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Posidonia oceanica, commonly known as Neptune Grass or Mediterranean tapeweed, is a marine flowering plant endemic to the Mediterranean Sea. Like other seagrasses, it forms large underwater meadows in the submerged photic zone of sheltered coastal waters (Pirog 2011). Due to its high rate of primary production and its ability to structure and stabilize the seabed, Neptune grass creates habitat for many other marine organisms and thus plays a significant role in littoral Mediterranean ecosystems (Gobert et al. 2006, Kendrick et al. 2005, Pergent et al. 1994).
Posidonia oceanica is one of the largest, slowest growing, and longest-lived plants. In a recent genetic study of 40 P. oceanica populations across the Mediterranean, Arnaud-Haond et al. (2012) found individual clones spanning up to 15 km (9.3 miles). Based on the plant's known growth rate, such individuals are likely to be thousands, possibly tens of thousands of years old (Arnaud-Haond et al. 2012) .
With their origin possibly dating back to the Pleistocene, some P. oceanica meadows have shown great resilience, persisting through great environmental changes over millennia. Yet, today P. oceanica populations are declining rapidly due to human-induced disturbances (Ardizzone et al. 2006, Duarte 2002, Marbà et al. 1996, 2005, Montefalcone et al. 2007, Waycott et al. 2009). Major threats include coastal construction (Badalamenti et al. 2006, Ruiz & Romero 2003), trawling (Gonzalez-Correa et al. 2005), fish farming (Díaz-Almela et al. 2008, Pergent-Martini 2006), and climate change (Marbà & Duarte 1997, 2009). Arnaud-Haond et al. (2012) warn that "the ancient meadows of P. oceanica are declining at a rate several hundred-fold faster (about 5%.yr−1, Marbà et al 2005, Waycott et al. 2009) than the rate over which they spread when forming (Doyle & Doyle 1987, Sintes 2006), a situation that this slow growing, long-lived species is poorly capable of recovering from."