Keeping up with the sponges: a microscope, a movie camera, and a laundry marker reveal sponges on the move

Natural History, Dec-Jan, 1997 by Calhoun Bond

Few creatures appear more stationary than a sponge. With no muscles, no nerves, and a body generally designed for low-flow filtration, a sponge seems so passive, so immobile, that one might easily forget it is an animal at all.

Anchored to a solid surface, a sponge feeds by filtering tiny particles from the water, which enters through the many pores that dot the animal's surface. Once inside, water travels through a system of internal canals, with specialized cells straining out food particles along the way. This canal system is supported by an internal skeleton made of spicules (bits of silica or calcium carbonate) embedded a tough protein known as spongin.

This simple design is appropriate for an animal with stay-at-home habits, and most textbooks describe sponges as completely sessile (permanently attached to the surface they "sit" on). Over the years that I have studied these animals, however, I have learned that the sedentary sponge is not so stuck as it seems. In fact, when the need arises, sponges can do what many other coral-reef inhabitants appear unable to do -- get up and go.

My involvement with sponges began while I was a graduate student at the University of North Carolina at Chapel Hill. As a biologist embarking on studies of animal development and design, I was first attracted to sponges by their reputation for simplicity and by the similarity between their cells and structures and certain cells and tissues found in more complex creatures. Also, in the laboratory, sponges can be induced to form transparent sheets thin enough to fit an intact, living animal under a microscope.

To gather specimens for my research, I ventured into the streams and ponds near Chapel Hill and collected small, freshwater sponges. Using time-lapse microscopes belonging to my adviser, Albert Harris, I began taking films of the sponges and soon saw things that contradicted the traditional view of sponges: the little creatures were slowly crawling across the bottom of their containers.

Both Harris and I were curious about whether other sponges could move about like my little freshwater subjects. Seizing the opportunity to spend time in warmer waters than those of central North Carolina, I headed toward the coast and obtained various species of marine sponges that range from Bermuda to the Caribbean. To my delight, these larger sponges also moved -- albeit slowly -- when placed in saltwater tanks.

Within one to four weeks after being placed in the aquariums, the sponges formed flattened protrusions. When these protrusions -- which became the leading margins of the sponge -- contacted a glass wall, they began to move along the glass, appearing to drag the rest of the animal behind. (Sponges usually climbed up the walls, giving the impression that they were trying to escape. More likely, the part of the sponge closest to the top of the tank was probably exposed to more water flow and thus more nutrients. As a result, this part of the sponge may have been healthier and better able to pun the rest of the animal along with it.)

Writing on the tank with a laundry marker, I periodically traced the sponges' progress along the aquarium glass. On average, the sponges traveled about two millimeters a day. Of the five species I measured, the champion speedster was the attractive lavender sponge Haliclona loosanoffi, which began moving a few days after capture and regularly clocked speeds in excess of four millimeters a day. Haliclona also had the intriguing habit of leaving behind large portions of its old skeleton as it moved.

Neither I nor Harris had ever seen or heard of anything like these crawling sponges before, but soon afterward, a colleague informed us of a 1949 paper that also reported on sponge locomotion. The author of this paper, Maurice Burton, was a longtime keeper (curator) at the British Museum in charge of the museum's large sponge collection (among other invertebrates). Burton had caused a small sensation in the press in 1933 by reporting at a meeting of the Zoological Society of London that some of the living sponges under his care were slowly moving around in their aquariums. His report inspired the British satirical magazine Punch to publish a letter about "Fluffy," a pet sponge who allegedly migrated from his owner's bathroom to a pail of water in the garage. Burton was invited to publish his results but demurred, thinking his findings weren't conclusive enough. (Perhaps he was also wary of more publicity.)

After World War II, Burton corroborated his aquarium findings with field studies. He scoured thousands of square feet along the rocky coast of southeast England and observed thousands of sponges in tidal pools and sea caves. His 1949 paper, published in the Proceedings of the Zoological Society of London, contained detailed observations of these sponges and numerous sketches depicting how the animals continually changed shape and moved. Burton writes of the "awkward and cramped positions" he endured while sketching his subjects, but he makes no mention of snorkel or scuba equipment. I picture him scrambling around treacherous, slippery rocks, just behind the receding tide, and sticking his head and body into submerged holes in an effort to catch sponges in the act of moving.