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Abstract The High‐Pressure Plankton Observatory (HiPPO) is designed to quantify motions of zooplankton for behavioral study, including swimming and metabolic responses to environmental perturbations. It builds on prior chamber designs while filling gaps in capability for resolving orientation of small (< 1 mm) plankton, tracking their movements over ecologically relevant spatial scales, and recording in flow‐through conditions on a vessel at sea. The HiPPO chamber has a direct light path for silhouette imaging of zooplankton as they move vertically and horizontally across a 3.56 cm diameter viewing area. Seawater forced by a high‐performance liquid chromatography pump is exchanged continuously through the chamber, but flushing of zooplankton is prevented by fine mesh at the ports. A high‐resolution camera/computer setup enables sustained imaging of plankton motions for quantitative analysis. Application of HiPPO to an investigation of larval behavior of deep‐sea hydrothermal vent species revealed swimming behaviors similar to those of shallow‐water species, including upward and downward helices, meandering, and short hovers. In conditions with microbial biofilm (a potential settlement cue) on a 2024 expedition, vent larvae unexpectedly swam rapidly upward in tight helices at velocities (0.15 cm s⁻¹) higher than those observed in prior experiments with no biofilm (0.03 cm s⁻¹). Many factors varied between the 2024 and earlier trials, so the difference cannot be attributed with certainty to a cue response. This study describes key new features of HiPPO and demonstrates the system's ability to document novel zooplankton behavior. 

Abstract Mass mortality of marine animals due to volcanic ash deposition is present in the fossil record but has rarely been documented in real time. Here, using remotely-operated vehicle video footage and analysis of ash collected at the seafloor, we describe the devastating effect of the record-breaking 2022 Hunga submarine volcanic eruption on endangered and vulnerable snail and mussel species that previously thrived at nearby deep-sea hydrothermal vents. In contrast to grazing, scavenging, filter-feeding, and predatory vent taxa, we observed mass mortality, likely due to smothering during burial by thick ash deposits, of the foundation species, which rely on symbiotic chemosynthetic bacteria for the bulk of their nutrition. This is important for our broad understanding of the natural disturbance of marine ecosystems by volcanic eruptions and for predicting the effects of anthropogenic disturbance, like deep-sea mining, on these unique seafloor habitats. 

Abstract Most volcanic eruptions on Earth take place below the ocean surface and remain largely unobserved. Reconstruction of past submerged eruptions has thus primarily been based on the study of seafloor deposits. Rarely before the 15 January 2022 eruption of Hunga volcano (Kingdom of Tonga) have we been able to categorically link deep‐sea deposits to a specific volcanic source. This eruption was the largest in the modern satellite era, producing a 58‐km‐tall plume, a 20‐m high tsunami, and a pressure wave that propagated around the world. The eruption induced the fastest submarine density currents ever measured, which destroyed submarine telecommunication cables and traveled at least 85 km to the west to the neighboring Lau Basin. Here we report findings from a series of remotely operated vehicle dives conducted 4 months after the eruption along the Eastern Lau Spreading Center‐Valu Fa Ridge. Hunga‐sourced volcaniclastic deposits 7–150 cm in thickness were found at nine sites, and collected. Study of the internal structure, grain size, componentry, glass chemistry, and microfossil assemblages of the cores show that these deposits are the distal portions of at least two ∼100‐km‐runout submarine density currents. We identify distinct physical characteristics of entrained microfossils that demonstrate the dynamics and pathways of the density currents. Microfossil evidence suggests that even the distal parts of the currents were erosive, remobilizing microfossil‐concentrated sediments across the Lau Basin. Remobilization by volcaniclastic submarine density currents may thus play a greater role in carbon transport into deep sea basins than previously thought.