Here be dragons - phylogeography ofPteraeolidia ianthina(Angas, 1864) reveals multiple species of photosynthetic nudibranchs (Aeolidina: Nudibranchia)

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The aeolid Pteraeolidia ianthina (Angas, 1864) is a strikingly-coloured aeolid nudibranch, informally known as the ‘Blue Dragon’. It is recognised as an unusually widespread Indo-Pacific species, with variation in colouration and morphology, and biogeographic differences in zooxanthellae (dinoflagellate symbionts of the genus Symbiodinium). This variation hints at possible cryptic species, which was tested here using phylogenetic analyses of mitochondrial DNA data (COI, 16S). Our results showed multiple well-supported clades with slight but consistent differences in radular morphology and colouration, and thus we clarify one of the three available names. A temperate NSW clade showed a more elongate and pointed central radular tooth and lacked white body colouration, in comparison to a more variable tropical clade, which had a shorter and more blunt central tooth. The type locality of Pteraeolidia ianthina is Sydney Harbour, New South Wales (NSW), Australia, and according to our study, does not occur outside NSW. Pteraeolidia semperi (Bergh, 1870) and P. scolopendrella (Risbec, 1928) are removed from synonymy with P. ianthina. Wider phylogeographic sampling is required before resolving the availability of the two remaining names, and subclades within the tropical clade, but there is evidence to suggest multiple cryptic species exist. The biogeographic differences in symbionts, and the importance of their role in life history, suggests that changes in symbiosis may have helped drive divergence via local adaptation in the host nudibranchs.