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Why do some plankton and fishes migrate upward at night and downward before dawn?

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A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Many plankton and fishes perform daily vertical migrations primarily in response to light levels. This behavior is considered an evolutionary advantage for survival.1

By remaining in the dim light or darkness beneath the photic zone during the day, plankton can avoid predators that rely on vision to locate their prey. After sunset, these organisms ascend to the surface waters, where food is more abundant, allowing them to feed safely under the cover of darkness.1

This cycle creates a rhythmic pattern throughout the water column:
* Daytime: Organisms descend to deeper, darker levels to hide from visual predators.12
* Dusk: As light fades, they migrate upward toward the surface to feed.312
* Night: They remain in the upper layers to exploit food resources.2
* Before Dawn: They return to the depths before the sun rises to avoid detection.31

Predatory fishes and other marine animals often follow this cycle, tracking their prey as they move through the water column.2