Sponges don't deserve their reputation as amorphous, uniform creatures,unable to show any coordinated behaviours. For Sally Leys from the University of Alberta, there is a lot more to sponges: `they are multi-cellular animals',she says, `and all multi-cellular animals must be able to coordinate behaviour'. When Leys and her colleague Glen Elliott looked at sponges expelling waste, they found that they coordinated contractions in their bodies to achieve this (p. 3736). `The intriguing thing is that they can do this without a nervous system', says Leys.
The team collected samples of the freshwater sponge Ephydatia muelleri and removed the asexual cysts, called gemmules, from their samples. They then placed the gemmules in growth medium so that they could grow small sponges for their experiments. Juvenile E. muelleri are small and see-through, so it is relatively easy to see what is going on. The small sponges `look like a garden tent, with a chimney coming out of the middle,' Leys explains. The `garden tent' contains a series of canals, tubes and bulges that all converge on a central chimney, which has an opening at the top called the osculum that lets waste out. The team grew the sponges either in the middle of a coverslip, so that they could see their overall movements,or sandwiched between two coverslips, which made it easier to measure how the different parts of the sponge moved to expel waste.
Using time-lapse imaging under the microscope the team stimulated sponges either with a drop of water soluble ink, or by shaking, and measured the changes in the diameter of the canals and pores as the sponges responded to the unpleasant stimuli. During the initial `inflation' phase of the response the canals inside the tent got bigger as they filled up with water, and the osculum at the top of the chimney was very small. During the next `plateau'phase the channels got bigger still, and this process lasted longer in bigger animals as there is more space to expand. During the final `contraction' phase the pores that let water into the sponge closed and the channels in the tent started to contract slowly from the outside in, the contractions increasing in rate towards the centre, filling the base of the chimney with water. For the final contraction, the canals and the osculum contracted, the tent lowered and all the water was quickly forced up the chimney as a package and out of the osculum at the top.
Examination of sponge samples under a fluorescence microscope showed that contractile actin filaments connected cells, which would allow waves of contraction to travel through the sponge. Because sponges don't have nerves,finding out what causes these coordinated contractions to happen is `the holy grail', says Leys. The team think that the most likely explanation is a wave of calcium ions travelling through the sponge, acting as a signalling molecule. Despite the many mysteries remaining, the team's results means that there is more to sponges than meets the eye; `we need to think about sponges differently', Leys says, `because they are animals, they act like animals, and have more animal characteristics than previously thought'.