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OPEN         Evidences of adaptive traits to rocky

                              substrates undermine paradigm of
            SUBJECT AREAS:
                              habitat preference of the Mediterranean
                              seagrass Posidonia oceanica
             PLANT ECOLOGY
                              Fabio Badalamenti , Adriana Alagna & Silvio Fici 2
             7 October 2014
                              CNR-IAMC, Istituto per l’Ambiente Marino Costiero, Via G. da Verrazzano 17, 91014 Castellammare del Golfo (TP), Italy,
                  Accepted    2 Dipartimento di Scienze Agrarie e Forestali, Universita ` di Palermo, Via Archirafi 38, 90123 Palermo, Italy.
            5 February 2015
                  Published   Posidonia oceanica meadows are acknowledged as one of the most valuable ecosystems of the Mediterranean
              5 March 2015    Sea. P. oceanica has been historically described as a species typically growing on mobile substrates whose
                              development requires precursor communities. Here we document for the first time the extensive presence of
                              sticky hairs covering P. oceanica seedling roots. Adhesive root hairs allow the seedlings to firmly anchor to
                              rocky substrates with anchorage strength values up to 5.23 N, regardless of the presence of algal cover and to
           Correspondence and  colonise bare rock without the need for precursor assemblages to facilitate settlement. Adhesive root hairs
           requests for materials  are a morphological trait common on plants living on rocks in high-energy habitats, such as the riverweed
          should be addressed to  Podostemaceae and the seagrass Phyllospadix scouleri. The presence of adhesive root hairs in P. oceanica
               A.A. (adriana.  juveniles suggests a preference of this species for hard substrates. Such an adaptation leads to hypothesize a
                              new microsite driven bottleneck in P. oceanica seedling survival linked to substrate features. The mechanism
                              described can favour plant establishment on rocky substrates, in contrast with traditional paradigms. This
             adriana.alagna@  feature may have strongly influenced P. oceanica pattern of colonisation through sexual propagules in both
          the past and present.

                                  eagrasses are marine flowering plants and represent the most recent stage of a long evolutionary process
                                  started from terrestrial monocotyledons. Several morphological and physiological adaptations have
                              S allowed terrestrial plants to return to the marine environment in the relatively recent past and to effectively
                              compete with algae .
                                Seagrasses belong to the monocotyledon subclass Alismatidae Takht. and are represented by 13 genera that
                              evolved in three separate hydrophilous lineages, i.e., marine Hydrocharitaceae Juss., Zosteraceae Dumort. and the
                              monophyletic group of Posidoniaceae Vines, Ruppiaceae Horan. and Cymodoceaceae Benth. & Hook. f., which
                              offer an extraordinary example of convergent evolution .
                                Although the morphology and anatomy of seagrasses varies among taxa as a result of different evolutionary
                              pathways, there is a suite of structural adaptations of this group to the marine environment. Among these traits
                              are strap-shaped leaves possessing fibre strands to withstand drag forces exerted by water movements, photosyn-
                              thesis occurring mainly in the leaf epidermis, loss of stomata and the development of aerenchyma in response to
                              reduced gaseous movements in the liquid medium, submarine pollination and the ability to disperse in the marine
                              environment through several unique mechanisms . Moreover seagrasses need an anchoring system sufficiently
                              developed to withstand the hydrodynamic disturbance produced by waves and currents . This is particularly
                              relevant for species living in high-energy environments.
                                Although among seagrasses other genera have been reported to grow on hard as well as soft bottoms (i.e
                              Amphibolis C. Agardh, Thalassia Banks ex K. D. Koenig and Thalassodendron Hartog) , Phyllospadix Hook. is
                              the only genus that grows predominantly in the littoral zone on hard substrates where it competes with macro-
                              algae for space. Comparing Phyllospadix spp. with the closely related species Zostera marina L. - a species that is
                              typically rooted in soft sediment - several anatomical and morphological traits can be identified as adaptive
                              features to rocky substrates and surf exposure . Among these, short and thickened roots with extensive root hairs
                              provide strong anchorage to hard substrates . As far as we know Phyllospadix scouleri Hook is the only species of
                              the genus for which short adhesive root hairs have been described .
          SCIENTIFIC REPORTS | 5 : 8804 | DOI: 10.1038/srep08804                                               1
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