Various morphological adaptations are associated with symbiotic relationships between organisms. One such adaptation is seen in the nemertean genus Malacobdella. All species in the genus are commensals of molluscan hosts, attaching to the surface of host mantles with a terminal sucker. Malacobdella possesses several unique characteristics within the order Monostilifera, exhibiting the terminal sucker and the absence of eyes and apical/cerebral organs, which are related to their adaptation to a commensal lifestyle. Nevertheless, the developmental processes that give rise to these morphological characteristics during their transition from free-living larvae to commensal adults remain uncertain. In the present study, therefore, we visualized the developmental processes of the internal morphologies during postembryonic larval stages using fluorescent molecular markers. We demonstrated the developmental processes, including the formation of the sucker primordium and the functional sucker. Furthermore, our data revealed that sensory organs, including apical/cerebral organs, formed in embryonic and early postembryonic stages but degenerated in the late postembryonic stage prior to settlement within their host using a terminal sucker. This study reveals the formation of the terminal sucker through tissue invagination, shedding light on its adhesion mechanism. Sucker muscle development likely originates from body wall muscles. Notably, M. japonica exhibits negative phototaxis despite lacking larval ocelli. This observation suggests a potential role for other sensory mechanisms, such as the apical and cerebral organs identified in the larvae, in facilitating settlement and adhesive behaviors. The loss of sensory organs during larval development might reflect a transition from planktonic feeding to a stable, host-associated lifestyle. This study also emphasizes the need for further studies to explore the phylogenetic relationships within the infraorder Amphiporiina and investigate the postembryonic development of neuromuscular systems in closely related taxa to gain a more comprehensive understanding of ecological adaptations in Nemertea.

中文翻译：

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共生丝带虫的胚胎后发育和生活方式转变


各种形态适应与生物体之间的共生关系有关。一种这样的适应出现在纽虫属 Malacobdella 中。该属中的所有物种都是软体动物宿主的共生体，通过末端吸盘附着在宿主外套膜的表面。 Malacobdella 在 Monostilifera 目中具有几个独特的特征，表现出末端吸盘并且没有眼睛和顶端/大脑器官，这与它们对共生生活方式的适应有关。然而，在它们从自由生活的幼虫到共生成虫的转变过程中产生这些形态特征的发育过程仍然不确定。因此，在本研究中，我们使用荧光分子标记可视化了胚胎后幼虫阶段内部形态的发育过程。我们展示了发育过程，包括吸盘原基和功能吸盘的形成。此外，我们的数据显示，感觉器官，包括顶端/大脑器官，在胚胎和胚胎后早期形成，但在胚胎后晚期退化，然后使用末端吸盘在宿主体内定居。这项研究揭示了末端吸盘通过组织内陷形成的过程，揭示了其粘附机制。吸盘肌肉的发育可能源自体壁肌肉。值得注意的是，尽管缺乏幼虫单眼，粳稻仍表现出负趋光性。这一观察结果表明其他感觉机制（例如幼虫中发现的顶端器官和大脑器官）在促进沉降和粘附行为方面具有潜在作用。 幼虫发育过程中感觉器官的丧失可能反映了从浮游摄食向稳定的、与宿主相关的生活方式的转变。这项研究还强调需要进一步研究探索下目双足动物内的系统发育关系，并研究密切相关类群中神经肌肉系统的胚胎后发育，以更全面地了解纽虫的生态适应。