Experiment of the month

In the Red Sea, stony corals such as Stylophora pistillata and Seriatopora hystrix grow relatively tall and slender when attached to horizontal surfaces but stouter when growing sideways from vertical rock walls or the flanks of sunken ships. This difference was thought to be largely an effect of light, which strikes the tops of upward-growing corals and the sides of horizontal ones.

However, Tel Aviv University biologist Efrat Meroz and colleagues suspected that gravity might also be at play. To test this idea, they forced young polyps (corals are colonies of individual polyps) to grow from either the walls or the bottom of an aquarium, with light coming either from above, or from the side, or from both directions. No matter what the light orientation, they found, the polyps that grew sideways ended up stouter than the others. The researchers also grew polyps in a centrifuge that spun constantly for one month. That experiment, which used centrifugal force to simulate hypergravity, showed that the greater the force exerted on the polyp, and the more sideways its direction, the stouter the polyp became. Thus, gravity can now be added to the list of factors--such as water currents and light--known to affect pattern formation in sessile marine invertebrates. ("The Effect of Gravity on Coral Morphology," Proceedings of the Royal Society of London B 269, 2002)