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1304 Genetica (2011) 139:1293–1308

match any discrete geographic areas. The high small-scale
geographical complexity of this area, which mostly
encompasses an archipelago with complex hydrodynamic
regimes (Pracchi and Terrosu Asole 1971; Gerigny et al.
2011), may shape the distribution of genetic variability.
In contrast to the patterns observed for the SCR group,
the pattern of ISSR genetic variability found in the SAS
group is rather homogeneous (Figs. 3a, b, 4d). The Mantel
correlogram obtained for the SAS group (Fig. 5d) dis-
played a shape that resembled IBD or an almost clinal
variation. Although we cannot exclude the possibility that
this pattern might reflect, in part, the less-dense geographic
coverage of sampling sites in some areas of the SAS group,
we nonetheless hypothesise that a high level of gene flow
occurs in the SAS group. The different degree of genetic
structuring may also depend on the larger population sizes
encountered in the SAS group than in the SCR group
(Casu, Rivera-Ingraham, pers. obs.; Espinosa 2009). The
lack of strong population fragmentation may allow a
stepping-stone fashioned gene flow (see Kimura and Weiss
1964) along the coastline, which may be favoured by the
surface directional current. For instance, the influence of
the Algerian Current, which flows eastward (Robinson
et al. 2001), may contribute to promote the spread of P.
ferruginea larvae. Indeed, this current has been cited as
responsible of the dispersion by rafting of the vermetid
gastropod with direct development Dendropoma petraeum
(Calvo et al. 2009) which shows low genetic structuring in
a region encompassing the Northern African and South-
Fig. 6 Mitochondrial DNA dataset: network based on COI Eastern Spanish coasts from the Atlantic to the Siculo-
sequences. Circle size is proportional to the number of individuals
sharing the same haplotype. The largest circle contains 169 specimens Tunisian Strait. Nonetheless, the presence of individuals
of P. ferruginea. Labels on branches indicate the position at which sharing the same genetic makeup as those found in the
mutations have occurred (mutated position: ARG 7: 13; MVE 6: 53; Zembra and the Sicilian islands in Alboran Island suggests
CDN 2: 252; CHA 2: 266; ALB 3 and CHA 5: 311; PAR2: 370; LIT that local currents, gyres and eddies (Send et al. 1999) may
3: 522; PAR 1: 577)
also favour an occasional larval dispersal westward.
Unlike the ISSR analysis, mtDNA sequences show no
levels of collection for human consumption. These facts significant genetic differentiation throughout the entire P.
lead to hypothesise that past and present human pressure is ferruginea distribution range. Several reasons may be
the main cause of genetic drift of these populations. Indeed, invoked to explain the differences observed between
several sites are presently characterised by both very low nuclear and mtDNA markers. First, ISSRs depict patterns
density and reduced specimen size (Casu, pers. obs.). of multilocus variations at many independent loci, whereas
Although P. ferruginea is known to be able to adjust the mtDNA reflects variation at a single locus. Furthermore,
timing of sex change according to the density of large ISSRs may detect higher levels of genetic variability and/or
individuals (Rivera-Ingraham et al. 2011a), the lack of a genetic structuring with respect to mtDNA markers. This
population of large-sized individuals affects the reproduc- may be related to the high potential for detecting differ-
tive rate (Espinosa et al. 2006). In Corsica and North- ences using random or semirandom primers that amplify
Eastern Sardinia, a less marked spatial genetic structure nuclear noncoding DNA sequences, which evolve faster
than that found in North-Western Sardinia has been and are less constrained by selection than mtDNA genes
observed (Figs. 3a, b, 4c). These findings suggest that gene (e.g., De Aranzamendi et al. 2008, 2009). Indeed, mtDNA
flow is sufficient enough to prevent extensive genetic dif- may be affected by natural selection (Ballard and Whitlock
ferentiation in this area. However, the pattern of genetic 2004), which may drastically reduce the number of hap-
distribution in North-Eastern Sardinia is quite complex, lotypes. In particular, Bazin et al. (2006) suggested that the
due to the overlap of two Bayesian subclusters that do not mtDNA variation in invertebrates is consistent with the

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