Lacking muscles or a nervous system, sponges are some of the simplest animals on the planet. Sitting quietly on the ocean floor, their peaceful lives are passed patiently filtering the passing currents for food. But take a closer look. Even these apparently static creatures are capable of movement,albeit at glacial speeds. And when Michael Nickel from Stuttgart University found a previously unidentified species of sponge in an aquarium at the Zoological Garden Wilhelma in Stuttgart, he realised that he had found an Olympian amongst sponges; the nameless species could attain a top speed of over 2 mm h-1. Curious to know how this new species, eventually christened Tethya wilhelma after the zoo, got around, Nickel rigged up a digital time-lapse camera to capture the creature's every move in an effort to understand the sponges'locomotion(p. 2839).

Nickel admits that capturing the animal's leisurely movements wasn't easy,as they only move in response to disturbance. Moving the animals to rest on three different surfaces, a piece of coral, plastic and glass, Nickel filmed the animals from above and the side, recording their slow progress once every 5 to 30 minutes over thousands of hours. Replaying the sponge's speeded-up progress, Nickel could clearly see the animal's pulsating movements, but realised that the animal was not moving by peristalis and that he needed to quantify the activity more precisely. Calculating the sponge's position from thousands of frames, Nickel reconstructed the animals' course and realised that they appeared to follow straight trajectories as they moved over the glass and plastic surfaces, averaging speeds between 0.186 mm h-1and 0.024 mm h-1. The sponge was also able to rotate, `changing direction instantly' says Nickel.

How the animals move is still a mystery, but Nickel suspects that they propel themselves by morphological changes in the cells that attach them to a surface. And how do the animals coordinate their progress without a nervous system? Nickel suggests that guide structures on the animals' surface may generate gradients of signalling molecules through the sponge's body and he is keen to learn more about the signalling molecules that keep the sponge on the straight and narrow.

References

Nickel, M. (
2006
). Like a `rolling stone':quantitative analysis of the body movement and skeletal dynamics of the sponge Tethya wilhelma.
J. Exp. Biol.
209
,
2839
-2846.