Anelasma squalicola is a barnacle found attached to deep-water lantern sharks of the family Etmopteridae and is the only known cirriped on fish hosts. While A. squalicola is equipped with mouth and thoracic appendages (cirri), which are used for suspension feeding in conventional barnacles, its attachment device (peduncle) appears to have evolved into a feeding device, embedded into the tissue of its host. Here we demonstrate, through comparisons of the feeding apparatuses between A. squalicola and conventional suspension-feeding barnacles, that mouthparts and cirri of A. squalicola are highly reduced, and incapable of suspension-feeding activities. We show that in conventional suspension-feeding barnacles strong symmetries exist within these vital trophic structures. In A. squalicola strong asymmetries are widespread, indicating that those structures have been uncoupled from natural selection. The digestive tract is consistently empty, suggesting that feeding via cirri does not occur in A. squalicola. In addition, comparisons of stable isotope ratios (δ13C and δ15N) between A. squalicola, its shark host, and a conventional suspending feeding barnacle indicate that A. squalicola is taking nutrition directly from its host shark and not from the surrounding water. Our results strongly indicate that this barnacle has abandoned suspension feeding and now solely relies on obtaining nutrition from its host by a de novo evolved feeding mechanism.

中文翻译：

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陷入困境：鲨鱼藤壶 Anelasma squalicola 最近改变进食策略的表型后果（Lovén，1844）

Anelasma squalicola 是一种藤壶，附着在 Etmopteridae 科的深水灯笼鲨身上，是唯一已知的寄生在鱼类宿主身上的藤壶。虽然 A. squalicola 配备了嘴巴和胸部附件 (cirri)，用于在传统藤壶中悬浮进食，但它的附着装置 (花梗) 似乎已经进化成一种进​​食装置，嵌入宿主的组织中。在这里，我们通过比较 A. squalicola 和传统悬浮摄食藤壶之间的摄食装置，证明 A. squalicola 的口器和卷毛大大减少，无法进行悬浮摄食活动。我们表明，在传统的悬浮喂养藤壶中，这些重要的营养结构中存在很强的对称性。在 A. squalicola 中，强烈的不对称性很普遍，表明这些结构已与自然选择脱钩。消化道始终是空的，这表明角鲨不发生通过卷毛进食。此外，鲨鱼宿主 A. squalicola 和传统悬浮摄食藤壶之间的稳定同位素比率（δ13C 和 δ15N）的比较表明，A. squalicola 直接从其宿主鲨鱼而不是从周围的水中获取营养。我们的结果有力地表明，这种藤壶已经放弃了悬浮进食，现在完全依赖于通过从头进化的进食机制从宿主那里获取营养。A. squalicola、其鲨鱼宿主和传统悬浮摄食藤壶之间稳定同位素比率（δ13C 和 δ15N）的比较表明，A. squalicola 直接从其宿主鲨鱼而不是从周围的水中获取营养。我们的结果有力地表明，这种藤壶已经放弃了悬浮进食，现在完全依赖于通过从头进化的进食机制从宿主那里获取营养。A. squalicola、其鲨鱼宿主和传统悬浮摄食藤壶之间稳定同位素比率（δ13C 和 δ15N）的比较表明，A. squalicola 直接从其宿主鲨鱼而不是从周围的水中获取营养。我们的结果有力地表明，这种藤壶已经放弃了悬浮进食，现在完全依赖于通过从头进化的进食机制从宿主那里获取营养。