Corals have evolved the ultimate communal lifestyle. Their structures are composed of thousands of individual polyps, each of which has budded from a nearby sibling. Uri Frank explains that the polyps of most species remain connected throughout their lives, eternally linked by structures that allow communication and cooperation within the commune. However, some species lose these tissue links, leaving each polyp to a solitary existence within the colony. But how are polyp individuals affected by this loss of contact? Frank,Yossi Loya, Itzchack Brickner and Uri Oren were curious to know if polyp clones still recognise siblings after severing the link and whether siblings still cooperate despite their loss of contact. Donning their scuba diving equipment, Frank, Oren and Brickner headed to the warm waters of the Red Sea to investigate the clonal coral Lobophyllia corymbosa(p. 1690).
First the team tethered intact polyps together to see whether or not they formed a bond. Frank explains that if the polyps fused they must be programmed to maintain a connection, but if they didn't, then they lacked the mechanisms to remain fused. However, after binding polyps from the same colony together for several weeks there was no sign that the individuals had attempted to form a connection; they are programmed to lose their links. But having become isolated, were individual clones still able to recognise their lost twins?
Descending again to the bottom of the sea, the team carefully removed sections from polyps and grafted them to sectioned polyps from their own colony. Returning 6 weeks later to see how the corals had fared, the team were pleased to see that the sectioned polyps had fused well with polyps from their own colony, but failed to connect with foreign polyps. The clones were able to recognise their siblings and accept the grafted tissue, while rejecting unrelated polyps.
Having found that coral polyps could recognise their siblings, the team wondered whether they could still cooperate, despite their physical isolation. Knowing that connected polyps could aid injured twins, Frank, Oren and Brickner decided to test whether the isolated polyps could help injured siblings too. Collecting intact polyps from the seabed, the team returned to the lab and allowed the polyps' symbiotic algae to photosynthesize in an aquarium supplied with radioactive carbon, incorporating the radiolabel into their metabolites before returning the `hot' polyps to their clusters. The team then caused a minor injury to one of the neighbouring polyps and then waited see if the `hot' individual came to its aid. Two days later the team returned to the polyp cluster to see if the injured neighbour had become radioactive and were amazed that it had. Not only were the polyps able to recognise siblings and communicate, but also they behaved as if they were still united, cooperating so that the injured polyp acquired its neighbour's radioactive label.
Frank admits that it was surprising that the individuals behaved as if they were still part of a united colony, and he is now curious to know how the polyps support their siblings in times of need.