Species delimitation is particularly challenging in Mediterranean nudibranchs because morphological distinction between closely related species is subtle and several cryptic species have been discovered. In this study, we apply an integrative approach to investigate taxonomic distinction of Flabellina cavolini and F. gaditana and assess their genetic and morphologic distinction. We combined a comprehensive phylogeographic survey from across their distribution range and compared genetic variation at mitochondrial and nuclear markers with the variation in colour pattern of cerata that is used to discriminate these two species. Phylogenetic, phylogeographic and species delimitation analyses consistently identify two evolutionary and taxonomic units that do not match with the morphospecies, demonstrating that Flabellina cavolini and F. gaditana are indeed two cryptic, closely related, species and that colour pattern of cerata is not a valid diagnostic character. This study emphasizes how the comparison of range-wide patterns of phylogenetic and morphological variation, with the inclusion of samples from the type localities, is essential for species delimitation of Mediterranean nudibranchs. Moreover, this study demonstrates the occurrence of Flabellina gaditana in the Mediterranean Sea, allowing to extend the known distribution range of the species, that was previously restricted to the eastern Atlantic Ocean, as well as increasing our inventory of Mediterranean nudibranch diversity.

中文翻译：

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当形态学与系统发育不匹配时：两个同胞裸鳃类动物（腹足类动物）的令人费解的案例

地中海裸鳃类动物的物种划定尤其具有挑战性，因为密切相关物种之间的形态差异很微妙，并且已经发现了几种隐匿物种。在这项研究中，我们采用综合方法研究Flabellina cavolini和F. gaditana 的分类学区别并评估它们的遗传和形态差异。我们结合了对它们分布范围内的全面系统地理学调查，并将线粒体和核标记的遗传变异与用于区分这两种物种的 cerata 颜色模式的变异进行了比较。系统发育、系统地理学和物种划分分析一致地确定了与形态种不匹配的两个进化和分类单位，证明Flabellina cavolini和F. gaditana确实是两个神秘的、密切相关的物种，而且 cerata 的颜色模式不是有效的诊断特征。这项研究强调了系统发育和形态变异的范围广泛模式的比较，包括来自类型地点的样本，对于地中海裸鳃类动物的物种定界至关重要。此外，这项研究证明了Flabellina gaditana在地中海的出现，从而扩大了该物种的已知分布范围，该分布范围以前仅限于东大西洋，并增加了我们对地中海裸鳃类动物多样性的库存。