Appendicularia consists of approximately 70 purely marine species that belong to Tunicata the probable sister taxon to Craniota. Therefore, Appendicularia plays a pivotal role for our understanding of chordate evolution. In addition, appendicularians are an important part of the epipelagic marine plankton. Nevertheless, little is known about appendicularian species, especially from deeper water. Using µCT, scanning electron microscopy, and digital 3D-reconstruction techniques we describe three pairs of complex oral sensory organs in the mesopelagic appendicularian Bathochordaeus stygius. The oral sensory organs are situated at the anterior and lateral margin of the mouth and inside the mouth cavity. A single organ consists of 22–90 secondary receptor cells that project apical cilia through a narrow hole in the epidermis. The receptor cells are innervated by branches of the second brain nerve. Based on position, morphology, and innervation we suggest that the oral sensory organs are homologues of the coronal organs in other tunicates. We discuss the hypothesized homology of coronal organs and the lateral line system of primary aquatic vertebrates. The complex oral sensory organs of B. stygius are unique in tunicates and could be adaptations to the more muffled environment of the mesopelagic.

中文翻译：

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Bathochordaeus stygius (Tunicata Appendillaryia) 的口腔感觉器官结构独特，与冠状器官同源


附肢动物由大约 70 种纯海洋物种组成，属于被囊动物（Tunicata），可能是颅动物的姊妹分类单元。因此，附肢对于我们理解脊索动物的进化起着至关重要的作用。此外，阑尾动物是表层海洋浮游生物的重要组成部分。然而，人们对阑尾类物种知之甚少，尤其是来自较深水域的阑尾类物种。使用 µCT、扫描电子显微镜和数字 3D 重建技术，我们描述了中层阑尾生物 Bathochordaeus stygius 的三对复杂的口腔感觉器官。口腔感觉器官位于口腔的前缘和侧缘以及口腔内部。单个器官由 22-90 个次级受体细胞组成，这些次级受体细胞通过表皮上的窄孔投射顶端纤毛。受体细胞由第二脑神经的分支支配。根据位置、形态和神经支配，我们认为口腔感觉器官与其他被囊动物的冠状器官是同源的。我们讨论了初级水生脊椎动物冠状器官和侧线系统的假设同源性。 B. stygius 复杂的口腔感觉器官在被囊类动物中是独一无二的，可能是为了适应中层更压抑的环境。