Health under water: Even sponges sneeze

People sneeze, as do dogs, cats, and birds.

Health under water: Even sponges sneeze

People sneeze, as do dogs, cats, and birds. Few would have thought that sponges are also capable of doing this. But actually: Sponges contract their bodies and expel mucus into the surrounding water. And it's quite popular with other sea creatures.

Sponges are among the oldest multicellular organisms on earth and play an important role in the food cycle of many marine ecosystems. Researchers at the University of Amsterdam have now discovered that the Caribbean stovepipe fungus (Aplysina archery) uses a common mechanism to keep its internal filter system clean: it sneezes. Much to the benefit of other aquatic organisms that use the sneezed out mucus and the waste particles it contains as food. This is what the team of scientists describes in the journal "Current Biology".

Sneezing out mucus is probably one of the oldest ways for organisms to rid themselves of unwanted debris. However, this mechanism has remained largely unexplored in sponges. The animals were thought to passively flush out waste products with the flow of water through the main body opening, called the osculum.

However, some studies have observed that when sponges are exposed to heavy debris, they contract their bodies and secrete mucus to remove sediment from their bodies and protect their duct system from clogging. The mucus was not secreted through the osculum, but through smaller pores in the top layer of the sponges, the ostia. However, the exact mechanism behind this phenomenon was previously unclear.

That is why marine biologist Jasper de Goeij and his team investigated the little-explored ostia of the stovepipe sponge. The small openings in the body absorb water and food particles and then direct them via the canal system into the central cavity of the sponge. But just like other animals, sponges cannot do with everything that gets into their bodies: bacteria, viruses, plankton or sediments have to get out again. Video footage of A. archeri clearly shows that the species excretes waste products through its ostia in the form of mucus. This often creates a net-like, transparent layer of mucus on the surface of the sponge, in which foreign particles are trapped and transported away.

The mucus collects and clumps at the nodes of this network. Every three to eight hours, the researchers observed coordinated contractions and relaxation phases of the sponge surface, which coincided with the closing of the ostia and the shedding of the clots of mucus. The researchers also observed this "sneezing" in sponges of the Chelonaplysilla genus. They assume that the majority of all sponge species use such a sneeze mechanism.

But how do the sponges manage to transport the slime against the inflowing water flow? In other animal species, small eyelash-like structures, the cilia, usually ensure that the mucus flows in the right direction. However, the cilia of the stovepipe sponge appear to be largely immobile.

Without muscles and nerve cells, a sponge also sneezes much more slowly than, say, a human: unlike us, it takes about half an hour to sneeze. It is therefore reasonable to assume that the scientists have discovered a new type of sneezing mechanism. The periodic contraction and relaxation of the sponge followed by a sneeze could represent an early step in the evolution of muscle cells, they suspect.

Sneezing doesn't just help the sponge keep itself clean. Video recordings by the research team also show that secreted clumps of mucus and the waste products they contain are a source of food for various sea creatures such as fish or worms. The team's results not only shed light on the metabolism of one of the oldest animal families, but also suggest that sponges most likely play an even greater role in the underwater food cycle than previously thought.

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