Radiocarbon dating suggests some deep-sea coral may live for thousands of years. Credit: Hawaii Undersea Research Laboratory

Some species of coral can live for over 4,000 years — longer than any other animal that lives in the ocean, a study has found.

Uncertainty over how to date coral makes estimates of their lifespan contentious. A radiocarbon-dating study published in 2006 by Brendan Roark, then at Stanford University in California, suggested that living colonies of Gerardia corals could be more than 2,700 years old1. But a 2002 study by Richard Grigg at the University of Hawaii had estimated the life span of the same species to be around 70 years2.

Much of the disagreement turns on where carbon in the corals is coming from. Roark, currently at Texas A&M University in College Station, now claims to have confirmed his earlier results by looking closely at how the corals obtain their carbon.

Carbon capture and storage

When growing their skeletons, corals use carbon that is found as either dissolved inorganic carbon or particulate organic carbon in the surrounding waters. As they get larger, corals grow in a tree-like fashion, forming growth rings. Grigg counted these rings to estimate age.

In contrast, Roark used radiocarbon dating in his 2006 study to look at the amounts of the isotope carbon-14 in the corals. 14C is produced in the upper atmosphere where it rapidly oxidizes into 14CO2. All living organisms take up 14C in proportion to the amount of it that is present when they are alive. Whereas carbon-12 and carbon-13 are stable isotopes, 14C decays to nitrogen with a half-life of 5,730 years. Monitoring the decay can give researchers an idea of how long ago the isotope formed — allowing old tissue to be dated.

But some researchers questioned Roark's methods, arguing that carbon in the food the corals were eating from the ocean floor and incorporating into their skeletons could be older than the corals themselves — giving rise to unduly ancient age estimates.

To determine if this was the case, Roark and his colleagues carefully analysed the outer few millimetres of recently deposited skeletons of both Gerardia and Leiopathes and compared the 14C values to those of the surrounding water and to that of shallow-water coral. Reporting in the Proceedings of the National Academy of Sciences, they reveal that the organic carbon that these corals are acquiring is fresh carbon that is rapidly transported to depth from surface waters3.

Their detailed study confirms the longest lived, Gerardia, to be 2,742 years old and Leiopathes to be 4,265 years old. This places them among other extremely long-lived organisms such as the bristlecone pine (Pinus longaeva), which can live for more than 4,800 years, and the mahogany clam (Arctica islandica) which can reach over 400 years.

Roark's radiocarbon analysis, however, is far from settling long-standing disagreements in the field. "The conclusions … are extremely biased if not flatly wrong," comments Grigg. He says the sample size of just three Leiopathes colonies is inadequate for any ecological study.

Very long lived species tend to have much lower reproductive output while shorter lived species tend to have higher output, he explains. "I've been taking submersible dives to look at these corals for 40 years and they are often full of developing eggs — this is hardly a characteristic of a thousand year old species," he adds.

Of greatest concern, says Grigg, is that the combination of live and dead material used in the study produces an age bias because live polyps feed on both modern and old carbon while dead material is subject to diffusion from old carbon dissolved in the water.

Help the aged?

Yet not everyone agrees. "This is a nice piece of work that reveals a lot about the lives of these corals," says marine biologist Murray Roberts at the Scottish Association for Marine Science in Oban, UK.

The animals' long lives and slow growth, with rates as sluggish as 5 µm a year, puts them at risk, as they are frequently collected for jewellery and are often damaged by deep-sea trawling, says Roark. "With such slow growth, human activity can potentially wipe them out before they can respond. [This creates] an increased impetus for the development of a coherent and effective international conservation strategy," he adds.

The authors may have analyzed some live twigs and some very old dead corals but their analyses overall smack of eco-extremism. Richard Grigg , University of Hawaii

But Grigg points out that Leiopathes has never been harvested commercially and is useless for jewellery. Gerardia has only been harvested selectively in two small locations in Hawaii, annual yields have never exceeded 3% of the standing crop, and 99% of the habitat of Gerardia in the Hawaiian Archipelago currently lies in federally protected waters. "The authors may have analyzed some live twigs and some very old dead corals but their analyses overall smack of eco-extremism," says Grigg.