It’s said that we know more about the surface of the moon than our own oceans, and the same may be true of the lifeforms that inhabit them. Cold seawater and unfathomable pressures create alien landscapes populated by creatures as strange as they are beautiful. They swim through a star-studded sea where, instead of supernovae exploding, the ominous glow of anglerfish lures blink on and off, portending death for a wayward fish; and instead of glimmering stars splashed across the sky, bioluminescent plankton turn the sea a brilliant, twinkling blue. Perhaps that's why we've spent millennia dreaming about what inhabits these underwater realms. Large, small, glowing or ghostly, the ocean’s animals have plied our literature with a never-ending cast of surreal characters. Tentacled Kraken, oceanic dinosaurs, bright-white Moby Dick, deadly Sirens and rainbow fish have led many a mind through imaginary seas to new worlds, or to the center of the Earth. But reality can be just as amazing as these facets of our shared mythology. The undersea creatures featured here are pulled from the work of photographer and marine biologist Alexander Semenov. They are just a few of the marine species that share this watery blue sphere with us. Though they burst with color and with life on these pages, most of these organisms will never be seen by a casual observer -- they're too small, live too far down, or are too well camouflaged. And some are already disappearing, casualties of a warming climate that’s threatening our oceans and the alien life they contain. Above: A Strange Worm This exquisite and intriguing creature is Pterosyllis finmarchica, a modern-day mystery. No one knows why it has those tendrils that look like tentacles, but we think this little guy is unbelievably beautiful. (The White Sea, Russia/Flickr)

A Worm-Snail Without a Shell The vermetid mollusk (Sepulorbis), a marine snail, lives inside a tube rather than a shell. Its hunting strategy is equally un-snaily. The mollusk catches prey by casting a sticky mucous web from the tube, which snares passing plankton and other critters. Then, the mollusk sucks the net and glued-on dinner back into the tube, where it enjoys its catch. (From the Great Barrier Reef, Australia/Flickr)

Slithering Sea Star What on Earth is this? Is it even alive? This tangle of sublime tendrils belongs to a basket star known as the Gorgon's Head, named after its resemblance to Medusa's scaly dreadlocks. Gorgonocelphaus arcticus is "the largest and most unusual ophiuroid in the White Sea," Semenov says (ophiroids are related to sea stars, but are creepier). Living at depths below 20 meters, this brittle star's central disk can be more than 10 centimeters across, with arms reaching more than 30 centimeters in length. The star looks like a roiling pit of snakes as it moves over the sea floor. (The White Sea, Russia/Flickr)

Cyanea Supernova Of all the critters in this gallery, Cyanea capillata -- the Lion's Mane jelly -- looks the most obviously astronomical. Its galactic doppelgänger, Tycho's supernova remnant, lies about 9,000 light-years away in the Milky Way. Above, Semenov has captured Cyanea from below, with its tentacles retracted as it floats. Below, a close-up of those beautiful tentacular curtains, which harbor the jelly's weaponized cells, called nematocysts. (The White Sea, Russia/Flickr/Flickr)

Head-Splitters Clione limacina, the Sea Angel, is a free-swimming mollusk that ditched its shell and replaced its foot with wings somewhere along the evolutionary path. Delicate and ravenous, these marine cherubs are fast swimmers. Their primary prey are sea butterflies (Limacina), which they catch using six pointy cones stored inside their otherwise seraphic heads. When it's time to eat, Clione heads open up and reveal the hexa-jaws of doom, which can inflate to be nearly half the length of the angel's body. Above, the transition from sea angel to Angel of Death. Below, a close-up of the Clione hunter. (The White Sea, Russia/Flickr/Flickr)

Sea Butterfly Limacina helicina, the sea butterfly, is a tiny mollusk that lives inside a dark shell and uses its "ears" as wings. Like its predator, the sea angel, Limacina has evolved wings in place of the normal mollusk foot. Unlike the sea angel, though, Limacina hunts with a big ball of secreted mucous, which it uses to catch tiny arthropods, bacteria and algae. Though it looks like a sea orchid, the sea butterfly's Russian name means "Sea Devil," and is derived from the shell's black coloration. (The Sea of Japan, Russia/Flickr)

Glow Worm This burrowing worm has a name that sounds like a dinosaur's -- Chaetopterus cautus -- and one of the most bizarre body shapes in this gallery, which at once resembles both an extraterrestrial flower and a spiny centipede. It also fluoresces. (The Sea of Japan, Russia/Flickr)

Alien Jelly This hydrozoan jelly looks a little bit like one of the alien invaders from Independence Day -- just much, much smaller and way more friendly. Bougainvillia superciliaris is only 1 centimeter long. It's an almost transparent, impossibly beautiful creature that spends most of its life hovering near the seafloor. (The White Sea, Russia/Flickr)

Scalloped Flatworm Brightly colored and with edges fluttering like flower petals, this flatworm is huge as far as flatworms go. Pseudoceros flowersi is about the size of your hand. (Great Barrier Reef, Australia/Flickr)

Burrowing Worm Alitta virens, a huge polychaete worm (like this one), that burrows beneath the sands of Arrakis (or lives on the seafloor). Hiding inside the worm are two sharp jaws that it can use to catch prey, in a frightening way: The worms shoots its pharynx -- a part of its throat -- at its quarry, clamps down, and retracts the whole sloppy mess. Above, a deceptively beautiful and friendly looking worm. Below, a closeup of the worm's incredible architecture. (The White Sea, Russia/Flickr/Flickr)

Monster Worm If you gazed deeply into the four eyes of Nereis virens, this is what would be staring back at you. A super closeup of this polychaete worm reveals the creature's multiple eyes, palps, and antennae. As is true for its cousin, Alitta virens, Nereis hunts by flinging its sharp-jawed pharynx (pictured below) at unfortunate prey. (The White Sea, Russia/Flickr/Flickr)

Shimmering Jelly Fast, beautiful, and nearly invisible except for red, fringe-like tentacles, Aglantha digitale are tiny hydrozoan jellies. The sausage-shaped things inside the critter are, appropriately, its gonads. (The White Sea, Russia/Flickr)

Colorful Cucumber This is not a slightly modified version of a spiny shell from Super Mario's universe. This is a sea cucumber (Aptostichopus japonicus). (The Sea of Japan, Russia/Flickr)

Fatal Flower A marvelous and deadly flower, Lucernaria quadricornis will paralyze you (if you're a small marine invertebrate) and pull you in for supper. Lucernaria are stalked jellies -- critters that, instead of swimming, are rooted to the seafloor by a stem-like foot. Don't let their graceful, cruciform tentacles fool you -- those tiny fingers store dozens of poisonous cnidocyte cells that immobilize prey with a gentle touch. (The White Sea, Russia/Flickr)

Twinkling Jelly Dryodora glandiformis, a tiny comb jelly with neon lights and long, sticky tentacles. (The White Sea, Russia/Flickr)

Crystalline Jelly This is a hydrozoan jelly with tentacles growing in the wrong direction. We think Aeginopsis laurenti looks just a little bit like the shrapwarden nasties from the classic Crystal Quest universe. The little orange blobs are parasites, which live inside some jellies and feed on them. Nice. (The White Sea, Russia/Flickr)

Moon Urchin You might think this sea urchin is looking at you, but you'd be wrong. That's not its eye. It's the other end of the urchin, from the Diadema genus. (The Red Sea, Egypt/Flickr)