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          Figure 5 | Conceptual diagram showing alternate hypotheses to be tested trough seed sowing experiments in order to (1) evaluate the influence of seed
          and microsite availability in sexual recruitment limitation of P. oceanica populations and to (2) identify characteristics of suitable microsite for P. oceanica
          seedling establishment. Influence of substrate type and predation are highlighted.

          stances (EPS) of bacterial and microalgal biofilms, which are com-  erage of adhesive hairs on roots allows P. oceanica seedlings to adhere
          posed of exopolysaccharides . Further investigations are needed to  to different types of consolidated substrates, such as vegetated and
          ascertain if P. oceanica adhesive root hairs maintain also a nutrient  bare rocks; further, adhesive hairs allow seedlings to settle on these
          uptake function.                                    substrates even in shallow, exposed sites. Each of these morpho-
           The sticky root hairs in vascular aquatic plants appear to represent  logical adaptations has been selected and maintained by evolutionary
          an adaptive trait for survival on hard substrates in high-energy envir-  processes because it may increase the probability of a new individual
          onments . The presence of morphologically and functionally similar  to remain in a favourable habitat for a given species, that has been
          adhesive root hairs in phylogenetically distant species such as the  successfully colonised by adult individuals . In the same way strong
          monocotyledon seagrasses Posidonia oceanica (Posidoniaceae) and  anchorage to hard substrates by adhesive root hairs could represent a
          Phyllospadix scouleri (Zosteraceae) and the freshwater dicotyledons  mechanism of habitat selection in P. oceanica seedlings.
          Podostemaceae can represent an example of convergent evolution in  Recruitment of juveniles to adult populations is considered a cru-
          response to life in habitats exposed to wave action or running water  cial stage in plant life history . Recruitment limitation models
          and characterised by hard substrates.               identify two main factors potentially affecting population size and
           It can be hypothesized that the adhesive properties of P. oceanica  species distribution: habitat carrying capacity and the intrinsic ability
          seedling roots represent an adaptive trait expressed under the right  of a population to grow . These two factors can be differentiated
          environmental stimuli. In the present work adhesive root hairs were  according to the spatial scale of interest. Propagule and suitable
          recorded in seedlings collected on different types of substrate i.e. bare  microsite availability limits recruitment at the local scale, while dis-
          cobbles, rock covered by algae and sand, suggesting that substrate  persal and habitat availability act at the regional scale . Seed limita-
          type does not play a role in root hair development. This observation  tion and microsite limitation represent the two end-points of a
          was confirmed by a subsequent work (Alagna et al., personal obser-  spectrum, with plant populations usually being both seed and micro-
          vation) in which P. oceanica seedlings were reared on soft vs hard  site limited .
          substrates in an indoor full replicated experimental system. Seedlings  Posidonia oceanica fruit production is reported to be rare in some
          produced sticky root hairs in all experimental groups, supporting the  region and quite regular in others 20,21 , therefore natural levels of
          hypothesis that substrate type does not influence the development of  seedling supply are expected to vary greatly according to year and
          adhesive root hairs. On the other hand seedling anchorage occurred  location and could represent a bottleneck for recruitment at the local
          only on consolidated substrates via root hair adhesion, supporting  scale (Fig. 5). However, correlative and manipulative experiments
          the hypothesis that sticky root hairs promote early seedling anchor-  have shown that even when seeds are available, recruitment is highly
          age on consolidated substrates. We cannot exclude that other  variable (0–70% survival) displaying strong patterning across habi-
          mechanisms allow P. oceanica seedling establishment on other types  tats 25,26,28,29 . These observations point out that microsite availability
          of substrate, however we failed to observe other mechanisms allow-  very likely plays an additional role in limiting P. oceanica seedling
          ing seedling anchorage on sand.                     recruitment (Fig. 5).
           In aquatic environments vascular plants have evolved specific  Characteristics of suitable microsites for P. oceanica seedling
          structures to assist with initial seed and seedling establishment.  recruitment remain to be defined. Substrate type seems to represent
          Among these are the arms covered by stiff bristles of Phyllospadix  a major driver in successful P. oceanica seedling establishment, with
          spp. fruits , which help seedlings to get entangled preferentially on  recruitment mainly occurring on firm substrates such as rock and
          intertidal branched algae , and the four-lobed comb of Amphibolis  dead matte with respect to unconsolidated ones such as sand, gravel
          spp. seedlings that grapples on adult plants and other structures  and pebbles 25,26,28,29 . Plantlets physical dislodgement by hydrodyn-
          present on the bottom . Similarly, the presence of an extensive cov-  amic forces is considered one of the main causes for failed seedling
          SCIENTIFIC REPORTS | 5 : 8804 | DOI: 10.1038/srep08804                                               4
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