Here’s a prediction. In the next month or so, a viral video will start circulating around the internet, depicting a mysterious underwater… thing. It will be a gelatinous specter appearing in front of confused divers, or a blobby phantom floating before a remotely controlled submersible, or a deflated lump washing up on a random beach. It won’t conform to your mental image of any animal; it probably won’t look like an animal at all. You, like most other people, will be confused. Even scientists, the headlines will claim, will be baffled.  

Except Steven Haddock. He will not be baffled.

Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute, is one of the few scientists who study the midwater—the huge volume of ocean sandwiched between the sunlit surface and the dark bottom. There’s a common misperception that this region is empty, but Haddock says that it’s understudied not unpopulated. “It’s by far the largest habitat on earth, and there’s only a small sisterhood and brotherhood of people who look at the organisms living in that zone,” he says. In terms of team leaders, “there are probably fewer than ten of us in the world, and we all know each other,” adds Karen Osborn at the Smithsonian National Museum of Natural History.

Most of the weird blobby creatures that star in those viral videos live in this zone. And while they’re unfamiliar and unidentifiable to most researchers, people like Haddock and Osborn can almost always instantly recognize them—if not to the species, then at least to the broad group. “They’re our familiar friends, like chickadees at a backyard feeder,” says Haddock. You’d be surprised at the number of interesting things you can find if you jump into the water in the middle of nowhere.”

There’s Deepstaria, a jellyfish that caused a viral sensation when it was filmed floating past an oil rig. It lacks the pulsating bell and trailing tentacles of most jellyfish, and is instead a brown, amorphous sac covered in a network of veiny lines. Some thought it was a discarded whale placenta, but no, it really is a jellyfish.

Nanomia swimming through green dye.
(Jack Costello)

There are jellyfish relatives called siphonophores, which live in colonies that can grow up to 40 meters in length. The members are all clones that bud off from each other and stay connected. These individuals can perform different roles, much like the organs in your body. In one siphonophore called Nanomia bijuga, there are clones that capture food with stinging cells and others that propel the colony by jetting water. Even among the latter set, some clones specialize in thrust, while others steer.

(This specialization can become so extreme that scientists have sometimes mistaken parts of a siphonophore for an entirely new animal. That happened in 2014 when scientists described a mushroom-shaped creature called Dendrogramma. They claimed that it could represent an entirely new group of animals; others suggested that it could "completely reshape the tree of life.” Well, no. It’s a siphonophore. It’s probably a bract—a structure that doubles as a float and a protective covering for more delicate body parts.)

There are salps—barrel-shaped animals that eat by filtering particles from the water that they pump through their bodies. Like siphonophores, they can clone themselves into long, connected chains that stretch for meters, or that coil into loops like transparent cinnamon rolls. But unlike siphonophores, they don’t sting and they aren’t related to jellyfish. In fact, they’re part of a group called the tunicates, which are closely related to back-boned animals like ourselves. If you ever see a chain of salps, think of them less as aliens and more as family.  

Another group of tunicates, the pyrosomes, also form large clonal, filter-feeding colonies. Theirs are shaped like socks—tubes that are sealed at one end and open at the other. Some are smaller than thimbles; others are 60-foot-long windsocks. The colony-members all face outwards: they each suck in water, sieves it for food, and expel the rest into the central part of the sock. Those inward jets also propel the colony along, albeit very slowly. A pyrosome won’t swim up to you, but some are large enough to swim into—and at least one penguin has died in this way.

In the midwater, even familiar groups of animals can have strange representatives. Osborn studies polychaete worms, the group that includes common earthworms. But her subjects have an unearthly beauty, with bright colors and undulating paddles, “Most people could stare at them for hours,” she says. “They’re really mesmerizing.” Some are literally mesmerizing: they dazzle or distract approaching predators by releasing “bombs” of glowing chemicals from their heads.  

