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The weird and wonderful world of the ocean twilight zone

Updated 10:44 AM ET, Fri July 2, 2021
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Plunge into the ocean twilight zone and you'll encounter an array of weird and wonderful species like nothing else found on earth. Scroll through the gallery to explore...

Phronima -- A small, translucent crustacean that turns its victims into its home. The phronima will skewer salps -- a gelatinous invertebrate -- and hollow out their insides, before climbing in. Female phronima lay their eggs inside before climbing out and pushing the remaining carcass around, lending it the nickname "pram bug."
Paul Caiger/Woods Hole Oceanographic Institution
Bean's bigscale -- This fish lives towards the bottom of the twilight zone, and has been found as far down as 4,000 meters (13,000 feet) beneath the surface. No looker, those bumps on its head are mucus-filled cavities. Known for its curious "rowing" style motion through the water, relying on its pectoral fins. Paul Caiger/Woods Hole Oceanographic Institution
Glass squid -- Glass squid are filled with ammonium chloride, a solution lighter than seawater, allowing them to float through the ocean in search of food and mates. Born in the surface ocean, they grow to full size at fourth months and will enter the twilight zone. Paul Caiger/Woods Hole Oceanographic Institution
Anguilliform leptocephalus -- Leptocephalus are tiny and grown up to become glass eels. Despite their size (sometimes less than 5 mm), these skeletal creatures still partake in the nightly vertical migration into shallower waters -- the larger the leptocephalus, the longer the migration. Paul Caiger/Woods Hole Oceanographic Institution
Barbeled dragonfish -- A fish with a fearsome mouth, the barbeled dragonfish has fangs embedded with nanocrystals that make its bite stronger than a shark, according to the Woods Hole Oceanographic Institution (WHOI). Females have a bioluminescent lure dangling from their chin. Grows to up to 20 inches (51 cm). Paul Caiger/Woods Hole Oceanogra
Strawberry squid -- Two eyes are better than one -- particularly in the case of the strawberry squid, whose eyes are different sizes. Its larger yellow eye is super-sensitive and picks out prey in waters above, while its smaller blue eye looks below for signs of bioluminescent fauna it will also eat.

Paul Caiger/Woods Hole Oceanogra
Slender snipe eel -- The snipe eel has a whole lot of backbone, with 750 vertebrae -- the most of any species on earth according to the WHOI. Their large eyes make up for a lack of bioluminescence when hunting for prey. Paul Caiger/Woods Hole Oceanographic Institution
Fangtooth -- According to the WHOI the fangtooth has the highest tooth to body ratio of any fish in the ocean. In fact, it's able to catch fish bigger than itself. Sockets in the roof of its mouth prevent the fish's teeth from puncturing its own brain in the process. Paul Caiger/Woods Hole Oceanogra
Hyperiid Amphipod (Pegohyperia princeps) -- This crustacean is a parasite to salps and other gelatinous creatures, and grow to just over one centimeter. Paul Caiger/Woods Hole Oceanogra
Transparent hatchetfish (Sternoptyx diaphana) -- The hatchetfish has evolved so that it has pale blue lights on its underside, camouflaging it against the surface from any predators loitering below. And with eyes tilted upwards, it's ready to prey on creatures above it. Some species are known to swim up from 1,500 meters (5,000 feet) into shallow waters to feed at night. Paul Caiger/Woods Hole Oceanogra
Helmet jellyfish -- Brainless and eyeless, the helmet jellyfish negotiates its surroundings thanks to a sensory "bulb." The pigment giving its red tones is damaged by sunlight, so the sensor tells the creature when it should retreat into the darkness of the ocean twilight zone.
Paul Caiger/Woods Hole Oceanographic Institution
Giant ostracod -- Although giant in name and more than 30 times the size of the average ostracod, this species is still only around an inch long. It has a slit-like mouth from which feathery antennae extend, which the ostracod uses to swim, feed and sense its surroundings. Paul Caiger/Woods Hole Oceanographic Institution
Copepods -- This crustacean's rowing-like movement lends it its name -- copepod means "oar-feet" in Latin. It migrates up and down in the ocean by adjusting the density of fats in its body, staying low and out of predators' paths during the day.
Paul Caiger/Woods Hole Oceanographic Institution
Bristlemouth -- There may be as many as a quadrillion (1,000 trillion) of these small fish in the ocean, making it the most abundant vertebrate on Earth. Bristlemouths do not take part in the nightly migration to the surface ocean. A clue as to why might be their swim bladders: while those creatures that make the migration have swim-bladders filled with air, the bristlemouth's is filled with fat. Paul Caiger/Woods Hole Oceanographic Institution
Acanthephyra sp. -- Acanthephyra is a genus of shrimp known for their vibrant color and bioluminescent properties. Paul Caiger/Woods Hole Oceanographic Institution
Pteropod -- Shelled varieties of these graceful creatures are known as "sea butterflies," while shell-less species are dubbed "sea angels." Pteropods take part in the nightly migration into the surface ocean and feed by spreading a mucous net around them, trapping plankton and other particles.

Paul Caiger/Woods Hole Oceanogra
Siphonophore -- Siphonophores are a colony comprised of connected "zooids." These form rope-like chains that can be longer than a blue whale and dangle tentacles that sting and trap their prey. Paul Caiger/Woods Hole Oceanographic Institution
Ctenophore -- These comb-like creatures are bioluminescent and move around via eight rows of cilia (narrow eyelash-like filaments), which "beat" in synchrony. Paul Caiger/Woods Hole Oceanographic Institution