🌗
Migratory Behavior
Diel Vertical Migration
Daily vertical journey between deep-water refuge and surface feeding zones - the largest coordinated animal migration on Earth by biomass.
Trigger: Predator Function: Feed at surface at night, hide in darkness by day
📖 Overview
Every day, billions of small fish and zooplankton climb from the mesopelagic twilight zone to feed at the surface at night, then descend to the darkness by dawn. This is diel vertical migration - the ocean's biggest and most reliable animal movement, involving more biomass than any terrestrial migration. Lanternfish alone constitute perhaps 65% of all deep-sea fish, and every one migrates daily.
⚙️ How It Works
Vertical distance of hundreds of metres each way, twice daily. Triggered by light levels. Requires physiological adjustment to changing pressure - swim bladders must vent gas on ascent and refill on descent.
🧭 Evolutionary Advantage
Feed in productive surface waters under cover of darkness. Escape visual predators during the day by descending to depths where light is scarce and hunting difficult.
🐟 Examples Across Species
Lanternfish (Myctophidae), hatchetfish, many mesopelagic species. Some tuna and swordfish reverse-migrate to hunt vertical migrators.
✨ Fun Fact
The layer of lanternfish reflecting sonar signals during their daily migration is so consistent it was once mistaken for the ocean floor - naval sonars call it the "deep scattering layer".