Pretty in Pink - Signs of coral reef recovery in the Phoenix Islands

#2: CRUSTOSE CORALLINE ALGAE: Next on the greatest hits of the Phoenix Islands (and these are in no particular order, by the way), is our CCA tour. CCA tour? What's that? Glad you asked.

Hydnophora rigida coral near Kanton Island (Photo: R. Rotjan)

CCA is science geek shorthand for Crustose Coralline Algae. Clear as mud, I know. CCA (now that you know what it stands for) is basically reef cement. It's an encrusting, hard, red algae (Rhodophyta) that covers dead reef substrate (among other things). CCAs are anything but slimy--they appear smooth, but also provide some texture and traction. This textural combination is a bit like sidewalk cement. It pours smoothly and will cover surfaces evenly, but has a bit of a grainy texture to the touch. Also similar to cement, CCA acts like mortar holding bricks together, only that CCAs cover loose pieces of dead coral rubble and holds them together.

With all of this construction/cement talk, you might be wondering about the pink part of this story. Molly Ringwald, Aerosmith, Pink Panther--this part is for you. CCAs are usually pink! The pinkness adds a lovely aesthetic to a coral reef, which is one nice feature.

PIPA reef in recovery. Can you spot the pink CCAs in this picture? (Photo: R. Rotjan)

More importantly, a pink reef is a reef ready for re-growth, since baby corals like to settle on CCA-covered substrates.

Coral recruits and juveniles growing on CCA-covered substrate (Photo: R. Rotjan)

Loosely, when corals reproduce, they either spawn or brood their next generation, and these eggs and sperm combine to create larvae which spend some time in the water column before settling on a permanent home on the reef floor. For a coral larvae, pink = prime real estate, a.k.a. smooth, available substrate with just enough texture to hang on while growing.

There is always some amount of CCA on reefs, and coral settlement is highly biased towards CCA. On a reef that has suffered some major challenge, in the case of the Phoenix Islands Protected Area (PIPA) reefs, the catastrophic coral bleaching event (mentioned in previous posts here, here and here) caused by the longest and highest thermal anomaly likely ever recorded on reefs. CCAs are a critical part of the recovery stage. Once corals bleach and die, their rubble often remains and gets strewn about on the reef floor, knocked around by waves and storms.

Without live coral animals continually building new skeleton, abandoned skeletons get quickly overrun by bioeroding organisms and become weakened to the point of collapse. This is why live, healthy, GROWING corals are critical to reef. Leftover skeletons can only provide structurally complex habitat for a short while. The point is that CCAs overgrow these dead skeletal frameworks and provide the perfect platform for larval settlement: a.k.a. coral re-growth. Hence, CCAs are a critical part of reef recovery.

Dead, standing coral rubble covered with pink CCA (photo: R. Rotjan)

CCAs themselves are only one part of a very complex recovery story. Briefly, for successful recovery recipe, a reef needs the following the ingredients: reproductive coral adults (or a nearby supply of coral larvae), a CCA blanket, clean and clear waters, and a healthy population of reef herbivores to graze the fleshy macroalgae that would otherwise cover the CCAs and outcompete baby corals for access to space and sunlight. A reef also needs to NOT have the following: disease, pollution, repeated disturbances, etc. Luckily for PIPA, there was only one major bleaching event. With almost no current influence from human populations, PIPA remains free from extensive pollution and disease.

A parrotfish feeding frenzy, "lawn mowers" (Photo: R. Rotjan)

PIPA also hosts a relatively intact fish population, which means that herbivores keep the reef substrate clean (think of herbivores as lawn mowers working to maintain a perfect corporate lawn). Weedy macroalgae is therefore only present in low abundance (because the herbivores graze it), and the reef stays closely cropped. Put all of this together, and the story is this: PIPA reefs are pretty in pink!

PIPA recovery in progress: newly growing coral overgrowing CCA substrate, which is kept clean by herbivores such as the surgeonfish shown here (Photo: R. Rotjan)

Sure enough, on this expedition, we saw lots of pretty pink CCA, and on top of the CCA: coral recruits and juveniles. PIPA is well on the way to recovery. Stay tuned for the next expedition (2011?!) to find out: Will coral be the new pink?



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