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The water column of the deep ocean is dark, cold, low in food, and under crushing pressures, yet it is full of diverse life. Due to its enormous volume, this mesopelagic zone is home to some of the most abundant animals on the planet. Rather than struggling to survive, they thrive—owing to a broad set of adaptations for feeding, behavior, and physiology. Our understanding of these adaptations is constrained by the tools available for exploring the deep sea, but this tool kit is expanding along with technological advances. Each time we apply a new method to the depths, we gain surprising insights about genetics, ecology, behavior, physiology, diversity, and the dynamics of change. These discoveries show structure within the seemingly uniform habitat, limits to the seemingly inexhaustible resources, and vulnerability in the seemingly impervious environment. To understand midwater ecology, we need to reimagine the rules that govern terrestrial ecosystems. By spending more time at depth—with whatever tools are available—we can fill the knowledge gaps and better link ecology to the environment throughout the water column. 

To the human observer, the deep sea is as extreme an environment as Earth has to offer. Below about 200 metres, there is no light from the surface, the water can be frigid (-2 to 5 ̊C), oxygen and food are scarce, and the pressure is staggering. Of course, to the countless species that inhabit the deep sea, these conditions are not so extreme, and in a statistical sense, they fall fairly close to average, since the deep comprises the planet’s largest habitat by volume. Despite its expanse, we know little about how life persists in an environment so different from our own. Only in the last half-century has technology emerged that allows us to collect and study live deep-sea animals. Diversity, Evolution and EcoPhysiology of Ctenophores (DEEPC, deepc.org, a US NSF-supported research effort) is opening a window on biochemistry in the deep, and specifically on its relationship to high pressure. By determining structural constraints on enzyme function under pressure, we aim to inform models focusing on deep-sea animal colonization, and to find general patterns of protein adaptation with possible applications in protein engineering and biocatalysis.