Sunday, January 11, 2015

Coral time-stamps

The clock started ticking on April 27, 2013. That's when Hannah and I put "time-stamps" on twenty coral colonies around Palau.
Staining a coral colony in Palau in April, 2013. 
Photo credit: Hannah Barkley

To a diver's eye, only the living surface of a coral colony is visible. But the living tissue is only a centimeter thick or less, just a thin veneer on top of the massive calcium carbonate skeleton below. In Palau, we are studying how quickly corals are able to build their skeletons, particularly in the relatively low pH regions of the Rock Islands. A typical technique to measure coral growth rates is derived from the annual high- and low-density bands that corals form in their skeletons. Like tree rings, we can count back the bands to determine the age and rate of skeletal extension. To be careful, though, we need a way to ground-truth the annual banding pattern, to make sure we are getting the right answer.

Perhaps the most direct method to measure coral growth rate is based on coral "staining". We cover a living coral colony with a plastic bag for two hours, and add a harmless red vegetable dye into the bag. This does no harm to the coral, and the red dye is actually incorporated into the skeleton. This serves as our "time-stamp" because we know the exact day that the dye was introduced.
Tom drilling a coral colony in Taiwan. Photo
credit: Anne Cohen

Two days ago, we collected short cores of skeleton from these colonies. The stain line is now well below the surface because the coral has built skeleton on top of it over the past 20 months. We know the stain line is April 27, 2013 and we know the top of the core is January 10, 2015. By measuring the distance from the the stain line to the top of the core, we can easily calculate the growth rate. This allows us to ground-truth our density banding estimates and to directly compare coral growth rates around Palau.

But we can learn even more from our staining experiment. The living coral tissue rests atop of thin, horizontal layer of skeleton, called a dissepiment. It's thought that the full moon serves as a cue to all the polyps of a colony to build a new dissepiment at the same time. This way, the dissepiments serve like rungs on a ladder, one formed each lunar month as the coral tissue climbs up and up. But the lunar timing of dissepiments has never been tested experimentally.
Two cores collected from stained coral colonies in Palau. The
pink lines are the stain lines, which were incorporated on
April 27, 2013. The thin green layer at the top is the coral
tissue layer. Each core is about 4 centimeters long. In this
case, one of these corals grew about twice as fast as the other.
Photo credit: Tom DeCarlo

We know that there were 21 full moons between our initial staining and our core collection. Does this match with the number of dissepiments? We will bring these cores back to our lab in Woods Hole, cut sections from the cores, and carefully count the dessipiments under a microscope. This will tell us whether or not dissepiment formation is consistent with timing of the lunar month.

- Tom DeCarlo

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