Rapa Expedition: Mystery at the Bottom of the Sea

As the Pristine Seas team explores the waters around the southern French Polynesian islands of Rapa and Marotiri, this subtropical ocean ecosystem is posing important questions and challenging our understanding of life in these environments. We love these challenges, though, as they are great opportunities to communicate ocean science and the importance of protecting these last truly wild places. Kike Ballesteros is our algae scientist, and here he reveals one marine mystery that has arisen.

By Kike Ballesteros

I’m writing this while we’re facing Marotiri Shoals, a tiny group of small islets 50 miles southeast of Rapa. We left Rapa at 5:30 this morning and will not arrive at the shoals until noon. So after five days of diving, it’s time to write some of my impressions about the marine life thriving in Rapa’s waters.

We made our first dive late on a windy and rainy afternoon in Rapa. Huge waterfalls were just in front of us as we jumped into the dark blue waters. The unpleasant weather above suddenly changed into a peaceful, slow-motion, crystal-clear, warm-blue environment. The group descending to 20 meters below the surface consisted of me; Alan Friedlander and Gilles Sui, our fish scientists; and Eric Brown, our coral specialist.

Without any seaweed visible, table coral stretches out uninterrupted. (Photo by Manu San Félix)

It took us only a few seconds to notice the amazing number of long-spined sea urchins (genus Diadema) on the bottom. There was no room for error as we floated over the sharp spines. There were some corals and coralline algae (since their hard calcareous skeletons allow them to survive the heavy grazing pressure of the urchins), but mostly the seafloor was literally covered by spines! And not a single seaweed as far as our eyes could see.

Coral specialist Eric Brown recovers the tape used to measure coral cover while avoiding scratching or impaling himself on the numerous urchin spines below. (Photo by Kike Ballesteros)
Coral specialist Eric Brown recovers the tape used to measure coral cover while avoiding scratching or impaling himself on the numerous urchin spines below. (Photo by Kike Ballesteros)

The next morning we made an early dive. And what did we see? A coral garden with hundreds of sea urchins. I measured densities of up to 15 sea urchins per square meter. Amazing! This lead me to think of Rapa as a paradise for sea urchins, which was not very appealing to us, given the extremely high likelihood of finishing our dive with several spines in our knees or in our hands.

However, everything changed on the third dive. We landed into a thick Sargassum seaweed bed, without a single sea urchin in sight! The seaweed was almost a meter high, waving with the swell. I enjoyed the dive, and being an algae specialist, I collected some specimens for our collections. But as soon as I had surfaced I was asking myself how was it possible to have reefs that were so different from each other?

Sargassum seaweed forest from Rapa (by Manu San Félix)
With food and cover abounding, the seaweed forest attracts fish both large and small. (Photo by Manu San Félix)

We dove again in the afternoon. At 20 meters we again had a sea urchin-covered seascape. This was exactly the same at depths of 18, 15, and even 12 meters, but it suddenly changed to a luxurious Sargassum forest at 10 meters. The line between the two environments was so evident that it seemed to be cut by a knife.

On one side you had the barren rock with corals and huge densities of sea urchins. On the other side was the brown forest, without a single sea urchin. For sure, no seaweed was growing on the “coral” side because the grazing activity of the sea urchins did not allow the seaweed to grow. But what was the factor not allowing sea urchins to enter the forest, eating the seaweed and turning the reef into another barren area?

Now we have conducted 11 dives in different sites around Rapa, and at each site you are either in a coral garden with hundreds of sea urchins or in a Sargassum seaweed forest—but never in between. You always expect Sargassum to be more abundant in the shallows, above depths of 12 meters, but sometimes it can go down to more than 20 meters or be completely absent even in the very shallow areas.

Algae specialist Kike Ballesteros measuring cover of seaweed and sea urchin density in a Sargassum forest (by Manu San Félix)
Algae specialist Kike Ballesteros uses a square of plastic pipe to isolate one square meter of seafloor and identify the species found there. (Photo by Manu San Félix)

Water movement likely plays a key role by enhancing algal growth. Urchins are probably not able to enter the seaweed forests because of the movement of the algae. Coral growth is enhanced in the barrens, as algal grazing by sea urchins facilitates the settlement and growth of small corals.

Whatever factors are ultimately at play, it is certain that we are only seeing a snapshot of processes that could take decades to establish. The existence of the two contrasting habitats (coral gardens with high sea urchin densities versus Sargassum seaweed forests devoid of sea urchins) increases the diversity of the seascape, adding additional value to the conservation of Rapa’s unique underwater ecosystems.


Kike’s Strange and Beautiful Algae Photos

Read All Posts From the Rapa 2014 Expedition

Learn More About Pristine Seas


The Pristine Seas expedition to Rapa is sponsored by Blancpain and Davidoff Cool Water.

Changing Planet

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Meet the Author
Paul Rose is an ardent explorer, television presenter, journalist, author, and Vice President of the Royal Geographical Society, and an Expedition Leader on the Pristine Seas team.