Despite having no nerves or muscles, sea sponges slowly contract to squeeze sand and debris out of the openings they feed through
10 August 2022
Sea sponges “sneeze” in slow motion to get rid of the sand and pollutants that they suck into their bodies, and the expelled mucus may be an important food source for other marine organisms.
Taking up to 50 minutes to complete, sponge sneezes use contractions to propel unwanted substances back out the same hole they entered, in long, sticky strings of mucus. These regular releases of “fresh biomass” may help explain why fish and crustaceans live in or near sea sponges, says Niklas Kornder at the University of Amsterdam.
Sponges suck in water and dissolved nutrients, like sugars, through small openings called ostia. Their internal filter system is believed to trap debris and expel it out of larger openings towards the top of the sponge, called oscula. But on a diving trip in the Caribbean Sea, Kornder noticed mucus covering the ostia of a few stove-pipe sponges (Aplysina archeri) — and then, passing by those same sponges an hour later, he saw that the mucus had disappeared.
Curious about what was happening, Kornder and his colleagues collected several stove-pipe sponges off the island of Curaҫao and recorded them in their laboratory over 24 hours using microscopic time-lapse video. They also filmed another stove-pipe sponge still in the sea, and studied a video of a Chelonaplysilla sponge filmed by a TV production company in Australia.
Each video showed particle-filled mucus travelling out of the ostia, against the direction of incoming water. At the surface, the mucus followed a network of “mucus highways” – white lines, probably made of collagen, intersecting at numerous junctions where the mucus would start to form globs. Then, the sponge would contract and the stringy clumps of mucus would release into the water.
The team’s analysis showed that trapped sediments made up 81 per cent of the weight of these mucus clumps, suggesting that sneezes probably help prevent the internal filters from getting clogged.
The remaining 19 per cent appeared to make good feeding material for other animals, says Kornder. The mucus contained 45 per cent more carbon and nitrogen than other kinds of natural waste in the nearby water, suggesting it was nourishing. The team observed shrimp and other small crustaceans eating the mucus, and even some small fish looking as though they were interested in it.
“Each time a sponge sneezes, there’s this whole resource that’s now available to these other organisms,” says Kornder. “And that might actually create some of the astonishing diversity we see on these very beautiful reefs, with their very complex ecosystems.”
At an average speed of 2 millionths of a metre per second, these sneezes may be so slow because sponges lack both muscle cells and nerve cells, Kornder says. However, they have primitive versions of such structures, including chemical signalling from one cell to the next, that generate contractions.
Journal reference: Current Biology , DOI: 10.1016/j.cub.2022.07.017
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