A stroll in the sponge gardens.

By Arthur Güth – biologist @ Universidade de São Paulo (USP) - Brazil

The HyBIS dives have held a great deal of expectation for everyone on board. Geologists are waiting to see the features shown during multibeam survey and biologists are expecting to spot the life that inhabits these depths.

So, it is during these dives that we keep learning how the relief of the Rio Grande Rise shapes the distribution of the animals on it.

A view from a terrace of FeMn crusts covered in colonies of S. oculata. Here the currents are higher, bringing more food.

Typically, we see “gardens” of the branching sponge Sarostegia oculata at the edges of terraces and rocky outcrops where the currents pickup bringing more food. Like other sponges, these animals are filter feeders, living on small food particles floating in water. But what is the most interesting about them is the association with a zoanthid – a coral relative. These zoanthids, Thoracactis topsenti, are spread along the sponge probably benefiting from the body structure and habitat chosen by Sarostegia.

The robotic arm of the HyBIS sampling near these colonies. Notice also the dead colonies on the bottom

To the untrained eye, the sponge and its hitchhiking polyps look like a coral colony. The typical branching and the spacing between the polyps can fool even an experienced biologist. But what is most intriguing is that this kind of association between cnidarians and sponges is not found anywhere else but in the deep sea. And it is still not clear whether this relationship is mutualistic, with both animals benefiting mutually, or that the zoanthid simply finds a very nice place to settle and live.

 

A colony of Sarostegia oculata obtained from a dredge (left). Detail of the sponge and its associated Thoracactis topsenti polyps (right).