Characteristics
General characteristics :
- Ability to live in a haline (about 33 g/l salt) and marine environment (up to 40 metres deep) and nowhere else,
- Ability to grow while submerged,
- Low growth rate per year (about 1 cm per year for vertical rhizomes and 3 to 6 cm per year for horizontal rhizomes),
- Ability to perform photosynthesis,
- Sexual reproduction (for flowers and fruits) and asexual reproduction (allowing the expansion of the herbarium),
- Flowering starts around October.
Morphological characteristics :
Posidonia oceanica has very thick, branched rhizomes. And at the base of each leaf bundle, there is a persistent bundle of fibrous scales like straw (Fig.5).
Its clusters consist of different types of leaves depending on their growth (Fig.6).
The so-called "juvenile" leaves are those less than 5 cm long. Intermediate" leaves are those over 5 cm without a basal sheath or petiole. When growth is complete, a basal sheath is formed and the leaf is then said to be 'adult' (Boudouresque et al., 2006).
The greenish leaves in the herbarium are on average 30-80 cm long (up to 120 cm in Rafraf, Tunisia - unpublished) and 7-12 mm wide.
Posidonia oceanica is a plant that sheds its leaves all year round, with a maximum in autumn. These accumulate on the shore and form banks of varying height (Picture 1).
The network of rhizomes, dead or alive, and the sediment filling the interstices constitute an extremely compact whole that is called "mat" (Fig.7).
Finally, the roots of Posidonia Oceanica meadows are relatively thick (up to 4 mm) and very branched (Boudouresque and Meinesz, 1982).
Figure 5. General appearance of Posidonia oceanica (Boudouresque and Meinesz, 1982)
Figure 6. Rhizome and leaf bundle (A). Different types of leaves: adult leaves (B), intermediate leaves (C and D) and juvenile leaves (E) (Boudouresque and Meinesz, 1982)
Picture 1. Posidonia leaf bank at Chott Meriem
Figure 7. Schematic block diagram showing a Posidonia oceanica mat (Boudouresque et Meinesz, 1982)
Reproduction :
Like all spermaphytes[1], Posidonia oceanica can bear flowers which appear in autumn, then become fruits by sexual reproduction before detaching from the mother plant, between May and July. These fruits are then carried away by the sea current and release the embryo, which will settle on the bottom and germinate if the nature of the substratum and the physicochemical factors are favourable.
However, as flowering is rare (it does not occur every year and is rare in the cold waters of the northern Mediterranean, for example), the main means of reproduction for Posidonia oceanica remains asexual reproduction.
[1] According to Futura Science: Spermatophytes are a division of the plant kingdom, including all species of seed plants.
Figure 8. Posidonia oceanica seed
Ecological preferences :
Posidonia oceanica meadows develop in luminous waters allowing photosynthetic activity. As regards the substrate, it requires a loose mineral sediment containing an adequate proportion of organic matter and prefers hard substrates covered with coarse sand with good water circulation (Ben Mustapha and Hattour, 1992).
Moreover, posidonia is very sensitive to variations in salinity and temperature.
Finally, its stability and dynamics depend on hydrodynamics and sedimentation, which are responsible for the construction of mattes and the formation of erosive structures such as intermattes...