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Preliminary analyses revealed the presence of at least five mitochondrial clades within the widespread sea urchin Echinocardium cordatum (Spatangoida). In this study, we analyzed the genetic (two mitochondrial and two nuclear sequence loci) and morphological characteristics (20 indices) from worldwide samples of this taxon to establish the species limits, morphological diversity and differentiation. Co-occurring spatangoid species were also analyzed with mitochondrial DNA. The nuclear sequences confirm that mitochondrial lineages correspond to true genetic entities and reveal that two clades (named A and B1) hybridize in their sympatry area, although a more closely related pair of clades (B1 and B2), whose distributions widely overlap, does not display hybridization. The morphology of all E. cordatum clade pairs was significantly differentiated, but no morphological diagnostic character was evidenced. By contrast, other spatangoid species pairs that diverged more recently than the E. cordatum clades display clear diagnostic characters. Morphological diversity thus appears responsible for the absence of diagnostic characters, ruling out stabilizing selection, a classical explanation for cryptic species. Alternative classical explanations are (i) environmental plasticity or (ii) a high diversity of genes determining morphology, maintained by varying environmental conditions. We suggest a new hypothesis that the observed morphological diversity is selectively neutral and reflects high effective population sizes in the E. cordatum complex. It is supported by the higher abundance of this taxon compared with other taxa, a trend for the genetic and morphological diversity to be correlated in Europe, and the higher genetic and morphological diversities found in clades of E cordatum (except B1) than in other spatangoid samples in Europe. However, the Pacific clades do not confirm these trends.

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