Abstract
The pelagic nudibranch Fiona pinnata (Mollusca: Gastropoda) occurs exclusively on macroalgal rafts and other floating substrata, and is found throughout tropical and temperate seas worldwide. Its cosmopolitan distribution has been attributed to its planktotrophic larval mode and propensity for passive rafting, and although it was one of the earliest aeolid nudibranchs to be described, this study produced the first molecular phylogeny for this ubiquitous invertebrate. Mitochondrial and nuclear DNA sequence data was generated from specimens collected worldwide in order to elucidate the genetic structure and diversity within this obligate rafter. Phylogeographic analyses revealed three distinct lineages that were geographically partitioned in concordance with oceanic circulation patterns. Two clades were abundant and widespread, with one displaying a circum-equatorial distribution and the other exhibiting an anti-tropical distribution throughout temperate zones of the Pacific Ocean. A third lineage based on a single Indonesian specimen was also detected, and the genetic divergences and largely allopatric distributions observed among these three clades suggest that they may represent a cryptic species complex. Long-distance dispersal in this nudibranch appears to be current-mediated, and the North-South disjunction detected within New Zealand is concordant with known marine biogeographic breaks. In contrast, populations sampled in Chile and the South Island of New Zealand displayed close phylogenetic relationships, indicating that the West Wind Drift has facilitated trans-oceanic gene flow in Fiona pinnata. All sampled individuals formed a well-supported monophyletic group that was recovered in phylogenetic analyses of several independent molecular markers. Although much ambiguity has surrounded Fiona pinnata’s taxonomic status since its original discovery in 1775, the molecular evidence of the current study confirms that this group is substantially divergent from even its closest relatives in the Fionoidea and thus upholds the systematic standing of the historically monotypic family Fionidae.