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           The family Posidoniaceae includes the single genus Posidonia K.
          D. Koenig, which has a disjunct Mediterranean (1 species) and
          Australian (8 species) distribution . Posidonia oceanica (L.) Delile
          is the dominant seagrass of the Mediterranean Sea, where it is
          endemic. Its meadows extend from the surface to 40 m depth, with
          an estimated surface ranging between 2.5 and 5 million hectares .
          These meadows represent one of the most valuable ecosystems of the
          Mediterranean Sea and provide essential goods and services to
          coastal communities .
           Posidonia oceanica has been described in historical literature as a  Figure 1 | Two- to three-month-old Posidonia oceanica seedlings settled
          species growing mainly on soft, nutrient-rich substrates 11–13 . P. ocea-  on volcanic cobbles at Ustica Island (Tyrrhenian Sea).
          nica meadows are considered the ‘‘climax community’’ of soft sub-
          littoral habitats in the Mediterranean Sea, whose development is
          facilitated by precursor communities such as Cymodocea nodosa  Results
          Asch. beds that accumulate sediment and organic matter  11–13 .  Morpho-anatomical and ultrastructural analyses. No significant
          According to these authors, under particular conditions, the develop-  differences in morphometric features were detected among seedl-
          ment of P. oceanica meadows is also possible on rocky reefs but is  ings collected at Ustica in July 1997 and at Favignana and Capo
          always preceded by the development of algal turf assemblages that  Feto in June and July 2004 (P . 0.05). The overall mean values
          collect sediment and organic matter followed by the development of  recorded were as follows: seed length 1.93 6 0.04 (cm 6 1 SE),
          a C. nodosa ‘‘pelouse’’ 11–14 .                     seed width 0.92 6 0.02, number of roots 4.50 6 0.29, maximum
           Posidonia oceanica propagates via both vegetative and sexual  root length 2.54 6 0.21, total number of leaves produced from
          reproduction. Vegetative propagation through rhizome elongation  germination 9.61 6 0.31, maximum leaf length 5.27 6 0.21and
          from well-established patches has been widely studied and is con-  maximum leaf width 0.53 6 0.01. The number of standing leaves
          sidered the dominant process by which seagrasses occupy space and  plus leaf sheaths indicated that seedlings were approximately 2–3
          maintain existing meadows 15,16 . Recent studies have shown that  months old (Fig. 1).
          vegetative fragment recruitment and patch formation does occur 17  The older portion of the adventitious roots was brown in colour,
          but takes place mainly on complex rocky substrates .  and the younger portion was cream. Above the root cap, a piliferous
           Posidonia oceanica flowers during autumn, followed by fruit  zone that extended for several centimetres and covered a large part of
          release in the late spring of the subsequent year . A long-term data  the root length was observed (Fig. 2 a, b). Root hairs were covered by
          series indicates that meadow flowering occurs on average every five  a sticky substance that stained positively to PAS, indicating the pres-
          years at a basin scale, with massive seed production events recorded  ence of polysaccharides and glycoproteins. Root hairs constituted an
          every 8–10 years linked to high summer temperatures . Moreover,  adhesive coat through which roots adhered to the substrate (rocks,
          flowering and fruiting are described as quite regular and diffuse  algae, and encrusting organisms) (Fig. 1 and 2 d) and to which sand
          phenomena in the southwestern Mediterranean 20,21 . The successful  grains remained attached (Fig. 2 c). Adhesive root hairs were
          recruitment of P. oceanica seeds has been consistently considered a  recorded on primary and adventitious roots and on the hypocotyl
          sporadic event in the past . Modern molecular approaches revealed  region of the seed. This pattern was consistent across locations.
          higher genetic variability in meadows than previously thought, lead-  Using SEM imaging, the transverse section of adventitious roots
          ing to the reassessment of the role of sexual reproduction in popu-  from the mature region showed that the epidermis possessed an
          lation dynamics .                                   extensive and dense root hair coverage (Fig. 3). The epidermal cells
           One-year survival represents a bottleneck in the seedling life cycle,  from which root hairs originated had a mean diameter of 19.46 6
          and successful plantlet recruitment has been recorded mainly on  0.76 (mm61 SE) [n 5 12] (Fig. 3). Root hairs were unicellular and
          habitats characterised by firm substrates such as rocks covered by  elongated, with a mean length of 707.67 6 24.09 mm and a maximum
          algae, dead matte 24–29  and surprisingly even at very shallow sites (0.5–  length of 2400 mm. The mean root hair width was 12.42 6 0.36 mm,
          3 m) characterised by high hydrodynamic regimes 24,27,28 .  with a maximum width of 14.17 mm (Fig. 3 and 4). Root hairs were
           Since 1997, we have observed in several occasions P. oceanica  branched or forked distally, with hair tips broadened into an
          seedlings on a variety of substrates at depths ranging from 0.5 to
          20 m. The plantlets appeared firmly secured to the sea bottom only
          when they occurred on consolidated substrates (e.g. rocky reef cov-
          ered by algae) as they provided strong resistance when pulled 27,28 .
          Seedling found on unconsolidated substrates (e.g. sand or gravel)
          instead did not offer any resistance when pulled 27,28 . Moreover, some
          seedlings where observed settled even at exposed sites 27,28 and on bare
           Although P. oceanica seedling anatomy and ultrastructure have
          been analysed in detail by several authors 30,32 , and the initial seedling
          development has been observed ex situ, in laboratory cultures 31,32 ,no
          morphological features have been described thus far that may explain
          the effective and strong anchorage observed in the field on hard
           Here, the morpho-anatomical and ultrastructural features of P.
          oceanica seedling root systems collected at different locations were
          analysed to identify specific traits that represent adaptations for
          establishment on rocky bottoms. Moreover the anchorage strength  Figure 2 | (a), (b) and (c) Posidonia oceanica seedling roots observed
          of seedlings settled on hard substrates in the field was measured.  under the stereomicroscope. (a) and (b) extension of the piliferous zone,
          Implications on P. oceanica habitat preference and on meadow eco-  (c) sand grains stuck to root hairs, (d) the adhesive coat constituted by the
          logical succession are then discussed.              sticky root hairs.

          SCIENTIFIC REPORTS | 5 : 8804 | DOI: 10.1038/srep08804                                               2
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