Introduction

The ecology of the mesopelagic and lower epipelagic zones around Antarctica is relatively understudied, in part because of the logistical and financial implications of operating at such depths. This can also be considered true for the Antarctic benthic environment below scientific SCUBA survey depths, typically around 50 m. The increasing deployment of recreational manned submersibles from expedition cruise vessels in the waters around the Antarctic Peninsula provides a novel opportunity to expand our understanding of the marine ecology of this understudied environment.

Submersibles have long been utilized for biological surveys in depths beyond that accessible by SCUBA (Fricke & Meischner 1985; Lorance et al. 2000; Lindsay & Hunt 2005; Baker et al. 2021). Typically, these submersibles have been owned by nation states and operated by national or academic research institutions (e.g., Grassle et al. 1975; Hissmann & Schauer 2017). However, from the late 1990s and early 2000s, several manufacturers (including U-Boat Worx, Personal Submersibles Organisation, Triton and others) began producing PSs. These PSs have now been deployed extensively throughout the world ocean for recreational (Fairley 2018) and military use, and for commercial purposes by the oil and gas industry (Love & York 2005; Meyer-Gutbrod et al. 2019). PS deployment for scientific use is increasingly common (Schrope 2013; Phillips et al. 2018), although in the polar regions, the majority of deployments have been for film production (Amsler Margaret et al. 2017) or, since 2019, for purely recreational use. The increasing deployment of PSs in the polar regions by the expedition cruise industry represents a novel platform for research when these vehicles are considered as vessels of opportunity. Especially when combined with the advanced research facilities found aboard several new expedition cruise ships, these vessels of opportunity are potentially powerful assets for the scientific community.

The large scyphozoan Stygiomedusa gigantea (Browne, 1910; Cnidaria: Scyphozoa: Semaeostomeae) is a primarily deep-water (1000–3000 m) species, though observed at shallower depths (0–1000 m) in the Southern Ocean. It has been recorded in all ocean basins except the Arctic. Stygiomedusa gigantea is currently the only known species within the genus Stygiomedusa (Matsumoto et al. 2003) since S. stauchi Repelin, 1967 and S. fabulosa Russell, 1959 are now considered synonyms of S. gigantea. The body form of S. gigantea consists of a smooth and rounded bell, curtained with a pleated skirt and four oral arms. Typically, this species appears a deep brown–red hue with a yellow shading to the underside of the bell. Stygiomedusa gigantea individuals can reach substantial sizes, with oral arm lengths of over 6 m and bell diameters of 1 m being not unusual for fully mature specimens (Benfield & Graham 2010). Stygiomedusa gigantea is believed to be viviparous (Russell & Rees 1960).

Despite their size, records of S. gigantea remain few, with only 126 recorded encounters (including individuals brought up in nets) since the species was described in 1910 (Benfield & Graham 2010; Tarling et al. 2012; Schiariti et al. 2018; Fig. 1a). The bathymetric distribution of recorded encounters suggests an increased tendency towards shallower depths when in high southern latitudes (Fig. 1c). As a result of the limited number of records, very little is known regarding this species’ full distribution, behaviour, environmental preferences, diet or reproductive cycle. It is therefore important for the scientific community to seize any new opportunity to directly observe S. gigantea in its natural habitat.

Here, we present data from direct observations of S. gigantea conducted from PSs deployed from the expedition cruise vessel Viking Octantis during the 2021/22 Antarctic summer season. Furthermore, we outline the opportunities for utilizing PS as vessels of opportunity for scientific research.

Observations

Twenty-five PS dives were undertaken in a U-Boat Worx Cruise Sub-7 MKII 300 from the expedition cruise vessel Viking Octantis off the Antarctic Peninsula over the period January–March 2022. Dives lasted a maximum of 84 minutes and reached a maximum depth of 300 m. These dives included both training dives and commercial dives with fee-paying guests aboard. During these dives, observations of the large scyphozoan Stygiomedusa gigantea were recorded via mobile telephone cameras and professional cameras. During each observation, the PS was hovered, and all lights remained on.

