Description: Incirrate octopods (those without fins) are among the larger megafauna inhabiting the benthic environments of all oceans, commonly in water depths down to about 3,000 m. They are known to protect and brood their eggs until the juveniles hatch, but to date there is little published information on octopod deep-sea life cycles and distribution. For this study, three manganese-crust and nodule-abundant regions of the deep Pacific were examined by remote operated-vehicle and towed camera surveys carried out between 2011 and 2016. Here, we report that the depth range of incirrate octopods can now be extended to at least 4,290 m. Octopods (twenty-nine individuals from two distinct species) were observed on the deep Ka'ena and Necker Ridges of the Hawaiian Archipelago, and in a nodule-abundant region of the Peru Basin. Two octopods were observed to be brooding clutches of eggs that were laid on stalks of dead sponges attached to nodules at depths exceeding 4,000 m. This is the first time such a specific mineral-biota association has been observed for incirrate octopods. Both broods consisted of approximately 30 large (2.0-2.7 cm) eggs. Given the low annual water temperature of 1.5°C, it is likely that egg development, and hence brooding, takes years [Robison et al. (2014), doi:10.1371/journal.pone.0103437]. Stalked-sponge fauna in the Peru Basin require the presence of manganese nodules as a substrate, and near total collapse of such sponge populations was observed following the experimental removal of nodules within the DISCOL (DISturbance and COLonisation) area of the Peru Basin [Bluhm (2001), doi:10.1016/S0967-0645(01)00070-4]. Stalked fauna are also abundant on the hard substrates of the Hawaiian archipelago. The brooding behavior of the octopods we observed suggests that, like the sponges, they may also be susceptible to habitat loss following the removal of nodule fields and crusts by commercial exploitation.

Description: The scientific work during SO242/2 (28. August - 01. October 2015) was part of the JPIO Pilot Action 'Ecological Aspects of Deep-Sea Mining'. The main goal was to study the potential long-term ecological impact o anthropogenic disturbances on the deep-sea floor from mining polymetallic Mn-nodules. The expedition SO242 built on studies of the former German TUSCH projects (1989- 1996) with four RV SONNE cruises to the DISCOL Experimental Area in the Peru Basin, South Pacific (7°S, 88.5° W; 4150 m water depth) between 1989 and 1996 (DISCOL and ATESEPP projects).The integrated ecological studies were carried out within and next to plough tracks of the original DISCOL experiment 1989, which mimicked seafloor disturbances similar to those occurring during nodule mining. Leg 242/2 extended the investigations started during leg 242/1 with a focus on biogeochemical and biological sampling and observations, including comparative studies of the composition of benthic communities (all size classes) as well as of ecosystem functions (remineralization rates, transfer of matter and energy in food webs, ecotoxicology). In addition, observations were continued of the physicochemical characteristics of the DEA, including the overlying benthic boundary layer. The nodule fields surrounding the DEA were used as references for undisturbed areas. A large proportion of the work was based on autonomous instruments and sensor modules that were deployed by means of ROV and lander systems. In addition, ROV-manipulated and telemetry guided instruments such as the Ocean Floor Observatory System (OFOS) were used for targeted sampling and surveys. Food-web experiments including some small-scale disturbances were carried out and sampled directly at the seafloor by the ROV. In this dataset, we present the raw image data collected by the OFOS system. The OFOS is a towed underwater camera system equipped with both a high-resolution photo-camera (iSiTEC, CANON EOS 5D Mark III) and a high-definition video-camera (iSiTEC, Sony FCB-H11) The cameras are mounted on a steel frame (140L x 92W x 135H cm), together with two strobe lights (iSiTEC UW-Blitz 250, TTL driven), three laser pointers at a distance of 50 cm from each other that were used to estimate the size of seafloor structures, four LED lights, a Tritech Altimeter, and a USBL positioning system (Posidonia) to track the position of the OFOS during deployments. In total, 21 OFOS deployments were carried out during the cruise. Deployments were mostly focused within and around the DISCOL Experimental Area (DEA), where disturbance experiments had been carried out during previous work in the area. The selection of the tracks was done in such a way as to survey areas of the seafloor with different levels of disturbance according to the classification proposed by Bluhm et al. (1995): disturbed areas (plough marks within the DEA), undisturbed areas (areas outside the plough marks but within the DEA) and the reference areas (outside the DEA). However, we created one additional category to define the areas of the DEA that were recently disturbed with the Epi-Benthic Sledge (EBS) during cruise SO242/1.