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The Living Ocean

Most of the planet is water, and most of that is dark, cold, and barely seen. Here is some of what lives down there, in the highest resolution I could find.

A silvery school of fish hovering over a green seagrass meadow in clear blue water.
Neptune grass · Posidonia oceanica · one clone ~100,000 yrs oldA single seagrass meadow off Ibiza is one continuous clone roughly 100,000 years old, older than our species' cave paintings.
A dense heap of purple and orange sea stars piled on intertidal rock.
Ochre sea stars · Pisaster ochraceusThis is the animal that gave science the term keystone species: pull it off the rock and the whole community collapses.
A small oval chiton with psychedelic pink, blue and orange zigzag stripes across its eight plates.
Lined chiton · Tonicella lineataIt rasps food with a tongue capped in magnetite, actual iron ore, a magnet's worth of metal in its mouth.
A sunlit coral outcrop of staghorn, plate and boulder corals with small fish hovering above in blue water.
Great Barrier Reef · Flynn Reef, Australia · ~2,900 reefsThe only living structure visible from space, and it isn't one reef but nearly 3,000, built by trillions of polyps over 20,000 years.
A small fish patterned in a psychedelic maze of electric orange and cyan-blue.
Mandarinfish · Synchiropus splendidusIt is one of only two animals that make blue with actual blue pigment cells, and it wears no scales, just a foul, toxic mucus.
A cobalt-blue eel with flared yellow nostril flaps curling out of a reef.
Blue ribbon eel · Rhinomuraena quaesitaEvery electric-blue ribbon eel is male; if it lives long enough it turns yellow and becomes female, flipping both sex and color.
A banded brown-and-white octopus with long arms fanned out over dark volcanic sand.
Mimic octopus · Thaumoctopus mimicus · copies 15+ animalsIt is the only animal known to impersonate other species at will, shape-shifting to copy sea snakes, lionfish and flatfish as the threat demands.
A small octopus with translucent veined webbing perched on a shell against dark sand.
Coconut octopus · Amphioctopus marginatusOne of the only invertebrates to use tools: it hoards coconut and clam shells, then reassembles them into a portable suit of armor.
A small ochre octopus covered in glowing electric-blue rings on dark ground.
Blue-ringed octopus · Hapalochlaena sp. · venom for 26 adultsSmaller than a golf ball, it carries enough toxin to kill 26 adults in minutes, and there is no antivenom; the blue rings flash a second before it bites.
A mantis shrimp airbrushed in every color at once, all reds, greens and blues, on the seabed.
Peacock mantis shrimp · Odontodactylus scyllarus · punch ~23 m/sIts club strikes so fast the water boils into a flash of light and heat, and the blow lands with about 1,500 newtons, enough to crack aquarium glass.
A velvety black sea slug piped in neon green and orange.
Variable neon slug · Nembrotha kubaryanaIt steals toxins from the sea squirts it eats and oozes them as a defensive slime, the same chemicals now studied as anticancer leads.
A small electric-blue sea slug with winged, finger-like appendages on pale sand.
Blue dragon · Glaucus atlanticusThis thumbnail-sized slug eats the Portuguese man o' war and stores its stinging cells, concentrating them so its own sting outdoes the jellyfish it robbed.
A small white crab covered in red polka dots sitting on a folded carpet anemone.
Porcelain crab in an anemone · Neopetrolisthes maculatusIt doesn't fight for its anemone home, it fishes from it, unfurling feathery nets to strain plankton, and snaps off a limb to escape if grabbed.
The electric blue and magenta gas float of a Portuguese man o' war at the surface.
Portuguese man o' war · Physalia physalisWhat looks like one animal is really a colony of tiny clones, none of which can live alone. Each is built for a single job: the purple gas float you see, the long stinging tentacles, the feeding mouths, and the breeding parts, all sailing the open ocean as one body.
A glowing, perfectly symmetrical glass dumbbell-shaped radiolarian skeleton under the microscope.
Radiolarian · Stylosphaera coronata laevis · single cellEach radiolarian is a single living cell that pulls silica from seawater and builds itself an ornate skeleton out of it, the same material as glass. When they die, the tiny glass shells sink and pack into deep-sea sediment hundreds of metres thick.
Star-shaped colonial diatoms radiating outward like a firework under the microscope.
Diatoms · light micrograph · ~20% of our oxygenDiatoms make about a fifth of the oxygen you breathe, roughly one breath in five, while drifting unseen in sunlit seawater.
A circular diatom shell forming a mandala of evenly spaced honeycomb pores.
Diatom shell · Stephanopyxis grunowiiIts pores are spaced so precisely that engineers study diatoms as natural blueprints for photonic crystals and nanoscale filters.
where the sunlight gives out

