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Corals Crawling With Crustaceans

Smithsonian scientist Laetitia Plaisance talks about her recent study and its finding that coral reefs support even more biodiversity that we thought

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Laetitia Plaisance searches for crustaceans in a piece of dead coral. Photo courtesy of Laetitia Plaisance.

Despite having offices just across the National Mall from each other, it was in the tiny town of Bocas del Toro, Panama, that I met Laetitia Plaisance. It was September 2009, and I was at the Smithsonian Tropical Research Institute’s field station in Bocas tagging along with coral reef biologist Nancy Knowlton, as she studied a coral reef’s mass spawning event. Plaisance, a marine ecologist at the National Museum of Natural History, was on Knowlton’s scuba diving team.

During her stay at STRI, Plaisance was also deploying devices called autonomous reef monitoring structures, or ARMS, off the coast, for the purposes of her own study. In the past few years, she has collected crustaceans—ranging in size from five millimeters to five centimeters—from dead coral heads or ARMs at depths of 26 to 39 feet in designated sites in the Indian, Pacific and Caribbean oceans. Recently, using DNA barcoding, she determined that a far greater number of crustaceans—as many as 525 different species—far higher than expected, lived in the 20.6 square feet of natural and manmade structures. The results of her globe-trotting research, a study titled “The Diversity of Coral Reefs: What Are We Missing?” is now available in the journal PLoS ONE. I caught up with her the other day to discuss the study.

What sites did you include, and how were those sites selected?

We selected the sites to span the range of diversity that we find on a reef. Typically, there are more species in the North (Lizard Island, Australia) than in the South (Heron Island, Australia) and in the West (Great Barrier Reef sites) than in the East (French Frigate Shoals, Hawaii – The Line Islands – Moorea, Frecnh Polynesia) in the Pacific Ocean. We also added two other ocean basins—the Eastern Indian Ocean (Ningaloo Reef, Australia) and the Caribbean (Bocas del Toro, Panama) that have very different evolutionary histories and biogeographies. The Caribbean reefs are very degraded and it was interesting to see how they compare with Indo-Pacific sites.

Can you explain what an ARMS is?

The ARMS were developed by NOAA in Hawaii. Basically, it is a little home for all the different species to settle in. It is about 20 centimeters with layers, completely opened or closed, for the species that prefer open layers with a lot of currents or the species that like little caves. You have all these different habitats in it. It is a great sampling device that supposed to mimic, roughly, the complexity of dead coral. We can use them in sand, grass beds, in all different sorts of habitats. We can process them very easily, and we can compare the results from site to site.

How often did you visit the sites? And, how did you go about your collecting at each?

I visited the sites once or twice depending on how and when the field trips were organized. Usually, we would dive in the morning. We tried to find live coral heads. Sometimes we were successful and sometimes not. Then, we’d take them back to the lab. I worked mainly alone, but sometimes I had volunteers helping. I would start in the lab, opening the coral head, breaking them down. It took about a day to examine the whole coral head because there were so many things living in it. I just grabbed all of the crustaceans that I could see and stored them under running saltwater. I would take the crustaceans, take pictures, record data and then take a bit of tissue for molecular studies and store the rest of the body for morphological studies later on. I didn’t do any morphological studies, but we have collaborations with people who do, so I would save the rest of the crustaceans for them. Then, I would take back to Washington only the tissues to work on the DNA sequencing.

Why did you choose to focus on crustaceans?

They are probably the most numerous group living in the coral heads. It is about half crustaceans and the rest would be mollusk and other things. Crustaceans were really diverse and abundant. But also they are very easy to sequence. Mollusks are a hassle to sequence. To avoid those technical problems, we chose the crustaceans.

How did you use DNA barcoding?

People have been using DNA barcoding now for about 10 years. It is a short sequence fragment, and we sequence the same fragment for everything. We have universal primers. It’s not that easy, of course. You always have problems. But it is easier than all the other molecular techniques right now. I sequenced that short fragment in each crustacean and then compared all those fragments for all the different species. Basically, if it is five percent different, it is two different species. If it is less than five percent different, it is the same species. So it was really easy to determine how many species we had.

In total, you found 525 different species. How many did you expect to find?

Yes. We really didn’t expect that much. Compared with diversity estimations in coral reefs, we found a lot. In the Great Barrier Reef, we had about 200 decapods, when the Great Barrier Reef is supposed to have 900—and we only sequenced two square meters. So it is just so much more compared to the estimates that have been published.

After I sequenced everything, I compared my sequences with the sequences that have been published and are available. Only a few of the crustaceans have been sequenced previously, and a lot of them have probably not been described yet.

What’s next for you?

The highlight of this research was really to be able to dive on the reef and witness the beauty of it. But the shocking part was to see how everything can be destroyed so fast. When we went back to Panama in 2010, the reef had bleached completely. The temperatures were really high. Where we actually had deployed the ARMS there, it was a dead zone a year later. There was nothing living anymore.

I think coral reefs are much more important than the general public knows and the government knows. They have so many threats right now, locally and globally. It is unbearable to see the destruction. That is why right now I am actually transitioning to conservation.

I am interested in the resiliency of the reefs. Reefs can undergo a phase shift. Basically, you have plenty of healthy corals and a few weeks later you just have algae that has overgrown the reef. I am trying to find solutions to reverse these phase shifts.

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