Direct observations of Stygiomedusa gigantea were made from PSs deployed from Viking Octantis three times (see Table 1 and Fig. 1b). Occurrences of S. gigantea were recorded at water depths of 80 m, 87 m and 280 m. On no occasion was more than a single S. gigantea individual observed during a dive. Still images of each individual were recorded (Fig. 2), along with additional video footage (Supplementary material). The three observations were established as different individuals from distinguishing marks on the bell. No quantitative measurements could be made, but each individual observed was estimated to be longer than the length of the PS (4.85 m), and at least double this length for one of the observations. No other species were found in association with S. gigantea individuals. One individual (Fig. 2d) bores a distinct knot in one of its oral arms. Previous observations of S. gigantea made from a PS deployed by the passenger ship the Scenic Eclipse have also been reported from Antarctic Peninsula waters (Fig. 1), although the metadata, including exact location and depth, has been lost (Gilbert, pers. comm. 2022).

Discussion

The information in this note highlights the scientific potential of PSs as vessels of opportunity in underwater ecological research. That depths of 50 to 300 m are now being accessed with relative frequency in the polar regions provides novel research opportunity for the scientific community. The multiple observations made of the rarely seen Stygiomedusa gigantea from just a limited number of PS dives provide a clear example of the scientific utility of PSs.

Previous to this publication, there were only 126 documented records (Benfield & Graham 2010; Tarling et al. 2012; Schiariti et al. 2018) of S. gigantea. Of these records, 69% were from the high latitude waters of the Southern Ocean and Antarctica (Benfield & Graham 2010). Previous authors have suggested that the larger proportion of sightings in the Southern Ocean may be due to increased zooplankton sampling effort there. However, the observations presented here add to the emerging impression that the colder waters of the southern high latitudes may be a key habitat for S. gigantea.

All of our observations were made at relatively shallow depths (≤280 m). This is in keeping with an apparent tendency towards shallower depths observed in other recorded encounters made at similar southern latitudes.

A symbiotic relationship has been reported between the ophidiiform fish Thalassobathia pelagica and S. gigantea (Harbison et al. 1973; Drazen & Robison 2004), and it is likely that S. gigantea benefits from the presence of T. pelagica in warmer waters through the removal of parasites (Purcell & Arai 2001). However, T. pelagica is not found in the Southern Ocean, and there are no observations of S. gigantea forming associations with alternative fish species found in the Southern Ocean. We did not observe any other species in association with S. gigantea. It is possible that the lower prevalence and slower growth of parasites in colder waters (Harvell et al. 2002) may mitigate the need for such a symbiosis when S. gigantea is located in southern high latitudes.

Conclusion

Underwater scientific surveys in Antarctica have been conducted via SCUBA since the 1950s (Pollock 2007), with recreational SCUBA operations becoming significant in the early 2000s (Lamers & Gelter 2012). This has led to a fairly good understanding of species found down to 50 m. Below 50 m, the Antarctic marine environment remains under-explored. The increasing deployment of PSs from Antarctic expedition cruise vessels presents a novel opportunity for the scientific community to access these under-explored waters. Additionally, most Antarctic tour operators that deploy PSs also operate Remotely Operated Vehicles for safety reasons, and this presents further opportunities for scientific data collection, as has been demonstrated in other regions of the world (Macreadie et al. 2018; McLean et al. 2020). PSs have the potential to be enhanced for scientific data collection relatively easily by the addition of conductivity–temperature–density sensors (Jonsson et al. 2013), advanced camera arrays (Phillips et al. 2016) and parallel laser systems (Laidig et al. 2013). PS deployment could also be scientifically optimized by dive plans that include same-depth or vertical transects.

Citizen science projects onboard expedition cruise vessels, such as those coordinated by The Polar Collective, are popular with guests and have contributed to numerous scientific publications (e.g., Taylor et al. 2019). There is great potential to harness data from citizen scientists following PS deployment, whereby any georeferenced imagery data are gathered across operators and collectively analysed to build our understanding of polar underwater biotopes and species. Consideration should be given in advance to additional data-gathering equipment required onboard PSs and the storage of data post-dive, including that gathered by citizen scientists.

Acknowledgements

The authors would like to thank all guests and crew of the Viking Octantis, without whom this work would not have been possible. In particular, we thank submarine pilots Mark Andrews, Remy Izendooren and Sven Sudar. We are also grateful to Prof. Julian Dowdeswell and Emily Cunningham for comments that helped to improve this manuscript. Finally, we wish to thank Dr Damon Stanwell-Smith for his scientific oversight at Viking Expeditions, which laid the foundations for this work.

References