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You are here: /main/research/NOWRAMP 2002/features/mystery of the corals


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Mystery of the Corals
Posted by Mark Heckman, Educator, Waikiki Aquarium/ University of Hawai'i - Manoa

French Frigate Shoals is a unique site. All that is left of the original volcanic island is a tiny spire of rock, La Perouse Pinnacle. The rest of the volcanic island has sunk beneath the sea or been eroded away over time. The pinnacle stands as a last reminder of the toehold the scalding magma left behind in this vast ocean.

But it is not over yet, like all of the Hawaiian Islands, corals and coralline alga grow here. Together with other types of sea life that make shells or skeletons, they have created a vast area of reef and islets that surround the pinnacle. Miles of rich reef are here, filled with fish and invertebrates. The reef is a strong matrix of coral skeletons and coralline algae, with sands between. Life, that contrary effort to entropy, forever building and organizing, has not yet given in. French Frigate Shoals is much older than the main Hawaiian Islands. This trend will continue as we move up the chain. Creation, erosion, subsidence, it is all just part of the history of these islands and reefs.

Acropora corals at French Frigate Shoals.  Photo by Jim Watt.

Of particular interest, are the extensive group of corals, the Acropora (1). These are the corals that people imagine when they think of coral reefs. Go to Fiji, Australia, the Indian Ocean, Palau, New Guinea, Paris (2), and you will find the Acropora. They come in plate forms over 12 feet in diameter and in branching forms like thickets, delicate as lace or thick as antlers.

But wait, what place did I miss in that list? Hawai`i, what about Hawai`i? It's warm enough, the water is clear enough. If Acropora is the main coral group in the rest of the world, what happened to Hawai'i? Herein lies a mystery. There are currently just a few Acropora coral heads in the entirety of the main Hawaiian Islands. It seems like they should be here. They are close, they have been in found in Hawai`i in the past, but they just aren't abundant now.

Various ideas have been proposed as to why these corals are so numerous at French Frigate Shoals. Certainly the easiest is one of proximity. If you are near a friend, you are in close proximity. If they have a cold, and they sneeze on you, that proximity will transmit an organism or two. If the friend is across the room, that same cold organism will probably not make it.

In the case of Acropora, the closest source for Hawai`i is (choose one): Okinawa (3), Bora Bora (4), Los Angeles (5), or Johnston Island (6)? Did you pick correctly? Currents must have a big impact here. Somehow, whether through eddies (current swirls) or larger sub-tropical counter currents, Acropora have made it across to French Frigate Shoals. They may have come as the planula larvae that corals release, or settled on objects that floated across.

The Acroporids may work their way back down to the main Hawaiian Islands. They were part of Hawai`i's geological past, fossil reefs on O`ahu have Acroporid corals. Perhaps they died off due to changes in sea level. In the past three ice ages, as the water cooled and was tied up in ice sheets and glaciers further north, coral reefs have been exposed as when sea levels dropped more than 300 feet. This was probably a bit much for relatively slow growing marine organisms that are sensitive to temperature and do not do well out of water.

Whatever the reasons, Acroporids are abundant at French Frigate Shoals. Diving is unlike anywhere else in the Hawaiian Archipelago. Take a look at the pictures; it does not look like the underwater environment in the main Hawaiian Islands for a variety of reasons.

Acropora on Rapture Reef at French Frigate Shoals.  Photo by Jim Watt.

Some fish, such as the Chevron butterflyfish, (Chaetodon trifacialis), depend on Acropora for food, they will eat nothing else. Lose the Acropora, lose the fish. Other fish and wildlife depend on corals such as the Acropora as well. Seaweeds grow underneath, urchins graze, and the labyrinth of spaces provides night and day hiding spaces for a wide variety of colorful reef fish. The beauty of an Acroporid reef is clear to any beholder.

References: Veron, JEN (2000), Corals of the World, Melbourne, Australia, and Expedition Research Scientists: Dr. Jim Maragos, US Fish and Wildlife Coral Reef Biologist, Dr. Don Potts, University of California, Santa Cruz.


(1) You can say "a crop poor a" if you are from the United States or "ackro pore a" if you are from Australia. Scientists can be a stubborn lot and sometimes don't agree on how to say the words they create. But it is not their fault, scientific terms are based on Latin, a language we know how to write, but no longer know how to speak. This leads to some confusion.

(2) Paris does not have any Acropora at present. It doesn't even have an ocean. But if you get to Paris, while you are sitting at a sidewalk café, consider that 40 million years ago, it was an ocean and Acroporids put in one of their first appearances there. They've 'been around.'

(3) Sorry - although Okinawa definitely has Acroporid corals, including all of the same species (and more) that are found at French Frigate Shoals, there are many such places in the Pacific. Distance is a big issue here, there are much closer sources.

(4) Bora Bora just has a great sound to it, doesn't it? Easy to imagine relaxing on a Tahitian beach, snorkeling in the beautiful waters with the beautiful branching corals. But, sorry, it is not the right answer. There are much closer sources. Try again.

(5) Not even close - Los Angeles barely has corals at all, too cold. Reef building corals do best in warmer waters.

(6) YES! In all of the Hawaiian archipelago, French Frigate Shoals is the closest to Johnston Atoll, a mere 450 miles away. Johnston also has all of the species found at French Frigate (and a few more). This is the closest source and makes the best match. Other close sources include Kingman Reef at 1,050 miles, Palmyra at 1,100 miles, and Wake Island at 1,550 miles.

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