Posidonia oceanica

Posidoniaceae is one of the 5 families of seagrasses, descendants of terrestrial plants that re-colonised the ocean between 100 and 65 million years ago. Seagrasses are monocotyledons (also known as monocots, one of two major groups of flowering plants or angiosperms that are traditionally recognized, the other being dicotyledons, or dicots) that are not true grasses (family Poaceae) but are closely related to the lily family, Magnolyophyta (http://www.marbef.org/wiki/mediterranean_seagrass_ecosystem). This kind of rich knowledge about plants accumulated over hundreds of years always amazes me.

The sole genus of the family Posidonia has 9 species: Posidonia angustifolia, P. australis, P. sinuosa, P. coriacea, P. denhartogii, P. kirkmanii, P. ostenfeldii, P. robertsonae and P. oceanica.

P. oceanica, common name Neptune grass, is named after the god of the sea. Poseidon is one of the twelve Olympian deities in Greek mythology. In Roman mythology he is Neptune.

My friends and I used to challenge ourselves to reach the darker waters above the P. oceanica meadows that were 150 or maybe more meters out from the shore at the Kadınlar Denizi beach in Kuşadası where we went swimming in the summers during the ‘60s and ‘70s. Several times a season decaying leaves from these meadows would hit the shore en masse. This phenomenon would usually take place at night and we would wake up to a beach line covered with dead dark brown leaves. This didn’t perturb us for we could easily walk over them and jump into clean clear waters with beautiful sand at the bottom all the way to the P. oceanica meadows. The grass would be gone the next day, carried away by the waves. Having Neptune grass in the sea is an indication of non-pollution in fact. This grass grows best in clean waters.

                              A picture of the town of Kuşadası (c. 1935) showing decaying Neptune grass that has hit the shore

There was another small bay nearby where the shore was permanently covered with dead leaves several meters high. I remember going out for a family picnic on this bed of grayish-brown dry leaves one time. The pile was soft and bouncy under our feet. It all depended on the currents, the winds and the compass direction the beaches were facing.

                                                          P. oceanica beds on the west coast of Corsica, in 2000

There is extensive research on P. oceanica and its significant role in littoral (coastal regions) Mediterranean ecosystems. Here are some facts about Neptune grass: It forms large underwater meadows in the submerged photic zone of sheltered coastal waters, photic meaning enough sunlight penetrates to permit photosynthesis; it has high rate of production which is the synthesis of organic compounds from carbon dioxide making it a high producer forming the base of the food chain; it structures and stabilizes the seabed thus creating habitat for many marine organisms.

P.oceanica is one of the largest, slowest growing, and longest-lived plants. In a genetic study of P. oceanica populations across the Mediterranean, individual clones spanning up to 15 km have been found. 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-http://eol.org/pages/1089001/overview).

This species is found only in the Mediterranean Sea occupying an area of only about 3% of the basin. This corresponds to a surface area of about 38,000 square kilometers. With their origin possibly dating back to the Pleistocene, some P.oceanica meadows have shown great resilience, persisting through environmental changes over millennia (http://eol.org/pages/1089001/hierarchy_entries/56317045/overview). Today, however, research indicates that P. oceanica populations are declining rapidly. This is due to human-induced disturbances such as coastal construction, trawling, fish farming, and climate change. Researchers warn that the ancient meadows of P. oceanica are declining at a rate several hundred-fold faster than the rate over which they spread causing an unfavorable situation this slow-growing, long-lived species of seagrass is poorly capable of recovering from.

                                                                                               dry rhizomes

As a lay admirer of this great plant I have been noticing for some time that the situation isn’t what it used to be in my youth. We used to find rhizomes of P.oceanica on the beach sand sometimes. There would be egagropili, the fibrous balls formed from materials of dry foliage and rhizomes shaped into balls by the under current, sitting on the sand. Most of them were 3-5 cm in diameter and perfectly spherical. They would feel like felt to touch. The one in the picture that I have kept is oval.

                            egagropili 

I do not remember seeing the fruit of P. oceanica. I am learning that the fruit is free floating and known in Italy as "the olive of the sea" (l'oliva di mare). They resemble small green olive drupes.


                   The Dilek Peninsula National Park and Kadinlar Denizi beaches on the Aegean Sea in Kuşadası

Kadinlar Denizi was one of the best beaches in the whole Mediterranean, I am sure. I don’t think that is quiet true anymore.

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