What is the twilight zone of the ocean?
Quick answer
The ocean’s twilight zone, scientifically the mesopelagic zone, lies from about 200 to 1,000 m (650–3,300 ft) deep. Some faint sunlight reaches it, but it’s too weak for photosynthesis, so no plants or algae grow there. Below 1,000 m is the midnight zone, where no sunlight reaches at all.
The ocean’s layers of light
| Zone | Depth | Light | Other name |
|---|---|---|---|
| Sunlight zone (epipelagic) | 0–200 m | Enough for photosynthesis | Euphotic zone |
| Twilight zone (mesopelagic) | 200–1,000 m | Dim; too weak for photosynthesis | Disphotic zone |
| Midnight zone (bathypelagic) | 1,000–4,000 m | None | Aphotic zone begins |
| Abyssal zone | 4,000–6,000 m | None | The abyss |
| Hadal zone | 6,000 m to about 11,000 m | None | Ocean trenches |
The aphotic zone means every layer with no sunlight at all, from about 1,000 m to the bottom. The deepest of all is in the Mariana Trench.
Life in dim light
Twilight-zone animals have adapted in remarkable ways:
- Big eyes to gather every bit of light, like the barreleye fish with its see-through head
- Bioluminescence: about three-quarters of animals in this zone can make their own light
- Counter-illumination: lanternfish glow on their bellies to hide their silhouette from predators below
- Silver or red bodies: red light doesn’t reach this deep, so red animals look black
The biggest migration on Earth
Every night, billions of fish, squid and tiny animals swim up from the twilight zone to feed near the surface, then return to the depths at dawn to avoid predators. This diel vertical migration is believed to be the largest animal migration on the planet by mass.
Why it matters for climate
Twilight-zone animals eat food near the surface and then release carbon in deeper water. This helps move carbon from the air into the deep ocean, a process scientists call the biological carbon pump.
Plants and algae live only in the sunlit layer; see what transpiration is for more on how plants work. For a broader view, read why the ocean is important.
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