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90 Gianmarco Ingrosso et al.

relies exclusively on larvae and propagules, whose duration is often limited in
time. However, this hypothesis requires testing, since the relationship
between pelagic larval duration (PLD) and genetic structure is far from linear,
and the larval/propagule biology of bioconstructors is itself often poorly
known. For animals, several recent reviews showed a poor correlation
between PLD and population genetic structure, suggesting that patterns of
connectivity in the marine realm are difficult to predict based on life history
features alone (Costantini et al., 2018; Riginos et al., 2011; Weersing and
Toonen, 2009). Usually, small-scale genetic structuring is consistent with
short PLD, often contrasting with species with long-living larvae (Pascual
et al., 2017). Moreover, at larger spatial scales, other biotic (e.g. presettlement
processes) and/or abiotic (e.g. hydrodynamics, geomorphology) variables may
shape the genetic structuring of populations.
So far, genetic connectivity of animal bioconstructors and benthic organ-
isms inhabiting Mediterranean bioconstructions is poorly known. The few
available studies have focused mainly on Porifera, Cnidaria, Echinodermata
and Tunicata, using mitochondrial and microsatellite genetic markers
(Costantini et al., 2017). An analysis of a wider array of invertebrates living
in bioconstructions is highly desirable, but is currently limited by the lack of
knowledge on life histories and distributions, and by the availability of
species-specific genetic markers (Adjeroud et al., 2014; Casado-Amezua
et al., 2011, 2012; Costantini et al., 2017).
The 2b-RAD Genotyping-by-Sequencing technique (Wang et al.,
2012) is useful for genotyping a high number of SNP markers across the
entire genome in nonmodel organisms (Paterno et al., 2017). Its application
to several hundred individuals of five of the most abundant species in
Mediterranean coralligenous reefs (Leptopsammia pruvoti Lacaze-Duthiers,
1897, C. caespitosa, Myriapora truncata (Pallas, 1766), Parazoanthus axinellae
(Schmidt, 1862) and P. clavata) collected in all Italian seas revealed a strong
and highly significant genetic structuring among populations for all species
(Costantini et al., 2018; L. Zane and E. Boscari, unpublished observations).
Each species showed geographical differentiation, with a high percentage of
genetic variation explained by population subdivision, indicating strong
isolation between samples. Genetic data for the five species indicated that
the strongest barriers to gene flow were between the western (Tyrrhenian
and Ligurian Seas) and the eastern (Adriatic and Ionian Seas) samples. This
result suggests that genetic data, besides showing isolation between sites, keep
a trace of historical processes and contemporary factors leading to and
maintaining the separation of the two basins. However, strong differences