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Chemistry and Ecology 259

Downloaded By: [Furnari, G.] At: 10:31 24 May 2010 [67] compared with the flowering performance estimated in Sicily showed that the latter area has
higher averages values [23]. However, in sites where P. oceanica grows near its upper limit of
thermal tolerance (Stagnone di Marsala lagoon), no flowering events have ever been detected over
a 24 year time series.

Flowering probability increases with age, reaching a maximum in 15-year-old shoots (with
an inflorescence frequency per age of 12.7%), as observed in Sicilian meadows [18]. Fifteen
years thus represents the ageing point at which the probability that a shoot can perform sexual
reproduction declines. This pattern partially confirms the conclusions of a study by Balestri and
Vallerini [87] on the north-western coast of Italy, but is not in agreement with data recorded in
other localities of western Mediterranean [67], showing that optimal age for flowering may change
with geographic area in the Mediterranean. However, the latter suggestion should be taken with
caution because the partial agreement could be derived from the difference in the time-length series
used in the literature (16–32 years old). It would be interesting to assess whether the threshold
age for sexual reproduction applies to single shoots or to the entire genotype from which the
shoot eventually branched. More insights in this direction could help in clarifying the observed
differences.

Previous studies suggest that flowering has a negative effect on leaf biometry [94,95], determin-
ing also a reduction in rhizome elongation and production lowered to ∼27 and 38%, respectively
[18]. According to these calculations, it is possible that the cost associated with sexual reproduc-
tion may be difficult to sustain for older shoots, suggesting that the species’ capacity to regulate
its internal resource economy to support flowering may be limited by other physiological stresses
linked to ageing.

8. Associated communities

Seagrass epiphytes play an important role in ecosystem functioning contributing significantly to
the primary production of the meadow [96,97] and representing an important food resource for
many organisms [98,99]. Furthermore, the abundance and composition of assemblages of seagrass
epiphytes are considered sensitive indicators of natural and anthropogenic disturbance [100,101].
Only studies on H. stipulacea and P. oceanica epiphyte assemblages have been carried out along
Sicilian coasts over the last 30 years.

The first study of the macroalgal epiphytic community on H. stipulacea, dating back to 1993
[28], was carried out on a meadow in Catania harbour. A total of 30 species (22 Rhodophyta,
3 Ochrophyta and 5 Chlorophyta) were found. From a qualitative point of view, the epiphytic
community on H. stipulacea results are very similar to those on P. oceanica leaves [13]. Among
the epiphytes identified the presence of Chondria pygmaea Garbary et Vandermeulen, described
in the Red Sea as epiphytic on H. stipulacea leaves [102], is noteworthy. This leads us to suppose
that this species migrated into the Mediterranean together with its host species [103].

Some years later, a meadow of H. stipulacea occurring near the harbour of the island of Vulcano
(Aeolian Islands) was studied to assess the associated community [32]. In that study, only nine
species of macroalgae were reported as epiphytic on H. stipulacea leaves.

Further data is provided by Di Martino et al. [58] in a study of a meadow of H. stipulacea
present at Capo Meli (Syracuse). In this article the authors report a total of 61 and 39 species
of macroalgal epiphytic on both rhizomes and leaves in autumn and spring, respectively. More
recently, Di Martino et al. [104] studied temporal variations in the algal assemblage associated
with a H. stipulacea meadow in the Maddalena Peninsula (Syracuse). A total of 110 species
(5 Cyanophyta, 81 Rhodophyta, 19 Ochrophyta and 5 Chlorophyta) epiphytic on H. stipulacea
leaves were found.