Search “ocean zones” online, and you will see hundreds of illustrations that depict the same vertical profile of the sea. The thin, top layer is the “sunlight” or epipelagic zone, which receives enough light for photosynthesis by phytoplankton, algae and some bacteria. Below it is the twilight zone, where the light fades but is still strong enough for some animals to see by and where many animals make their own light through bioluminescence. Next is the midnight zone, with no measurable light, followed by the relentlessly cold abyss. Finally, there are the incredibly deep seafloor trenches known as the hadal zone, named after Hades, Greek god of the underworld.
In this classic view, the amount of light and the water pressure—which increases steadily with depth—largely define which creatures live where. Those factors are important, but so are water temperature, salinity, amounts of oxygen and nitrogen, and the changing currents. Data collected worldwide have revealed that ocean dynamics, and ocean life, are far more complex than we thought, surprising us again and again as we explore.