Sneezing is far from a uniquely human behavior. Maybe you’ve seen your dog or cat do it, or you’ve seen a YouTube video of a giraffe sneezing on an unsuspecting toddler at the zoo. For sneezing, you don’t even need a nervous system, let alone a nose, and it goes 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 I know because it’s so old and it’s everywhere,” says Jasper de Goeij, marine ecologist at the University of Amsterdam. As filter feeders, sponges play a vital role in their aquatic ecosystems, attracting, processing and releasing water filled with varied organic matter as waste that organisms such as snails, brittle stars and tube worms feed on. “A sponge is actually an animal with many small mouths and one or more larger outlet openings,” says Dr. de Goeij. Those “little mouths” are called ostia, and the openings from which the water flows are oscula.
For years, scientists knew that sponges can regulate their water flow with a contraction of many minutes — i.e., a “sneeze” — but now Dr. de Goeij and colleagues found that sponges appear to sneeze as a form of self-cleaning, releasing waste particles into the mucus through their ostia. The work was published Wednesday in Current Biology.
The researchers encountered sponges sneezing snot while working on a project investigating the role sponges play in moving nutrients through a reef ecosystem. The work required Niklas Kornder, another marine ecologist in Amsterdam, to spend a lot of time with sponges. “I would just look at the surface of it all day long; it was pretty boring,” he recalls. (Mr. Kornder was diving in the Caribbean at the time.)
Fortunately, things got more interesting when he saw opaque fibrous material coming out of the sponges. “Then I’d come back to it later, and the stringy stuff would be gone,” he said.
To find out what those “stringy things” might be, the researchers captured time-lapse images of sponges, specifically the Caribbean tube sponge Aplysina archeri. In the lab, they were able to identify the wires as streams of slime containing waste. They would emerge from the sponge’s ostia, move across the organism’s surface and clump together into clumps that could be released with a sneeze, then quickly gobbled up by other ocean critters.
When she first viewed the time-lapse images, Yuki Esser – a bioinformatics student in Amsterdam at the time and co-author of the study – was disappointed, thinking that the movement she was seeing (i.e., the sneezing) was just a camera error. “I thought there must be a drop of water or something like that on the camera lens that caused this,” she said. But she soon realized it wasn’t a mistake. And when Ms. Esser and her colleagues discovered they had captured nearly identical time-lapse video of A. archeri off the coast of Curaçao, shooting images became “a kind of sport,” she said. “Like, ‘Maybe we sneezed on camera one more time!'”
The researchers believe that sneezing waste-laden mucus is a widespread tactic among sponges around the world. And the study raises more questions, said Sally Leys, an evolutionary biologist at the University of Alberta and a co-author of the study.
“The slime,” she said. “Is it similar to the slime of other animals? And which cells make it?” She also wants to know what is causing the sneezing. “If our noses are dripping, we’ll get the Kleenex out,” she said. “But how does a sponge know that now is the time to sneeze?”
Studying this slime could improve scientists’ understanding of how microbes, and possibly diseases, 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 is also struck by what this study can teach us about our own evolution.
“This could give us clues as to how early life evolved from these squishy mindless things to these complex organisms that build spaceships,” said Dr. Ushijima.
