Aeolid nudibranchs are well-known for their ability to incorporate cnidarian nematocysts and use them for defense; this process is tightly linked with the feeding preferences of molluscs. As many nudibranch groups show signs of ecology-based adaptive radiation, studies of prey-based defensive mechanisms can provide valuable insight into details of nudibranch evolutionary history. The main goal of this study is to test the correlation of ecological traits, feeding mechanisms, and prey preferences with cnidosac fine morphology and to pinpoint the phylogenetic value of these traits. We study the cnidosac morphology in thirteen species—representatives of the main lineages within the family Fionidae s.l. The morphological analysis includes histological sections, transmission electron microscopy, confocal laser scanning microscopy, and scanning electron microscopy. For phylogenetic study, available molecular data from public repositories were used, and phylogenetic trees were produced based on Bayesian Inference and Maximum likelihood analysis for a concatenated dataset of three molecular markers (COI, 16S, H3). In general, fionid cnidosacs fit the common aeolid pattern, but among different species we detected a high variation in type of obtained nematocysts, their arrangement within cnidophages, and in number of cell types within cnidosacs. We report on presence of cellules speciale in the haemocoel of all studied species, and for the first time, we report on cells with chitinous spindles in the haemocoel of all fionids except Eubranchus. The function of both these cell types remains unknown. The loss of functional cnidosacs occurred at least three times within Fionidae, and in case of the genera Phestilla, Calma, and Fiona, this loss is linked to their non-cnidarian diet. The diversity of cnidosac fine structure within Fionidae s.l. correlates with that of the radular morphology and feeding preferences of each species. Prey shifts between cnidarian and non-cnidarian prey (both through evolutionary shifts and individual variation) rarely occur within Fionidae s.l.; however, microevolutionary shifts between different hydrozoan species within a single genus are more common. Cnidosac morphology demonstrates considerable resulting changes even when switching between similar hydrozoan species, or changing the feeding site on same prey species. These data indicate that cnidosac morphology likely follows microevolutionary prey shifts—in other words, it is affected by switches in prey species and changes in feeding sites with a single prey species. Thus, the cnidosac morphology may be a useful indicator when studying ecological features of particular species.

中文翻译：

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考虑到生态特性和进化，Fionidae 科（腹足纲：Nudibranchia）内的线虫囊封存

Aeolid nudibranchs 以其吸收刺胞动物线虫囊并将其用于防御的能力而闻名；这个过程与软体动物的摄食偏好密切相关。由于许多裸鳃亚目群体显示出基于生态学的适应性辐射迹象，因此对基于猎物的防御机制的研究可以提供对裸鳃亚目进化史细节的宝贵见解。本研究的主要目标是测试生态特征、摄食机制和猎物偏好与刺胞动物精细形态的相关性，并确定这些特征的系统发育价值。我们研究了 13 个物种的 cnidosac 形态——Fionidae sl 中主要谱系的代表。形态学分析包括组织切片、透射电子显微镜、共聚焦激光扫描显微镜、和扫描电子显微镜。对于系统发育研究，使用了来自公共存储库的可用分子数据，并根据贝叶斯推理和最大似然分析对三个分子标记（COI、16S、H3）的串联数据集生成了系统发育树。一般来说，fionid cnidosacs 符合常见的 aeolid 模式，但在不同物种中，我们检测到获得的线虫囊类型、它们在 cnidophages 内的排列以及 cnidosacs 内的细胞类型数量存在很大差异。我们报告了所有研究物种的血腔中都存在特殊细胞，并且我们第一次报告了除真鳃亚纲外所有 fionids 的血腔中具有几丁质纺锤体的细胞。这两种细胞类型的功能仍然未知。在 Fionidae 中，功能性 cnidosacs 的丢失至少发生了 3 次，对于 Phestilla、Calma 和 Fiona 属，这种损失与它们的非刺胞动物饮食有关。Fionidae sl 内 cnidosac 精细结构的多样性与每个物种的鳞状形态和摄食偏好相关。刺胞动物和非刺胞动物猎物之间的猎物转换（通过进化转换和个体变异）很少发生在 Fionidae sl 中；然而，单个属内不同水螅动物物种之间的微进化转变更为常见。即使在相似的水螅动物物种之间切换，或改变同一猎物物种的进食地点，刺胞囊的形态也会发生相当大的变化。这些数据表明 cnidosac 的形态可能遵循微进化猎物的变化——换句话说，它受到猎物物种转换和单一猎物物种觅食地点变化的影响。因此，在研究特定物种的生态特征时，刺胞囊形态可能是一个有用的指标。