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Sneeze by Sneeze, Sponges Fill the Seas With Their Mucus

Sneezing is far from a uniquely human behavior. Maybe you’ve seen your dog or cat do it, or watched a YouTube video of a giraffe sneezing on an unsuspecting toddler at the zoo. In fact, sneezing doesn’t even require a nervous system, let alone a nose, and dates back to some of the first multicellular animals: sponges.

The sponge has been around for at least 600 million years. “It’s the most successful animal that I know of, because it’s so old, and it’s everywhere,” said Jasper de Goeij, a marine ecologist at the University of Amsterdam. As filter feeders, sponges play a crucial role in their aquatic ecosystems, drawing in water filled with varied organic matter, processing it and releasing it as waste on which organisms like snails, brittle stars and tube worms feed. “A sponge is basically an animal that has a lot of little mouths and one, or several, larger outflow openings,” said Dr. de Goeij. Those “little mouths” are called ostia, and the openings where water flows out are oscula.

For years, scientists have known that sponges can regulate their water flow with a many-minutes-long body contraction — i.e., a “sneeze” — but now, Dr. de Goeij and colleagues have found that sponges appear to sneeze as a form of self-cleaning, releasing waste particles in mucus through their ostia. The work was published in Current Biology on Wednesday.

The researchers came across sponges sneezing snot while working on a project investigating the role played by sponges in moving nutrients through a reef ecosystem. The work required Niklas Kornder, another marine ecologist at Amsterdam, to spend a lot of time with sponges. “I would spend entire days just looking at the surface of them; it was quite boring,” he recalled. (Mr. Kornder was scuba diving in the Caribbean at the time.)

Fortunately, things got more interesting when he started seeing opaque stringy material coming from the sponges. “Then I’d come back to it later, and the stringy things would be gone,” he said.

To figure out what those “stringy things” could be, the researchers recorded time-lapse footage of sponges, specifically the Caribbean tube sponge Aplysina archeri. In the lab, they were able to identify the threads as streams of mucus carrying waste. They would come out of the sponge’s ostia, move across the organism’s surface and aggregate into clumps that could be released with a sneeze, and then quickly gobbled up by other ocean critters.

When first reviewing the time-lapse footage, Yuki Esser — a bioinformatics graduate student at Amsterdam at the time and a study co-author — was disappointed, thinking that the movement she was seeing (i.e., the sneeze) was just a camera focusing error. “I thought there must be a drop of water or something on the camera lens causing this,” she said. But she soon realized it wasn’t a mistake. And once Ms. Esser and her colleagues found they had captured nearly identical time-lapse video of A. archeri off the coast of Curaçao, recording footage “became kind of a sport,” she said. “Like, ‘Maybe we caught another sneeze on camera!’”

The researchers believe sneezing out waste-laden mucus is a widespread tactic among sponges all over the world. And the study stirs up more questions, said Sally Leys, an evolutionary biologist at the University of Alberta and a co-author of the study.

“The mucus,” she said. “Is it similar to other animals’ mucus? And what cells are making it?” She also wants to know what triggers the sneeze. “When our nose is dripping, we bring the Kleenex out,” she said. “But how does a sponge know that this is the moment to sneeze?”

Studying this mucus might improve scientists’ understanding of how microbes, and possibly disease, are transmitted in reef ecosystems, said Blake Ushijima, who studies corals at the University of North Carolina Wilmington and was not involved in the new research. He’s also struck by what this study could teach us about our own evolution.

“This could give us hints of how early life evolved from these squishy brainless things into these complex organisms building spaceships,” Dr. Ushijima said.

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