American Samoa’s island of giants: massive Porites colonies at Ta’u island
KeywordsCoral Coloni Coral Growth Polyp Size Skeletal Density Hermatypic Coral
One of the largest and oldest single hermatypic coral colonies known is a massive Porites coral located on the southwest corner of Ta’u island (14°15′S, 169°30′W) in the Manu’a group of American Samoa. The coral is believed to be Porites lutea based on colony and corallite morphology; however, Porites taxonomy is notoriously difficult and recent genetic work has revealed surprising levels of cryptic diversity (Forsman et al. 2009). Small (1 cm) cores were collected for genetic and microscopic work, and to determine skeletal density. DNA sequence for mitochondrial (COI, COIII) and nuclear (ITS region) markers were deposited in GenBank (accession numbers ITS-FJ416523-6, mt COI-FJ423967, and mt control FJ427369), and skeletal voucher specimens have been deposited at the Bishop Museum (SC4164).
The colony is composed of around 200 million polyps, assuming a smooth hemispherical surface and a polyp size of 1 mm2, and has a skeletal density of 1.4 g/cm3 (~52% average porosity). Using the volume and density estimates, the skeleton was estimated to have a dryweight of approximately 129 metric tons. Based on published growth rates for massive Porites (Lough and Barnes 2000; Potts et al. 1985), the core skeletal density, and annual mean water temperature, this coral is estimated to be between 360 and 800 years old, though it may be considerably older. It is surrounded by several smaller colonies, which measure between 4 and 28 m in circumference. Approximately 1 km to the south of this site are another dozen massive Porites measuring 16–24 m in circumference as well as several this size on the northeast corner of the island, and a few others in the waters of nearby islands.
While these corals in Ta’u may not be as old as some deep water corals, they are among the oldest known on tropical hermatypic reefs. Their large size, good health, and close proximity to each other implies that this location has had conditions conducive to coral growth for a long time.
The authors thank R. Brainard and A. Green for their help on this project and for their critical review and comments. Dedicated in memory of F. Tuilagi, who discovered this coral.