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Phylogeography and Cryptic Speciation of the Bivalved Sea Slug Genus Julia Gould, 1862
Juliidae E.A. Smith, 1885 is a family of Sacoglossan bivalve gastropods mistakenly placed in Bivalvia when initially discovered. These are the only known gastropods with bivalve shells, making them a morphologically unique group. A recent morphological and molecular study for the entire family Juliidae incorporated 21 samples of Julia (J. exquisita, J. zebra, and J. sp.) from six general localities worldwide and found support for unrecognized species diversity within the genus (three candidate species). Julia species were also previously reported to have interesting disjunct geographic distributions across the Indo-Pacific, often with overlapping ranges. These preliminary molecular results, in addition to their widespread distributions, warranted further investigation into Julia, especially since other recent studies on heterobranch sea slugs have revealed cryptic and pseudocryptic species in groups with large distributions. The objectives of this study were to use molecular sequence data to identify and delineate species of Julia using a more comprehensive representation of individuals across their ranges and supplement these data with morphological analyses of the bivalve shells to aid in potential species descriptions. The majority of the molecular data for this study were obtained from historical collections from several natural history museums, using DNA extraction methods that allowed the use of these dried, unpreserved specimens that were previously thought to yield insufficient or no DNA. One nuclear (H3) and two mitochondrial genes (CO1 and 16S) were used to establish a Bayesian and Maximum Likelihood phylogenetic hypothesis for members of Julia. Haplotype networks using the CO1 gene were created to visualize geographic differentiation. An Automatic Barcode Gap Discovery (ABGD) analysis recovered a total of 20 candidate species that coincide with monophyletic clades on the phylogenetic trees. Candidate species of Julia were found to be sympatric and haplotype networks showed little geographic differentiation among disjunct individuals within the same species. Morphological characteristics were found to be different among species complexes, however more data are needed to make conclusions about defining characteristics for candidate species recovered with genetic data. This study not only highlights the importance of museum collections in documenting species diversity, but also provides a framework for studying the evolution and biogeographical patterns of a group whose taxonomy, ecology, and overall biology have proven to be obscure since its initial discovery. This study provides an updated taxonomy of a morphologically interesting group that can help in future research of biogeographic and speciation patterns in other groups of organisms that share similar diversity and distributions.