James Bradley at Literary Hub: Each night, as the line that separates day from night sweeps across the face of the ocean, a vast wave of life rises from the ocean’s depths behind it. Made up of an astonishing diversity of animals—myriad species of minute zooplankton, jellyfish and krill, savage squid and a confusion of fish species ranging from lanternfish to viperfish and eels, as well as stranger creatures such as translucent larvaceans and snotlike salps—this world-spanning tide travels surfaceward to feed in the safety of the dark, before retreating to the depths again at dawn.
Known as the diel vertical migration, this nightly cycle is the single largest movement of life on Earth, with some estimates suggesting the biomass of the animals that make the journey may total 10 billion tons or more. So dense is this cloud of bodies, in fact, that in World War II, scientists working on early sonar were perplexed by readings showing a phantom sea floor that rose and fell at dusk and dawn. Now dubbed the deep scattering layer, this phenomenon unsettled many commanders, and later gave rise to research into the possibility submarines might be able to disguise themselves within the obscuring fog.
The animals that make this journey mostly reside in the twilight of the mesopelagic zone. Beginning approximately 200 meters below the surface and extending to approximately a kilometer down, the mesopelagic is the transition between the well-lit upper layer of the ocean and the stygian gloom below. Its upper boundary is defined as the point where a mere 1 per cent of the Sun’s light still penetrates; its lower limit lies at the point where all light is finally extinguished.
This lightlessness is both sanctuary and trap. Down in the gloom of the mesopelagic, zooplankton and other animals are safe from the predators that patrol the sunny waters above. But no sunlight also means no photosynthesis, and no food save that which drifts down from above in the slow rain of dead plankton, decaying fish and other organic matter known as marine snow. And so the residents of the mesopelagic wait, safe in the dark, until night falls above, and then rise to feast, some on the plankton, some on their fellow migrants from the deep.
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