Tentacles are remarkable anatomical structures in invertebrates for their diversity of form and function. In bivalves, tentacular organs are commonly associated with protective, secretory, and sensory roles. However, anatomical details are available for only a few species, rendering the diversity and evolution of bivalve tentacles still obscure. In Pteriomorphia, a clade including oysters, scallops, pearl oysters, and relatives, tentacles are abundant and diverse. We investigated tentacle anatomy in the group to understand variation, infer functions, and investigate patterns in tentacle diversity. Six species from four pteriomorphian families (Ostreidae, Pinnidae, Pteriidae, and Spondylidae) were collected and thoroughly investigated with integrative microscopy techniques, including histology, scanning electron microscopy, and confocal microscopy. Tentacles can be classified as middle fold tentacles (MFT) and inner fold tentacles (IFT) according to their position with respect to the folds of the mantle margin. While MFT morphology indicates intense secretion of mucosubstances, no evidence for secretory activity was found for IFT. However, both tentacle types have appropriate ciliary distribution and length to promote mucus transportation for cleaning and lubrication. Protective and sensory functions are discussed based on different lines of evidence, including secretion, cilia distribution, musculature, and innervation. Our results support the homology of MFT and IFT only for Pterioidea and Ostreoidea, considering their morphology, the presence of ciliated receptors at the tips, and branched innervation pattern. This is in accordance with recent phylogenetic hypotheses that support the close relationship between these superfamilies. In contrast, major structural differences indicate that MFT and IFT are probably not homologous across all pteriomorphians. By applying integrative microscopy, we were able to reveal anatomical elements that are essential for the understanding of homology and function when dealing with such superficially similar structures.

中文翻译：

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翼形双壳类动物触手的形式和功能

触手是无脊椎动物中非凡的解剖结构，因为它们的形式和功能多种多样。在双壳类动物中，触手器官通常具有保护、分泌和感觉作用。然而，只有少数物种的解剖细节是可用的，这使得双壳类触手的多样性和进化仍然模糊不清。在 Pteriomorphia 中，一个分支包括牡蛎、扇贝、珍珠贝和近亲，触手丰富多样。我们调查了小组中的触手解剖结构，以了解变化、推断功能并研究触手多样性的模式。收集了来自四个翼形科（Ostreidae、Pinnidae、Pteriidae 和 Spondylidae）的六个物种，并使用综合显微镜技术（包括组织学、扫描电子显微镜和共聚焦显微镜）进行了彻底研究。触手根据它们相对于地幔边缘褶皱的位置可分为中折触手（MFT）和内折触手（IFT）。虽然 MFT 形态表明粘液物质的强烈分泌，但没有发现 IFT 分泌活动的证据。然而，两种触手类型都有适当的纤毛分布和长度，以促进粘液运输以进行清洁和润滑。基于不同的证据线讨论了保护和感觉功能，包括分泌、纤毛分布、肌肉组织和神经支配。我们的结果支持 MFT 和 IFT 的同源性，仅适用于 Pterioidea 和 Ostreoidea，考虑到它们的形态、尖端纤毛受体的存在以及分支的神经支配模式。这与最近支持这些超家族之间密切关系的系统发育假设是一致的。相比之下，主要的结构差异表明 MFT 和 IFT 在所有翼形动物中可能并非同源。通过应用集成显微镜，我们能够揭示在处理这种表面相似的结构时对于理解同源性和功能至关重要的解剖学元素。