Page 2 - Alagna_Fici_alii_2015

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The family Posidoniaceae includes the single genus Posidonia K.
D. Koenig, which has a disjunct Mediterranean (1 species) and
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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
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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
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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,
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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
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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
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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
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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
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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
rocks.
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
substrates.
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