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Sponge spicules. Credit: Van Soest et al, 2015. PLOS ONE

The Surprisingly Complicated Construction Work of Simple Sponges

Sponges are animals that do incredible impressions of inanimate objects. They have no nervous, digestive, or circulatory systems. They have no symmetry—no left or right, no front or back. And their bodies comprise just two layers of cells, sandwiching a jelly-like filling.

With such simple body plans, you might expect sponges to be flaccid and soft. In fact, they can be sturdy and tough, because their jelly-like middle is often full of microscopic pieces of hard minerals, known as spicules. They come in a beautiful range of shapes: spines, grappling hooks, jacks, pollen-like spheres, and coralline branches. The spicules act like an internal skeleton, giving the sponges shape and support.

Now, Noriko Funayama’s group at Kyoto University has discovered how sponges build this skeleton, and his findings reiterate an important lesson about these creatures: they may look simple, but looks can be deceiving.

The team studied a sponge called Ephydatia fluviatilis, which can be found growing on freshwater rocks and extending green, finger-like projections into the water. This species can reproduce without sex, by budding off a little packet of cells called a gemmule, which then develops into another sponge. As this happens, cells called sclerocytes start making long, spiny spicules.

The spicules aren’t created haphazardly. Instead, Funayama’s team found that the sponge lays them down in a precise and regimented way. When they are first produced, they lie almost flat. Then the sponge moves them horizontally, sometimes sending them to the opposite side of its body. It then hoists the spicules upwards and pushes them outwards. This pushes the sponge’s outer layer outwards too, so the creature becomes bigger and almost tent-like in shape. Eventually, the spicules are cemented in place.

The sponge then repeats this process, hoisting even more spicules up the ones that have already gone vertical. Some spicules from this next wave form horizontal crossbars that stabilise the structure; others remain vertical and expand the sponge’s body even further.

These steps are all carried out by different classes of cells. The sclerocytes manufacture the spicules. A separate guild of “transport cells” then fix themselves to the spicules and shuttle them around the sponge. The basopinacocytes cement the spicules in place. And no one knows which cells raise them up.

This division of labour is reminiscent of human construction crews. Each cell in a growing sponge has its own specific role, and work together to build lasting structures out of local materials.

But unlike human crews, sponge cells don’t have any architectural plans to draw from. Instead, they somehow organise themselves and their spicules, likely behaving by very simple rules that produce unnervingly complex behaviour. In this way, the sponges are perhaps closer to termites, which can build skyscraping mounds through the collective actions of many unthinking, specialised units.

Reference: Nakayama, Arima, Kawai, Mohri, Inui, Sugano, Koba, Tamada, Nakata, Kishimoto, Arai-Shindo, Kojima, Matsumoto, Fujimori, Agata & Funayama. 2015. Dynamic Transport and Cementation of Skeletal Elements Build Up the Pole-and-Beam Structured Skeleton of Sponges. Current Biology