Don't judge a sponge by its cover

Finely tuned to their environment, sponges replace their cells daily, new study demonstrates.

Katie Willis - 29 November 2016

When it comes first impressions, sponges are much more complicated than meets the eye.

Contrary to popular belief, new research indicates that sponges are complex animals that use stem cells to replace their tissues. These findings fly in the face of previous assertions that sponges function more like large colonies of single-celled organisms than like other animals.

"Sponges are often thought to lack many of the features of more complex animals," explains Amanda Kahn, postdoctoral fellow at the University of Alberta in the Department of Biological Sciences. "What we found is that instead of dividing like a colony of cells, sponges replace their tissues using stem cells and the rate they do this depends on the type of sponge and where it lives."

Soaking up the implications

This has important implications for evolutionary biology.

Sponges are one of the first groups of multicellular animals to evolve so understanding how they work sheds light on how multicellular animals arose from a single-celled ancestor. In most animals cell division is finely controlled to prevent unwanted growth, like cancers. Kahn found that sponges also use stem cells to replace cells, implying that regulating how new tissues are formed is much more ancient than previously thought.

Their research has implications for ecology as well. Recent studies have suggested that reef dwelling species turnover cells so fast that a third of their body mass is replaced each day. But such a massive production and loss of tissues could be costly.

The study, conducted by then PhD student Kahn and Sally Leys, professor in the Department of Biological Sciences and renowned expert of sponge science and evolutionary biology, tested whether fast replacement of tissues was typical for colder water species taken from the lakes and ocean near the Bamfield Marine Sciences Centre in British Columbia, as well as deep-sea sponges from the Strait of Georgia.

"We studied the amount of energy that sponges put toward maintaining and replacing their cells to determine whether there were differences among species or depending on food availability, time of year, and temperature," says Kahn.

The results, she explains, gave a very different view of how sponges function.

Deep-sea sponges live where food is scarce, so new cells are only formed at the growing tip, rather than all over the body, to conserve energy. Further, sponges are able to slow cell replacement to conserve energy when food is scarce. When in a food-rich environment, on the other hand, sponges can put more energy into maintaining cells in their body.

"Whereas the previous thinking says that sponges have a constant and very high replacement rate for cells, our results show that cell replacement in different species is far from constant," explains Leys. "Cell replacement depends on the type of sponge, time of year and habitat. Most importantly, cell replacement is costly."

Next steps

So, what does this mean for the adaptive and efficient sponge?

"We want to tease apart the main factors that contribute to cost of living for a sponge," says Leys. "Understanding the different ways they make a living will help us understand what is important about their habitats-and allow us to preserve those better."

As for preservation, finding out ways to keep sponges happy is critical for other life on the ocean floor. "Sponges are extremely important in transferring food through ecosystems, because they filter so much water and excrete a massive amount out as wastes," explains Kahn.

The paper, "The role of cell replacement in benthic-pelagic coupling by suspension feeders," was published in Royal Society Open Science in November 2016.