How the sponge stays slim
One species' rapid cell shedding explains its huge carbon-catching capacity.
Biologists have discovered how a reef-dwelling species of sponge can filter enormous amounts of carbon without growing in size.
The sponge Halisarca caerulea can absorb up to two-thirds of its own weight in carbon each day by shedding cells at a rapid rate, according to research by Jasper De Goeij at the Royal Netherlands Institute for Sea Research in Texel, the Netherlands, and his colleagues. In a study published in the Journal of Experimental Biology1, the team proposes that this fast turnover may be a tactic for coping with the toxins and pathogens that are frequently encountered by the filter-feeders in their environment.
H. caerulea is found on tropical reefs in the waters of the Caribbean, and feeds by filtering carbon-rich organic matter from the water. Earlier work by De Goeij had shown that the sponges could absorb prodigious amounts of carbon yet didn't seem to grow in size. "We calculated that a sponge should double in biomass every three days, but found that H. caerulea hardly grows at all," says De Goeij.
To find out why, De Goeij and his colleagues collected growing sponges and stained them continuously with a chemical that labels actively dividing cells. The team cut samples from the sponge bodies at different time points up to 10 hours after the first exposure and analysed them for signs of the labelling agent.
They found that certain cells that pump and filter water in the sponge divided very quickly — once every five hours or so1. But adding a second chemical to search for dead cells revealed only a few in the chambers, which left them with a puzzle: the sponge wasn't growing, but the cells didn't seem to be dying off.
De Goeij was stumped until he gave a presentation on the sponge's impressive cell division to a group of cancer researchers. They immediately recognized the rapid division behaviour as being very similar to that of the human gastrointestinal tract. Cells in the epithelium of the colon are replaced every 12–24 hours by rapid proliferation and then shedding. "Once we looked at the sponge tissue with the human colon in mind, we found massive amounts of shedding," says De Goeij.
Other marine biologists are surprised by the find. "Nobody has looked at cell sloughing in sponges like this before," says Malcolm Hill, a marine ecologist who specializes in sponge evolution at the University of Richmond in Virginia.
De Goeij and his colleagues suggest that the rapid growth and shedding of the cells may be an evolutionary response to the harsh conditions in which H. caerulea grows. To survive in the nutrient-sparse Caribbean Sea, the sponge must filter vast quantities of water, which is likely to increase its exposure to toxins and pathogens. Rapid shedding might prevent such problematic materials from building up and damaging the filtering system, De Goeij says.
"Now that we know this, we need to look at other sponges to see whether this is a general phenomenon or just an idiosyncrasy of this particular species," says molecular palaeobiologist Gert Wörheide of Ludwig-Maximilians-University in Munich.
Hill believes that sponge biology could reveal useful information that is relevant to human diseases. "If this proves to be more than just the behaviour of one odd species, then studying the ways that sponges control cell growth could provide insight into controlling abnormal cell growth like cancer," he says.
- De Goeij, J. M. et al. J. Exp. Biol. 212, 3892-3900 (2009).