Copepod species of the genus Calanus (Calanus hereafter) are rice grain-sized crustaceans, distant relatives of crabs and lobsters, that occur throughout the Arctic Ocean consuming enormous quantities of microscopic algae (phytoplankton). These tiny animals represent the primary food source for many Arctic fish, seabirds and whales. During early spring they gorge on extensive seasonal blooms of diatoms, fat-rich phytoplankton that proliferate both beneath the sea ice and in the open ocean. This allows Calanus to rapidly obtain sufficient fat to survive during the many months of food scarcity during the Arctic winter. Diatoms also produce one of the main marine omega-3 polyunsaturated fatty acids that Calanus require to successfully survive and reproduce in the frozen Arctic waters. Calanus seasonally migrate into deeper waters to save energy and reduce their losses to predation in an overwintering process called diapause that is fuelled entirely by carbon-rich fat (lipids). This vertical ‘lipid pump’ transfers vast quantities of carbon into the ocean’s interior and ultimately represents the draw-down of atmospheric carbon dioxide (CO2), an important process within the global carbon cycle. Continued global warming throughout the 21st century is expected to exert a strong influence on the timing, magnitude and spatial distribution of diatom productivity in the Arctic Ocean. Little is known about how Calanus will respond to these changes, making it difficult to understand how the wider Arctic ecosystem and its biogeochemistry will be affected by climate change.