ABSTRACT
Within the spatangoid genus Echinocardium, the two most derived species, E. cordatum (E.c.) and E. fenauxi (E.f.) are well defined by architectural and morphological characteristics. But the occurrence of intermediate forms leads to suspicions of polytypism. This study combines morphological and molecular approaches to explore that polytypism. The studied specimens come from Brittany and Normandy (E.c.), Western Mediterranean Sea (E.c. and E.f.), Japan (E.f.), Autralia and New-Zealand (E.c.). E. mortenseni and E. mediterraneum were used as close outgroups, Brissopsis lyrifera and Abatus cordatus as more distant outgroups. Morphological analyses are based on superimposition methods using landmarks. The morphology of sea urchins is represented by 49 landmarks digitized on the apical side. Paired distances are computed and clustered into phenetic trees. The obtained trees are congruent with a cladistic hypothesis based on the architecture, but the largest inter-node distances do not coincide with the separation between Ex. and E.f. In the molecular approach, various regions of the 28S ribosomal RNA molecule are sequenced. The 5’-end, including the C1, D1, C2 and D2 domains and the D8 domain provide indications of the phylogenetic relationships between the considered species. Regardless of origin, E.c. and E.f. form a cluster. However, within that group, specimens are clustered according to locality (Pacific Ocean, Mediterranean Sea or Atlantic Ocean) rather than according to their specific assignment. The discrepencies recorded between the morphological and molecular approaches are discussed with respect to (1) the lack of a sequence of each type of E. cordatum and E. fenauxi, (2) the probable insufficient variation of the LSU-rRNA considered domains.
