Ocean depth zones
From the sunlit surface to dark trenches — 11 kilometres of exceptional biodiversity
11,034 m
Max depth (Mariana)
6
main depth zones
80%
of the ocean unexplored
Intertidal Zone
Physical conditions
Zone description
The intertidal zone is the coastal strip flooded at high tide and dry at low tide. Despite harsh conditions (heat, drying, waves) it is among the most productive and diverse habitats on Earth. Its animals have evolved exceptional survival mechanisms.
- ▸Sharp temperature swings (5–40 °C in a single day)
- ▸Waves and currents deliver nutrients continuously
- ▸Animals are adapted to drying every 6 hours
- ▸Home to mussels, barnacles, and small starfish
Notable species
Mussels
Mytilidae
Seals its valves tightly at low tide
Barnacles
Cirripedia
Attaches with a cement stronger than epoxy
Small starfish
Asterias rubens
Stays moist under rocks
Shore crab
Carcinus maenas
Carries water in its gills
Diver access:
Do not dive the intertidal — it is too shallow and hazardous with waves and rocks
Sunlight Zone
Physical conditions
Zone description
The sunlight zone is the upper ocean layer where light is sufficient for effective photosynthesis. Though it is less than 2% of ocean volume, it holds 90% of marine life. Most coral reefs and commercial fish live here.
- ▸Blue and green light reach 200 m
- ▸Red and orange colours disappear after 15 m
- ▸Surface currents distribute nutrients and oxygen
- ▸90% of the ocean’s primary production happens here
Notable species
Stony corals
Scleractinia
Need light for their symbiotic algae
Grey reef shark
Carcharhinus amblyrhynchos
Hunts in this zone
Green sea turtle
Chelonia mydas
Grazes seagrass in shallow water
Dolphins
Delphinidae
Hunts surface fish schools
Whale shark
Rhincodon typus
Filters surface plankton
Diver access:
Main recreational zone — most dives 0–40 m; technical divers to 130 m
Twilight Zone
Physical conditions
Zone description
The twilight zone is a strange world between light and dark — light is present but far too weak for photosynthesis. Animals evolved huge eyes to use any glimmer, and remarkable bioluminescence. It is among the least studied regions in marine science.
- ▸Some fish migrate vertically 100–500 m daily to feed
- ▸90% of animals produce biological light in the dark
- ▸Pressure is 20–100 times surface pressure
- ▸Temperature drops sharply at the thermocline
Notable species
Cock-eyed squid
Histioteuthis
Its eyes are different sizes
Lanternfishes
Myctophidae
Lights with spots along the flank
Cookiecutter shark
Isistius brasiliensis
Cuts a circular plug from prey
Glass shrimp
Sergestidae
Fully transparent like glass
Diver access:
Technical-dive limit — advanced divers reach ~300 m with specialised gear
Midnight Zone
Physical conditions
Zone description
The midnight zone is permanently dark — no sunlight reaches it. Pressure crushes anything unadapted. Temperatures are near freezing. Even so, fantastically shaped animals live here. There are fewer of them, but their biology is highly specialised.
- ▸No photosynthesis at all — energy comes from decaying sinking matter
- ▸Marine snow — organic particles falling from above — is the main food
- ▸Most fishes have gelatinous bodies to withstand pressure
- ▸Hunting encounters are rare — many wait for prey to pass
Notable species
Anglerfish
Melanocetus johnsonii
Dangles a lure to attract prey
Sperm whale
Physeter macrocephalus
Dives 3,000 m to hunt squid
Giant squid
Architeuthis dux
Largest invertebrate — 13 m long
Gulper eel
Saccopharynx ampullaceus
Jaw expands to swallow prey larger than itself
Diver access:
Impossible for humans — remotely operated vehicles (ROVs) only
Abyssal Zone
Physical conditions
Zone description
The abyssal zone covers 60% of the global seafloor — the largest habitat on the planet. It is cold, dark, and under enormous pressure. It depends entirely on marine snow from above. Primitive-bodied animals dating back 500 million years have been found here.
- ▸The largest share of the global seafloor (60%)
- ▸Brine pools — hypersaline lakes on the bottom
- ▸Hydrothermal vents independent of sunlight
- ▸Thermohaline currents move the water very slowly
Notable species
Abyssal sea cucumber
Holothuria
Extracts nutrients from mud
Tripod fish
Bathypterois
Stands on three fin-legs
Glass sponge
Hexactinellida
Skeleton of glassy silica
Abyssal anemones
Actiniaria
Live thousands of years without moving
Diver access:
Impossible — only bathyscaphe-class submersibles have reached these depths (11,034 m in the Mariana)
Hadal Zone
Physical conditions
Zone description
The hadal zone is the floor of oceanic trenches — the deepest, darkest, and most pressurised place on Earth. The Mariana Trench (11,034 m) is deeper than Everest is tall. Unexpected life has been found there — animals that endure these extreme conditions.
- ▸Mariana Trench — the deepest point on Earth (11,034 m)
- ▸The Red Sea reaches only 2,211 m (no hadal trenches there)
- ▸Pressure crushes metal spheres like paper
- ▸Bacteria have been found that use hydrostatic pressure as energy
Notable species
Hadal amphipod
Hirondellea gigas
Lives naturally at 11,000 m
Snailfish
Pseudoliparis swirei
Deepest known fish (8,336 m)
Hadal sea cucumber
Holothuriidae
90% of animals at these depths
Polychaete worms
Polychaeta
Live in abyssal mud
Diver access:
Only two expeditions have reached the hadal floor (1960 Piccard/Walsh, 2012 Cameron)
Red Sea depths specifically
The Red Sea is relatively shallow compared with the oceans — maximum depth reaches 2,211 m in the central basin. That means it has only the sunlight and twilight zones and a small part of the midnight zone.
Its floor is a distinctive mountain chain — the product of crustal rifting over 25 million years. It widens by 1–2 cm/year because it is a young, still-growing ocean.