Department of Ecology, Evolution, and Organismal Biology, Brown University, Providence, RI 02912, USA.
Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637, USA.
Integr Comp Biol. 2022 Oct 29;62(4):865-877. doi: 10.1093/icb/icac109.
Research on the water-to-land transition tends to focus on the locomotor changes necessary for terrestriality. However, the evolution from water breathing to air breathing was also a necessary precursor to the invasion of land. Air is approximately 1000 times less dense and 50 times less viscous, and contains hundreds of times more oxygen than water. However, unlike the transition to terrestrial locomotion, breathing air does not require body weight support, so the evolution of air breathing may have necessitated smaller changes to morphology and function. We used X-ray reconstruction of moving morphology to compare the cranial kinematics of aquatic buccal pumping, such as that seen in suction feeding, with the aerial buccal pumping required for lung ventilation in the West African lungfish (Protopterus annectens). During buccal pumping behaviors, the cranial bones and associated soft tissues act as valves and pumps, and the sequence of their motions controls the pattern of fluid flow. Both behaviors are characterized by an anterior-to-posterior wave of expansion and an anterior-to-posterior wave of compression. We found that the pectoral girdle and cranial rib rotate consistently during air breathing and suction feeding, and that the muscle between them shortens during buccal expansion. Overall, we conclude that the major cranial bones maintain the same basic functions (i.e., acting as valves or pumps, or transmitting power) across aquatic and aerial buccal pumping. The cranial morphology that enables aquatic buccal pumping is well suited to perform air breathing and accommodates the physical differences between air and water.
水到陆的转变研究往往侧重于为陆地生活所需的运动变化。然而，从水中呼吸到空气中呼吸的进化也是陆地入侵的必要前提。空气的密度约为水的 1000 倍，粘性约为水的 50 倍，并且氧气含量是水的数百倍。然而，与向陆地运动的转变不同，空气呼吸不需要体重支撑，因此空气呼吸的进化可能需要对形态和功能进行较小的改变。我们使用运动形态的 X 射线重建来比较水生口腔抽吸（如吸水喂养）的颅部运动学与西非肺鱼（Protopterus annectens）进行肺通气所需的空气口腔抽吸。在口腔抽吸行为中，颅骨和相关软组织充当阀门和泵，它们的运动顺序控制着流体流动的模式。这两种行为的特征都是前向后的扩张波和前向后的压缩波。我们发现，在空气呼吸和吸水喂养过程中，肩带和颅肋始终旋转，并且在口腔扩张期间它们之间的肌肉会缩短。总的来说，我们得出结论，主要的颅骨在水生和空气口腔抽吸中保持相同的基本功能（即充当阀门或泵，或传递动力）。使水生口腔抽吸成为可能的颅形态非常适合进行空气呼吸，并适应空气和水之间的物理差异。
