Page 1 - Alagna_Fici_alii_2015

OPEN Evidences of adaptive traits to rocky

substrates undermine paradigm of
SUBJECT AREAS:
habitat preference of the Mediterranean
ECOPHYSIOLOGY
EVOLUTIONARY ECOLOGY
seagrass Posidonia oceanica
PLANT ECOLOGY
BIOOCEANOGRAPHY
Fabio Badalamenti , Adriana Alagna & Silvio Fici 2
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Received
7 October 2014
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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
alagna@iamc.cnr.it;
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
gmail.com) 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
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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 .
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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
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environment through several unique mechanisms . Moreover seagrasses need an anchoring system sufficiently
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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
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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
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features to rocky substrates and surf exposure . Among these, short and thickened roots with extensive root hairs
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provide strong anchorage to hard substrates . As far as we know Phyllospadix scouleri Hook is the only species of
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the genus for which short adhesive root hairs have been described .
SCIENTIFIC REPORTS | 5 : 8804 | DOI: 10.1038/srep08804 1