How We Know

Ocean zone

Twilight Zone

Mesopelagic · 200–1,000 m

Reference answer

A dim global layer where enormous numbers of animals migrate upward at night and descend before dawn.

LightFading sunlight; too little for photosynthesis
TemperatureCold and declining
Pressure at base≈ 101 atm

Defining conditions

  • No photosynthesis
  • Bioluminescence is common
  • Diel vertical migration moves animals and carbon

Representative creatures

Deep-sea anglerfishA diverse group best known for a lure that brings prey within striking distance in dark water.Vampire squidA low-energy cephalopod that eats marine snow rather than hunting like a typical squid.Barreleye fishA fish with tubular eyes inside a transparent, fluid-filled shield that helps it look upward and track prey.Giant squidA rarely observed open-ocean squid with exceptionally large eyes and long feeding tentacles.Deep-sea dragonfishA small predatory fish with long teeth, sensitive eyes and light-producing organs.Giant isopodA large crustacean scavenger that feeds on animal falls and other food reaching the seafloor.Frilled sharkA rare eel-shaped shark with six pairs of gill slits, three-pronged teeth, and flexible jaws.Colossal squidThe heaviest known invertebrate, with enormous eyes and rotating hooks on the tentacle clubs.

Evidence

Sources reviewed

  1. Twilight zoneWoods Hole Oceanographic Institution · checked 2026-07-30
  2. What is vertical migration of zooplankton and why does it matter?NOAA Ocean Exploration · checked 2026-07-30