Echinocardium cordatum was taken as an exemplar of indirect development within the Spatangoida. Investigation of its genetic structure showed that there was no significant genetic differentiation in this spatangoid among Atlantic sites, more than 1000 km apart. However, there was strong genetic differentiation between Atlantic sites and Mediterranean ones. In spite of this, morphologists noted ... [Show full abstract] that individuals from some Atlantic and Mediterranean sites shared common morphological characters with Echinocardium fenauxi. To clarify the status of this species, we have collected specimens in the type area of E. fenauxi (vicinity of Marseille). The GPI locus gave 2 very different patterns : 1) a typical one from E. cordatum, common to all E. cordatum studied from Golfe du Lion, Galicia, Brittany, Scotland ; and 2) a totally different one, typical of some Echinocardium collected in Marseille. The fact that no individual from Marseille showed a combination of alleles from both patterns strongly suggests reproductive isolation between the two groups. In addition to differences in structure of the protein, this leads us to consider GPI as a diagnostic locus for a species of Echinocardium different from E. cordatum. However, this new taxon cannot unequivocally be assigned to E. fenauxi because of the lack of an unambiguous and well-supported morphological diagnosis.