Into the Dark

A deep-sea barreleye fish with two glowing green tubular eyes visible through its clear head.
Barreleye · Macropinna microstomaThose glowing green orbs are its eyes, aimed straight up through a transparent forehead so fragile it is destroyed the moment the fish is hauled to the surface.
An orange-and-glass rocket-shaped siphonophore colony hanging in black water.
Siphonophore · Marrus orthocannaIt is not one animal but a swimming colony of identical clones, each built for one job; some of its relatives grow longer than a blue whale.
A pale segmented worm crowned with scarlet gill filaments on a black background.
Pompeii worm · Alvinella pompejana · tail near 176 FMaybe the most heat-tolerant animal alive. It lives in a papery tube it builds on the wall of a deep-sea hydrothermal vent, the towering mineral chimney called a black smoker: its tail bathes near 176 F down in the chimney while its head pokes out into 72 F water, a gap of about 100 F across a single body.
A close portrait of a leopard seal's reptilian head and wide, smiling jaw at the surface.
Leopard seal · Hydrurga leptonyxIt is one of the only seals ever known to kill a person, yet a wild one once spent four days trying to feed a National Geographic photographer instead, bringing him live penguins, then dead ones, apparently deciding the clumsy human was a hopeless hunter.
A pod of sperm whales hanging vertically in deep blue open water like floating monoliths.
Sperm whales · Physeter macrocephalus · largest brain on EarthThey sleep standing up in the water, in ten-minute naps, the least sleep of any mammal, and carry the largest brain of any animal that has ever lived.
A manta ray seen head-on with cephalic fins unfurled and mouth open into a dark oval, mid barrel-roll.
Giant manta ray · Mobula birostrisMantas have the largest brain-to-body ratio of any fish and have passed the mirror test for self-recognition, a club otherwise limited to apes, dolphins and elephants.
A great white shark at the surface with its jaws agape, ragged teeth fully exposed.
Great white shark · Carcharodon carchariasTagged off South Africa, a great white named Nicole swam the entire width of the Indian Ocean to Australia, about 11,000 km in 99 days, then turned around and was back home within nine months, the longest journey ever recorded for a shark.
A green sea turtle hanging in glassy blue water in a clean light gradient.
Green sea turtle · Chelonia mydasA green turtle feeding off the coast of Brazil will swim more than 2,000 km across the open Atlantic to Ascension Island, a speck of land a few miles wide, and nest on the very beach where she hatched decades earlier, often within a few kilometres of the exact spot.
the ocean is still mostly unmet

Discovered Just Now

A translucent, faintly glowing deep-sea slug with a billowing hood and a fringed tail in black water.
Described October 2024Mystery mollusc · Bathydevius caudactylusRobots filmed this glowing slug 157 times over 20 years before anyone could name it. Naming a new species means physically catching one to study its body and DNA, and this jelly-soft animal of the midnight zone was so hard to collect intact that the proof took two decades: a slug that left the seafloor to swim, catching its food in a billowing hood.
A pale gold wrasse with a bold inky spot near its tail.
Described 28 February 2024Tailspot wrasse · Halichoeres sancheziThe two fish here are the same species: (A) the red-headed adult male, and (B) a paler phase carrying the black blotch at the tail base, the 'tailspot' it is named for. It slipped past science until 2024 because it changes outfit as it grows, so the young, the females and the males look like completely different fish.
An electric pink, magenta and orange wrasse against deep blue twilight-reef water.
Described 8 March 2022Rose-veiled fairy wrasse · Cirrhilabrus finifenmaaThis is a comparison plate, not a single fish: it sets the rose-veiled fairy wrasse beside the near-identical species it had been mistaken for, each shown alive in full colour (the vivid panels) and again after preservation (the pale blue-grey panels), when the colour drains away. Its name, finifenmaa, means 'rose' in Dhivehi, and it is the first new species ever formally named by a Maldivian scientist, in their own language.
A ghostly gem-like comb jelly with two long trailing tentacles, lit by an ROV in the dark.
Described 18 November 2020Duobrachium comb jelly · Duobrachium sparksaeIt is the first animal ever named as a new species entirely from high-definition video, with no specimen caught; it drifts almost 4 km down like a tethered balloon.
A pale glass sponge on a thin stalk with a bulbous head and eye-like holes on bare deep seabed.
Described 9 July 2020E.T. sponge · Advhena magnificaIt looks like a jellyfish on a plant stem, but it is neither. It is a glass sponge, an animal whose body is the bulbous, eye-holed head, raised on a thin glassy stalk so it can sieve food from the passing current. Its name means magnificent alien, because it resembled E.T. phoning home so closely that scientists gave it a whole new genus, Advhena.
A neon pink-and-yellow striped fish in a dark crevice of a deep twilight reef.
Described 25 September 2018Aphrodite anthias · Tosanoides aphroditeHere the male glows in full neon colour (A) above a paler female (B). The fish so dazzled the divers who found it that they failed to notice a sixgill shark circling above them, and they named it for Aphrodite, the Greek goddess of beauty.

Sources

Every photograph here is public domain or Creative Commons, gathered from Wikimedia Commons, NOAA, MBARI, and other open archives. Each links to its source and license.