Is this seagrass patch really as old as they say?

Recently, newspapers were abuzz with a new headline claiming that the oldest living thing on earth had been discovered,1 but this was not the first time statements like this have been made. Previous claimants to the title include bristlecone pines,2,3  huon pines,4,5 and Norway spruces.6 This time, the contender is a seagrass. It is yet another plant, but probably unfamiliar to most people. It’s claimed to be up to 200,000 years old.

Seagrasses live throughout the world’s oceans. Their slender green blades tend to bend and wave in the current, creating pleasantly idyllic scenes for snorkelers and swimmers who happen upon a seagrass meadow in clear, shallow water. They are also a rich habitat harboring many marine species, such as the stinging hydroids that have surprised many a snorkeler foolish enough to brush against the swaying fronds without looking. They are also a primary food source for many marine creatures. Thus, they are important for many of the world’s coastal salt water ecosystems.

What is it about them that made recent headlines? A team of Australian scientists studying patches of the seagrass, Posidonia oceanica, in the Mediterranean Sea were astonished to find genetically identical plants separated by up to 15 km. Because they are flowering plants (yes, underwater!), seagrasses can reproduce sexually. However, to get plants with identical genotypes, asexual reproduction must occur and this is usually due to breakup of an original clump of plants by waves or herbivores. Plant fragments can drift to another location, put down roots, and begin to grow again. Alternatively, plants can send out runners and sprout new leaves some distance from the original plant. Vegetative growth like this can create monospecific patches (i.e., all plants are genetically identical), but this species also grows slowly. Computer models indicated the clones could be as old as 10,000 to several tens of thousands of years….

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