Molecular characterization and phylogenetic position of the giant deep-sea oyster Neopycnodonte zibrowii Gofas, Salas & Taviani, 2009

The giant deep-sea oyster Neopycnodonte zibrowii Gofas, C. Salas & Taviani, 2009 is a keystone deep-sea habitat builder species. Discovered about fifteen years ago in the Azores, it has been described and assigned to the genus Neopycnodonte Fischer von Waldheim, 1835 based on morphological features. In this study, we generated DNA sequence data for both mitochondrial (COI and 16S) and nuclear (ITS2 and 28S) markers based on the holotype specimen of N. zibrowii to establish a molecular phylogenetic framework for the systematic assessment of this species and to provide a reliable (i.e., holotype-based) reference sequence set for multilocus DNA barcoding approaches. Molecular data provide compelling evidence that the giant deep-sea oyster is a distinct species, rather than a deep-water ecophenotype of Neopycnodonte cochlear (Poli, 1795), with extremely high genetic divergence from any other gryphaeid. Multilocus phylogenetic analyses place the giant deep-sea oyster within the clade "Neopycnodonte/Pycnodonte" with closer affinity to N. cochlear rather than to P. taniguchii Hayami & Kase, 1992, thus supporting its assignment to the genus Neopycnodonte. Relationships within this clade are not well supported because mitochondrial variation is inflated by saturation that eroded phylogenetic signal, implying an old split between taxa within this clade. Finally, the set of reference barcode sequences of N. zibrowii generated in this study will be useful for a wide plethora of barcoding applications in deep-sea biodiversity surveys. Molecular validation of recent records of deep-sea oysters from the Atlantic Ocean and the Mediterranean Sea will be crucial to clarify the distribution of N. zibrowii and assess the phenotypic variation and ecology of this enigmatic species.