Ancient Corals of Ta'u, American Samoa

by Douglas Fenner

The largest known coral colony
(Photograph by Paul Brown, NPAS)

On the southwest coast of Ta’u Island, American Samoa, there is a coral of massive proportions. It is a smooth hemispherical coral in the genus Porites. It measures 7 m (23 feet) tall and has a circumference of an amazing 41 m (135 feet). It is in near-perfect condition, with one narrow cleft that is dead and one low tumor the size of person. The tumor retains the color and polyps of the normal coral, it is just raised a little, and these types of tumors appear not to hurt the coral. It is the largest circumference coral we know of in the world (so far), although it is not the tallest. There is another coral in Taiwan that is taller.

This ancient coral is estimated to have 200 million tiny polyps, and to weigh 129 metric tons. It is clearly old, but we don’t know for sure how old. Australian researchers have come up with a formula for how fast this type of coral grows, based on water temperature. Due to relatively high water temperatures in American Samoa, corals grow faster than elsewhere. The formula indicates it should be about 360 years old. The only way to find out for sure is to remove a core from it, which has not been done. Whatever its age, we know from its good health and size that conditions there have been favorable for corals in this area for a longtime.

Diver next to the tumor on the
largest known coral
(Photograph by Paul Brown)

Small samples of the skeleton show that it is in the Porites lutea group of corals. Genetics indicates that there are at least three species in this group, all of which have similar skeletal details. Coral species identification is based on details of the skeleton.

This coral was originally pointed out to researcher Dr. Alison Green by a Samoan employee of National Parks, Fale Tuilagi. Subsequently, CRED has found more corals of similar size on the east side of Ta’u. Ta’u is a shield volcano like Mauna Loa in Hawaii, the youngest island in the Samoan archipelago at a mere 100,000 years, and home to the village from which voyagers set out over a thousand years ago to settle all of the Polynesian islands. It is also where Margaret Mead did her research that led to her famous anthropological book, “Growing up in Samoa.”

Reference:

Brown, D. P., Basch, L.,Barshis, D., Foresman, Z., Fenner, D., Goldberg, J. 2009.

American Samoa’s island of giants: massive Porites colonies at Ta’u island. Coral

Reefs 28: 735.