Classification, Distribution and Morphology of Caribbean Reefs

What is it about?

Darwin's theory of how coral reefs transform from Fringing reefs, to Barrier reefs to Atolls, is now a classic example of deductive reasoning solving the riddle of how simple vertical reef growth around sinking volcanic islands produced the Pacific’s most spectacular landform—Atolls. But what’s been glossed over is the trouble Darwin had when it came to fitting Caribbean reefs into this theory. The ubiquity of shallow banks and lack of sinking volcanoes in the Caribbean meant that he was unable to identify reefs that clearly fit into these three stages, and suggested instead they were poorly developed cousins. And although several alternatives have been proposed since Darwin's time, the problem of how Caribbean reefs develop and why they are different remains unresolved to this day. Our work reported in this paper shows that, instead of Darwin’s one reef-type with three stages of development, Caribbean reefs actually consist of two distinct types with different morphologies, geographic distributions and internal structures. The first type we call Flat-Type reefs, which have wide flat intertidal back-reef zones, are found predominantly in the southern Caribbean, and have an internal structure of stacked interlocking coral skeletons, features which are also found in their Pacific cousins. The second type we call Crest-Type reefs, which have sloping subtidal back-reefs instead of flats, are found predominantly in the northern Caribbean, and have an internal structure composed not of stacked colonies but of large skeletal fragments.

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Photo by Karsten Winegeart on Unsplash

Photo by Karsten Winegeart on Unsplash

Why is it important?

These puzzling differences obviously raise questions about why two reef types exist in the Caribbean instead of one, and what implications this might have for their uncertain future. In the paper, we suggest that the presence of crest-type reefs in the north clearly has something to do with hurricanes, which are more prevalent in the north than the south. For example, we already know that the rubbly internal structure of crest-type reefs is the result of the cyclic destruction and regeneration of coral colonies by hundreds of hurricanes over thousands of years. But this only explains half of the data and what’s unclear is why flat-type reefs are restricted to the southern Caribbean. Finding answers is important because the two reef types probably have different ecological communities and might respond differently to the future’s double jeopardy of climate warming and sea-level rise. At this stage, we’re just happy to solve Darwin's original problem and explain how Caribbean reefs are different. Figuring out why will be a difficult lock to pick, but the key could open several doors.

Perspectives