Description: Invertebrates in polar and deep-sea environments that have complex life histories are exposed to unique environmental conditions that may favor non-pelagic development and K-strategist reproduction. Although many polar species follow this strategy, the numerically most abundant species tend to have more r-strategist life-history characteristics. We deployed artificial substrata over 3 years in the Arctic deep sea and collected hundreds of specimens of the athecate hydroid Bouillonia cornucopia. While this species has previously been described as rare, we report dense, patchy recruitment on artificial substrata, suggesting that B. cornucopia is highly opportunistic. This species has rapid growth compared to other sessile invertebrates in the study area, high fecundity, and continuous reproduction—all characteristics of an r-selected life history. The species’ gonophores are simple, lacking an obvious spadix or radial canals. We observed nurse cells in histological sections of female gonophores, but no male gonophores were observed. Gonophores break away from the blastostyles in mature specimens and appear to have fertilization envelopes, suggesting that each gonophore is composed of a single oocyte and that embryological evelopment occurs in the water column. Hydroids are typically the first invertebrates to recruit to substrata in the Arctic but are easily overgrown. The opportunistic life histories of B. cornucopia and other hydroids may be adaptive for maintaining populations in the face of high mortality. Our study demonstrates the utility of artificial substrata for collections of otherwise rare opportunistic species.

Description: The second leg of the Polarstern expedition PS93 to the Arctic started on 21st July 2015 in Tromsø (Norway) and ended on 15th August 2015, again in Tromsø (Fig. 1.1). The cruise led to the deep-sea observatory HAUSGARTEN in the Fram Strait (approx. 78°30’N - 80°00’N, 05°00’W - 11°00’E). The total duration of the expedition was 24 days; about 285 hours (almost 12 days) were spent for station work. The remaining time was used to reach the study area, for steaming between individual stations, and for the transit from 79°N back to Tromsø. More than 30 stations were sampled, thereby usually deploying several scientific instruments per site. The total length of the expedition was approx. 1,800 nautical miles. The scientific work during expedition PS93.2 supported the time-series studies at the LTER (Long-Term Ecological Research) observatory HAUSGARTEN, where we document Global Change induced environmental variations on a polar deep-water ecosystem. This work was carried out in close co-operation between the HGF-MPG Joint Research Group on Deep-Sea Ecology and Technology, the PEBCAO Group (Phytoplankton Ecology and Biogeochemistry in the Changing Arctic Ocean) at AWI, and the Helmholtz Young Investigators Group SEAPUMP (Seasonal and regional food web interactions with the biological pump), representing a joint effort between AWI and the MARUM - Center for Marine Environmental Sciences at the University of Bremen. The expedition was also used to accomplish installations for the HGF infrastructure project FRAM (FRontiers in Arctic marine Monitoring). The FRAM Ocean Observing System aims at permanent presence at sea, from surface to depth, for the provision of near real-time data on Earth system dynamics, climate variability and ecosystem change. It serves national and international tasks towards a better understanding of the effects of change in ocean circulation, water mass properties and sea-ice retreat on Arctic marine ecosystems and their main functions and services. Within the framework of a ‘Trans-National Access’ (TNA) initiative of the European project FixO3 (Fixed-point Open Ocean Observatories), the expedition provided access to the FRAM Ocean Observing System thereby supporting external and joint scientific projects logistically. Beside the more “traditional” instruments used for marine research (e.g. water sampler, plankton nets, sediment sampler, moorings, bottom-lander) we operated a deep-diving Work-Class Remotely Operated Vehicle (ROV) as well as an Autonomous Underwater Vehicle (AUV) and different autonomous Unmanned Aerial Vehicles (UAVs) during the overall technically and logistically very challenging expedition. The effective cooperation between the scientific party and the ship’s crew, in combination with perfect weather conditions during most times of the cruise, made this expedition a great success.