Different but the same: DNA identification reveals a striking colour variability in a Mediterranean eolid sea slug specimen (Mollusca: Nudibranchia)

A peculiar eolid nudibranch showing an unknown chromatic array was found in a rocky bottom of Santa Maria al Bagno, in the Salento peninsula, Ionian Sea (Central Mediterranean Sea). This specimen, initially identified as Piseinotecus sp., was observed in situ and photographed while feeding and laying eggs close to individuals belonging to the Mediterranean Piseinotecus soussi. To assess the identity of this unexpected Piseinotecus ‘white morph’, a DNA identification approach was carried out using mitochondrial cytochrome c oxidase subunit I (COI), as it is the molecular marker mostly used to distinguish nudibranchs species. The molecular analysis unambiguously identified this specimen as Piseinotecus soussi and helped to shed lights on the striking intraspecific colour variability characterizing this rare species.