Last week, corals all over the world "leaped" higher up on their skeletons. What was the special occasion? Just another full moon. My latest publication in the journal Coral Reefs shows that Porites corals build new sheets of skeleton, called “dissepiments”, each lunar month. Dissepiments are essentially the ladder rungs in the skeleton that the living coral polyps uses to climb. The living polyp rests on the upper-most dissepiment for one lunar month while building the rest of its skeleton, and then builds a new dissepiment during the full moon. Why the full moon? Many corals, including Porites, are perforate, meaning that the skeleton does not completely enclose each polyp. Rather, the polyps across the colony are connected, and thus their growth needs to be synchronized. It seems that the full moon provides a cue to synchronize the dissepiment-building process. One key outcome of this finding is that we can use dissepiments as monthly time markers in the skeleton. In the paper, we demonstrate using dissepiments that seasonal changes in skeletal extension rates are responsible for producing the annual density bands in Porites corals. Further, we can detect stressful events in the history of a colony based on anomalies in the spacing between consecutive dissepiments.
I am especially proud of this research because it required a lot of patience of foresight. Due to the nature of this study, it took my entire PhD to complete. That lunar rhythms exist in coral skeletal growth was not an entirely new idea and it had been hypothesized for decades that dissepiments form on a lunar basis. But this had not been tested in a formal way. Previous attempts to test the lunar rhythm hypothesis were somewhat ambiguous because they tracked dissepiments over just a couple months. We knew we needed a longer study. We designed a study in which we first stained living Porites colonies in Palau with a stain that is incorporated into the skeleton. And then we waited for the corals to build their dissepiments. We waited 6 months before we returned to collected the first skeletal samples. Then we waited another 15 months before we collected another set of samples. This allowed us to track the formation of dissepiments over multiple periods of time. All told, by the time I analyzed all the samples, 4 years had passed since I first started designing the study.
A nice commentary piece was recently published in JGR-Oceans that highlights not only the key findings of our recent study on Dongsha Atoll, but also the important questions that emerged from our analyses. Yuichiro Takeshita did a great job in explaining that while we have a good understanding how community metabolism affects seawater chemistry, we have much less understanding of how seawater chemistry feeds back to influence community metabolism. This is a key question that we need to address if we are to accurately predict the sensitivity of coral reef communities to ocean acidification.