These animals build palaces out of their own snot

Every animal has its own idea of the perfect home. For the giant larvacean, that home is pretty darn weird—and new research shows that their slimy homes are even wackier than we thought. With their bulbous heads and flat tails, these gelatinous invertebrates look like big, ghostly tadpoles. Though they grow to reach the size of a breakfast muffin, they are usually surrounded by self-constructed “snot palaces” as big as 1 meter in length.

Now, new insight published Wednesday in Nature shows new structural details hidden in all the goop. Scientists mounted laser technology to remotely-operated submersible vehicles, launching them into the ocean 400 meters down to scan those delicate homes.By digitally reconstructing what they observed, the scientists found giant larvacean snot palaces have two nested structures: an inner house and an outer house.

“One way to think about it is like the brain and then the skull surrounding the brain,” says Kakani Katija, a principal engineer at the Monterey Bay Aquarium Research Institute (MBARI) who led the research. The outer house protects them from predators—making them harder to spot while providing a squishy, protective barrier—but also as a primary water filter, she says. It surrounds the inner house, which collects all the food away from the water, and they are separated by a small inlet channel. Katija says the whole thing is an elaborate feeding apparatus.

What’s particularly awe-inducing is how these larvaceans construct such intricate structures. Before they build their homes, larvaceans have cells lining their heads that only exist to secrete mucus. Then, all in one go, they release that mucus and inflate it like “a bouncy castle,” Katija says.

“Think of a balloon that hasn’t been blown up,” she continues. The whole mucus house is secreted at once, flat and formless, and then blown up in less than an hour to be fully inflated and operational. “It’s pretty incredible to watch,” she says, especially since larvaceans have no appendages to work with. “A spider has eight legs to build and lay down material for its web. This animal really just has a head and a tail.”

As intricate as these jelly castles are, they aren’t built to last. Within just one or two days, the filters eventually clog and the larvaceans abandon ship, promptly setting off to make a new palace. The discarded house, stuffed-up with nutrients sinks down to the ocean floor, where it nourishes the ocean’s bottom-feeders.

While larvacean mucus houses have been observed as early as the 1960s, scientists have had a hard time getting a critical look. These gelatinous structures are so delicate that you can’t capture them and drag them up to the surface the same way you would with other creatures. The only way to get an accurate look at these animals is to watch them where they live—a logistical and technological challenge.

MBARI overcame this barrier with the DeepPIV, a scanning instrument that uses laser technology developed by Katija and her team. DeepPIV emits a laser “sheet”—gelatinous material lights up when hit by the laser, allowing the scientists to note how the larvacean and its house are positioned. Every time the instrument was repositioned, the team saw a new two-dimensional “slice” of a larvacean house. By scanning the laser back and forth and compiling each slice of information, the team was able to non-invasively visualize the animals while digitally reconstructing a three-dimensional model.

How exactly these strange ocean animals manage to build their mucus houses in their small, simple bodies is still a mystery, but Katija knows there is a lot to learn. “Larvaceans can filter a wide range of particle sizes… and they can do this at a very high efficiency, like 95-99%. How well do our own engineering systems do in that regard?” Katija says. “Is there some feature or some mechanism that these mucus structures are using?”

Katija hopes that as they continue their discoveries, these structures will inspire future filtration technology or medical applications. There are so many captivating animals from the ocean’s midwaters that are understudied, she says. Now that there’s a way for them to reconstruct their body shapes, it opens up so many more opportunities for understanding them. ”That’s really what we’re hoping for when we illuminate something novel in the deep sea.”