Evolutionary transitions from benthic to pelagic habitats are major adaptive shifts. Investigations into such shifts are critical for understanding the complex interaction between co-opting existing traits for new functions and novel traits that originate during or post-transition. Gastropod mollusks are of particular interest in regard to benthic-pelagic evolutionary transitions, as shifts from benthic to pelagic habitats are uncommon. Phylliroe is one such pelagic lineage in Phylliroidae, a family of holoplanktonic nudibranchs with a highly aberrant morphology that appears to be adapted for life in the pelagic zone. However, the phylogenetic placement of this enigmatic group of pelagic nudibranchs has never been investigated. Here we present phylogenomic analyses which place Phylliroe within a group of nudibranchs called Dendronotida sensu stricto. We also discuss a subset of the morphological and behavioral features that Phylliroe shares with other closely related lineages (Dendronotidae, Tethyidae, and Scyllaeidae) and some that are unique to Phylliroe. Based on these data, and a literature review, we find a number of unique features found in Phylliroe that are adaptations to a pelagic environment, such as a fish-like body plan, highly reduced connective and muscular tissue in the notum, and elongated rhinophores. However, we were able to identify only a single commonality among Phylliroe and its closely related lineages, which is the presence of left-right swimming behavior. We further hypothesize that swimming behavior in this group likely represents an important trait that facilitated the transition from benthic to pelagic environments, and thus may provide evidence that major pelagic lifestyle transitions can rely on behavioral exaptations. These new insights into the origins of Phylliroe now provide a phylogenetic framework for testing for adaptations necessary for the benthic-pelagic transition in this group.

中文翻译：

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系统基因组学分析和形态学数据表明，在远洋叶状科（Gastropoda：Nudibranchia）起源之前就已经进化出左右游泳行为

从底栖到远洋栖息地的进化转变是主要的适应性转变。对此类转变的调查对于理解为新功能选择现有特征与源自过渡期间或过渡后的新特征之间的复杂相互作用至关重要。腹足类软体动物对底栖-远洋进化转变特别感兴趣，因为从底栖到远洋栖息地的转变并不常见。Phylliroe 是 Phylliroidae 中的一个这样的远洋谱系，这是一个全浮游裸鳃类动物，具有高度异常的形态，似乎适合在远洋地区生活。然而，从未研究过这个神秘的远洋裸鳃类动物群的系统发育位置。在这里，我们提出了将 Phylliroe 置于一组称为 Dendronotida sensu stricto 的裸鳃类中的系统发育分析。我们还讨论了 Phylliroe 与其他密切相关的谱系（Dendronotidae、Tethyidae 和 Scyllaeidae）以及一些 Phylliroe 独有的形态和行为特征的子集。基于这些数据和文献综述，我们在 Phylliroe 中发现了许多适应远洋环境的独特特征，例如鱼状的身体计划、notum 中高度减少的结缔组织和肌肉组织，以及拉长的犀牛. 然而，我们只能确定 Phylliroe 及其密切相关谱系之间的一个共同点，即左右游泳行为的存在。我们进一步假设，该组的游泳行为可能代表了促进从底栖环境到远洋环境过渡的重要特征，因此可能提供证据表明主要的远洋生活方式转变可以依赖于行为的改变。这些对 Phylliroe 起源的新见解现在提供了一个系统发育框架，用于测试该组底栖 - 远洋过渡所需的适应性。