“These really are the familiar animals,” says Haddock. “They are things that we identify pretty quickly.” That familiarity is often buried in news reports that like to focus on mystery. For example, when Haddock was recently interviewed for a documentary on those viral oceanic blobs, “they kept asking me to hold off saying that we immediately knew what something was when we saw the videos.”

He can only think of a few genuinely baffling cases. In 2000, MBARI scientists discovered a weird floating blob that they billed as the “mystery mollusk”; they now know that it’s some kind of sea slug. There’s also a creature called Pseudusa that looks like a jellyfish, but is actually a relative of salps and pyrosomes. Instead of filtering detritus from the water, it actively catches its prey by snapping its disc-shaped body shut, much like a Venus fly trap.

Egg masses can be hard, too. Fish, squid, worms, and many other marine animals lay eggs in huge clusters. In 2006, Danna Staaf from Stanford University noticed a 3-meter wide “semi-transparent grey cloud” off the coast of California; it was the egg mass of a Humboldt squid, and it contained up to 2 million eggs. Last summer, Turkish divers filmed an even bigger another cluster of squid eggs, this one 22 meters across. And last spring, divers spotted a huge pink sock floating off the Philippines; it was originally identified as a pyrosome, but was more likely an egg mass from a diamond-shaped squid. Such blobs are clearly eggs to those in the know, but the mystery lies in figuring out who the parents were.

And just as midwater species can perplex scientists who specialize on the ocean floor, or benthos, the fauna of the floor can also stump midwater connoisseurs. This summer, scientists aboard the research ship E/V Nautilus spotted a purple blob off the coast of California. They had no idea what it was, but after bringing it back to their ship, they realized that it was a coiled-up sea slug. “I didn’t know what it was either, but I give myself a pass on the benthic organisms,” says Haddock.

Midwater animals are harder to study because they are soft and fragile. Eschewing hard shells or stiff skeletons, they tend to craft their bodies from gelatin, which is cheap to make and neutrally buoyant. That allows them to save energy on growth and movement, but it also means that they get easily shredded by nets and other collecting tools. “They’re like Jello that isn’t quite set,” says Haddock. “If you collect them with trawls, they don’t hold up.

To study these animals, biologists need to go into their world. That means using remotely operated vehicles, or ROVs, which are piloted from large ships. “You need the proper toys, and they’re expensive” says Osborn. “And imagine dragging a camera through a mall and trying to explain our society through that quick little view. It would be very pieced-together. That’s what it’s like to study the midwater. It can be a little frustrating.”

Equipped with suction nozzles and plastic cages, the ROVs can capture midwater animals and bring them back to ships for further study. But keeping such creatures alive at the surface is hard, and scientists often have little time to collect tissue samples and take photographs. “People who study the midwater pretty much all end up being photographers,” says Haddock. “These animals shrivel and become unrecognizable when you try to preserve them, so the photos become our way of documenting them.”

Collecting these creatures also tends to wipe out any clues about their interactions. “With nets, once things are jumbled in nets, you don’t know who’s eating whom or who’s interacting with whom,” says Osborn. There are animals that eat jellyfish, or ride on them. There are comb jellies that eat salps, grabbing one end of the colonial chains and then slurping them down one by one. These life-and-death struggles, these rivalries and alliances, are all but impossible to study unless you stumble across them.  

On that note, how do these animals stumble across each other? “There are animals that are so rare we’ll only see one in every 20 dives,” says Haddock. “But then we’ll see another one a few meters away. How are these things finding each other in the darkness? How do they ever reproduce?” There’s a squid called Planktiteuthis that looks like a Christmas tree, with a long, tapered tail, green fringes, and red tips—it’s rare, but Haddock recently saw two on a single dive. There’s an octopus that possibly eats jellyfish; Haddock has only seen three in 20 years, but he thinks they’re relatively common.

“It’s frustrating to me seeing research on some minute detail of how a monkey does something when we don’t know some really basic stuff of how these animals live, reproduce, function,” says Osborn. Even though the creatures of the midwater are more identifiable than viral videos would suggest, they still have mysteries to spare.