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Mediterranean Bioconstructions Along the Italian Coast 91

were also evident within each geographic area, since all the pairwise compar-
isons were highly significant, including those involving the closest sampling
sites (about 70km for C. caespitosa, M. truncata and P. clavata, and about 5km
for L. pruvoti). Similarly, previous studies on coralligenous cnidarians, based
on microsatellites (C. rubrum, E. singularis and P. clavata)(Cannas et al., 2015,
2016; Costantini et al., 2016a,b, 2017; Mokhtar-Jamaı ¨ et al., 2011;
Pilczynska et al., 2016), showed populations structuring at different spatial scales
(from local to Mediterranean) and a strong differentiation among patches
separated by distances ranging from tens of kilometres to metres. Thus, both
the 2b-RAD approach and previous microsatellite results support the hypoth-
esis of negligible dispersal, even at the very local scale, which may be caused by
the very short duration of the free-swimming larval stage. These results indicate
a much higher structuring in bioconstructors than is shown in fish and other
invertebrates over comparable areas (Boissin et al., 2016; Paterno et al., 2017).
Information concerning genetic connectivity in algal bioconstructors is
limited. In general, molecular data for Mediterranean macroalgae are still
scarce (and have been produced mainly in studies with a taxonomic focus).
Coralline algae of the genera Lithophyllum, Mesophyllum, Lithothamnion and
Neogoniolithon produce the largest algal bioconstructions in the Mediterra-
nean Sea, and these sessile algae disperse by means of nonflagellate spores
(bispores, carpospores, tetraspores). Red algal spores are small cells with lim-
ited active movement and are devoid of storage substances: they settle only
on hard substrata and remain viable for only a few days (Norton, 1992;
Santelices, 1990). Although their dispersal is substantially affected by the
velocity of waves and water currents, these propagules do not disperse
more than a few kilometres, with most of them settling much closer to
the parental source (Gaylord et al., 2002). Furthermore, the calcareous
nature of the thallus prevents long distance dispersal by floating, a mecha-
nism known to be important for other morphological types of seaweeds
(Norton, 1992). A strong genetic structuring can be expected for Mediter-
ranean coralline bioconstructors, and this hypothesis is supported by data
available for the only species investigated in detail, L. byssoides. Pezzolesi
et al. (2017) sequenced 2 organellar markers in populations of this species
from 15 sites in the Central Mediterranean, for which they found high
haplotype diversity. The observed patterns did not show a clear correspon-
dence with the main biogeographic boundaries recognized in this area
(Patarnello et al., 2007); they were interpreted as a result of both past climatic
events in the hydrogeological history of the Mediterranean and by poor
propagule dispersal. L. byssoides spores are able to settle and germinate only