The chapter explores multi-species interactions between humans and marine life. Mariculture (marine organism farming) has experienced a gigantic global production increase since the mid-twentieth century. The chapter addresses the questions of why and how mariculture-related coevolution between humans and marine species has aimed to reorient coastal marine food webs towards human consumption. The argument of the chapter is that the construction of floating mariculture structures—or new dual-habitat structures for marine species and humans—facilitated human access to submerged habitats, enabling large biomass concentration. The new plastic structures of the Age of Oil were designed to vertically integrate two habitats, each located on one side of the sea surface. They intentionally simplified and localized the previously complex interactions within marine ecosystems. The process of Japanese mariculture pioneers designing these new dual-habitat structures after WWII was also encouraged and inhibited by multiple social and environmental problems, including food security, land reclamation destroying nursery grounds, petrochemical pollution, overfertilization, and coastal overfishing.