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424 DANILO SCUDERI ET ALII

Table 2. Materials of P. radiata and P. fucata utilized in the present study.

& Galinou-Mitsoudi (2013: fig. 4B), and is in rapid But populations constituted by these morpho-
spreading in the Mediterranean, with large popula- logically different specimens tend to remain quite
tions in certain localities, as could be argued by the constantly uniform with respect to those of other
growth rate of the species here attested. geographic regions.
Different species of Pinctada in the world So, another interpretation is that they represent
seem to live in different geographical districts, but different species, which could hybridize themselves
genetic differences between species seem not very when populations overlap their distributional areas.
high and molecular studies appeared controver- So, these intermediates represent hybrid speci-
sial. mens between two species.
Moreover in geographical zones where the dis- Among bivalve molluscs, Mytylidae comprise
tribution areas overlap, morphological intermediates examples of the same problem.
appear, adding confusions both to morphological Genetic studies attested the existence of three
and genetic studies. different european species of Mytylus: M. gallo-
Among the large amount of materials studied, provincialis in the Mediterranean Sea and adjacent
we have found few intermediates between P. radi- areas, M. edulis in North-Western coasts of Europe,
ata and P. fucata as well (Fig. 9). So, how can we and M. trossulus along Scandinavian coasts
interpret these intermediates? (Koehn, 1991).
On the one hand, intermediates could represent Intermediates between these species occur
morphs resulting by mating of different morphs of along overlapping ridges of geographical distribu-
one only quite polymorphic species, which often tion areas (Gardner et al., 1993; Gosling et al.,
live in different geographical areas. 2008).