggests artificial illumination was also not a significant attractant. The behaviour that we presume to be foraging consisted of brief “hops” from place to place, separated by centimetres, on the bottom. Upon contact with the bottom, the arms and webs were spread until the octopod again lifted off the bottom a few seconds later. No observed behaviours suggested that either of these individuals were resting or moribund.
The observations of the cirrate octopod of the genus Grimpoteuthis at 6957 m extends the reliably documented maximum depth range for cephalopods by 1812 m (Fig. 3a). Some of the deepest cephalopods captured to date are indeed species of Grimpoteuthis (Collins 2003; Collins and Villanueva 2006), but these new observations extend the range for this genus by over 2000 m (Fig. 3b). It also unequivocally places a cirrate octopod as a member of the hadal community.
Both cirrate octopods observed in this study are from the same area, therefore it is difficult to confirm whether their hadal presence is global or the result of chance encounter with an abnormally deep population. However, these new in situ observations provide evidence that may substantiate the reports from the former-Soviet trawling campaigns (Uschakov 1952; Birstein and Vinogradov 1955; Akimushkin 1963), which, if are correct, puts the deepest records slightly deeper again at 8100 m. It also provides credence to the Grimpoteuthis sp. from 7280 m in the Cayman Trench (cited in Belyaev 1989). Indeed, it appears that cephalopods, at least Grimpoteuthis, are perhaps more prevalent in the upper hadal trenches than previously anticipated.
Cirrate octopods are not uncommon in photographic surveys of deep habitats (Roper and Brundage 1972; Vecchione 2017). However, photo-based taxonomy of cephalopods is difficult because many of their generic and specific characters require close morphological examination, often internal, and are not visible in imagery (Vecchione and Roper 1991). The salient point in this study is the unequivocal bathymetric range extension of the molluscan class Cephalopoda, represented by the family Opisthoteuthidae, to well inside the hadal zone—to at least ca. 7000 m. The significance of this bathymetric range extension is perhaps best demonstrated by converting the depth to area of benthic habitat available for these demersal cephalopods. A distribution of 0–5145 m, gives octopods access to ~ 75% of the total ocean floor. This new extension to 7000 m adds another 24%, bringing the theoretically accessible area of global seafloor to 99%; all but the deepest few thousand metres that diminish exponentially in area with increasing depth (Stewart and Jamieson 2018).