The Surprising Elegance of Midwater Sea Life

Imagine a hollow cube, measuring one foot on each side, suspended in the ocean at a depth roughly three football fields below the surface. From here, the seafloor is another 4,000 feet (or 1,200 meters) down. There is nothing to cling to or hide behind, only wide open space. The current flows very slowly, about 6 cm of water per second, which shakes out to about a mile and a quarter of seawater moving through the cube over a 24-hour period. At this depth, also called the Twilight Zone, light is scarce and everything looks dark to the human eye. What kinds of creatures would pass through the cube? What sea life can survive at this depth?

Karen Osborn, a curator and zoologist at the Smithsonian National Museum of Natural History, is investigating these questions through her collaborations with scientists at the Monterey Bay Aquarium Research Institute and the use of their remotely operated vehicles (ROVs). ROVs, underwater robots that are controlled from on board a ship, allow scientists to explore the deep, open water they refer to as the midwater. The midwater is the slice of ocean below the surface and above the seafloor. This vast habitat is filled with billions of animals, many unlike anything you’ve ever seen thriving in a place difficult to directly compare to other habitats.

To help make this comparison, Osborn teamed up with photographer and artist David Liittschwager, to examine what would pass through a one cubic foot frame—called a “biocube.” These “biocubes” give naturalists a standard way to compare the numbers and kinds of animals found in the midwater to those in other habitats. The biocube was suspended 300 meters (980 feet) below the ocean surface over the course of an April day off the coast of Central California. The team came prepared with 25 years’ worth of transect data at this location to determine what kind and how many animals they would see over 24 hours. They used the ROV’s high definition camera to watch the traffic of life commuting through the cube. If an animal was small enough, the team collected it with the ROV, brought to the surface and photographed it on board the ship. Several animals observed in the biocube were photographed with the ROV because of their large size and extremely delicate gelatinous bodies.

As in all habitats, most animals in the midwater are tiny—microscopic crustaceans like copepods and amphipods, worms, sea butterflies, and a myriad of other unusual species. Animals found in the midwater have evolved unearthly features and adaptations key to their survival in the harsh habitat.

Click through the gallery below to see 15 of Osborn’s favorite images—beautiful and strange animals that call the midwater their home.

Editor’s Note: With the help of Smithsonian scientists, photographer David Liittschwager is documenting what biodiversity can be photographed inside one cubic foot in a variety of habitats like coral reefs and the open ocean. Photography collaborator and midwater expert Karen Osborn provided an exclusive look at sea life living 300 meters below the surface of the ocean off the coast of Central California. You can learn more about biocubes in person at the exhibit “Life In One Cubic Foot” inside the Sant Ocean Hall at the National Museum of Natural History in Washington, D.C.

Aegina, a type of jellyfish, swims through the water.
Corolla moves through the water by flapping a wing plate nearly twice the size of its gelatinous shell, giving it the nickname “sea butterfly”.Known as “viperfish”, these cryptic fish use their giant teeth to prey on other fish and can grow to be 2 feet long (24 inches).Bathochordaeus stygius are filter feeders closely related to sea squirts in the phylum Chordata, the same phylum to which humans belong.Aegina citrea are stealth predators that push their tentacles ahead of their body or bell as they swim through the water.Chuniphyes are siphonophores—colonial organisms in the same phylum as jellyfish, working together to survive. Cystisoma (a type of hyperiid crustacean) has one huge pair of eyes.This female hyperiid (Phronima sedentaria), a type of marine crustacean, is surrounded by her young inside the hollowed out barrel-shaped body cavity of a salp. These shrimp-like crustaceans called Gnathophausia are incredibly hardy and only need to eat every few months. This young Gonatus squid is still translucent, making it tricky for predators to spot it.Like a typical jelly, the tentacles of Haliscera biglowi trail behind it as it moves, causing water flow to push small organisms into the path of the jelly’s deadly stinging grip.Hastigerinella digitata's many spines support sticky, hair-like “fishing lines” made from cytoplasm (the living part of a cell – mostly fluid, salts and proteins) that entangle prey.In this photo, the jellyfish Pandea rubra may have been surprised by a camera light or in a defensive state, causing it to contract its red mantle and reveal its complex muscle anatomy.Rhabdosoma’s uncommon eye arrangement allows the animal to see 360 degrees.Tomopteris's bristles produce yellow bioluminescence, a unusual color among midwater animals, nearly all of which make some sort of light.