How sponges get rid of annoying mucus

by time news

Hatchi! If there is dust or pepper in the air, a strong sneeze can have a relieving effect. Apparently, it’s not just us humans that feel this way: sponges also use a similar mechanism to clean their delicate filter system. They are further away from us in the family tree of life than any other animal. They differ from the rest of the fauna by a very simple structure. Although they have differently specialized cells, they do not have cells for sensory perception, let alone stimulus transmission. Sponge cells can also crawl around independently.

Most sponges specialize in fishing tiny food particles out of the water, such as bacteria and viruses. Their highly efficient filter systems consist of small chambers lined with collared flagella cells. Their flexible flagella drive water through the sponge, in through very small pores and out again through much larger openings. The collar of these cells filters nutrients out of the water.

According to conventional wisdom, sponges are criss-crossed by one-way streets. Remains of the metabolism and indigestible things would therefore always have to be transported through a narrow canal system to the outlet. But how can blockages be avoided in this way? During a research stay on Curaçao, Niklas A. Kornder and Yuki Esser from the University of Amsterdam discovered that at least some sponges can use their inflow openings instead of the outflow openings for waste disposal. In coastal reefs, they studied the stovepipe sponge (Aplysina archeri), which is native to the entire Caribbean. Stately specimens of this horn-silica sponge attract attention with their vertical, meter-long tubes. For laboratory studies, however, the Dutch marine biologists chose smaller ones that they could house temporarily in aquariums.

Since sponges tend to act more slowly due to the lack of nerve and muscle cells, it was filmed with time lapse. In the field as well as in the laboratory, it was possible to document how small particles appear at the inflow openings and are taken along by slime tracks gliding over them. It is still unclear how the sponge keeps these conveyor belts going. But their speed could be measured: on average, it was two thousandths of a millimeter per second. Thanks to detailed image analysis together with six experienced colleagues, Kornder and Esser also discovered that the slime bands converge on bumps on the sponge’s surface. There, the particles enclosed in it clump together to form more or less long strands.

Cleaning mechanism is similar to that of our airways

The research team reports in the journal Current Biology how the stovepipe sponge finally gets rid of its waste: At intervals of three to eight hours, relatively brisk contraction waves run across the surface of the sponge at speeds of up to a tenth of a millimeter per second. The inflow openings close in a locally coordinated manner in such a way that they literally expel the debris packed in slime.

Presumably, indigestible substances are already being transported inside the sponge on slime bands – against the flow direction of the water. In principle, the stovepipe sponge would use a mechanism for self-cleaning that is very similar to that of our respiratory tract. In this case, it is the epithelial cells with their cilia that constantly sluice the dust particles trapped in the mucus in the direction of the exit.

Handling waste disposal via the inflow openings appears particularly advantageous for the stovepipe sponge. Because an outflow opening placed high up guarantees that thoroughly filtered water does not run through the sponge a second time. However, entrained waste particles would often sink to the bottom of the long tube before they reached the exit at the top end. If sponges transport indigestible matter out of the inflow openings, they should also be able to cope better with swirled-up sediments: what enters with the water can be sent back again immediately. This prevents a blockage in the canal system.

The stovepipe sponge is not the only sponge that transports indigestible particles to the outside through the inflow openings and gets rid of them with a kind of sneeze. Kornder and colleagues also observed such behavior in sponges of the genus Chelonaplysilla, whose outflow openings do not protrude as tubes. However, these sponges show shorter cleaning cycles and the slimy conveyor belts are weaker. How widespread the newly discovered method of garbage disposal is in the group of animals with the longest evolutionary history remains an open question.

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