Amniotes originated on land, but aquatic/amphibious groups emerged multiple times independently in amniotes. On becoming aquatic, species with different phylogenetic backgrounds and body plans have to adapt themselves to handle similar problems inflicted by their new environment, and this makes aquatic adaptation of amniotes one of the greatest natural experiments. Particularly, evolution of the sense of smell upon aquatic adaptation is of great interest because receptors required for underwater olfaction differ remarkably from those for terrestrial olfaction. Here, I review the olfactory capabilities of aquatic/amphibious amniotes, especially those of cetaceans and sea snakes. Most aquatic/amphibious amniotes show reduced olfactory organs, receptor gene repertoires, and olfactory capabilities. Remarkably, cetaceans and sea snakes show extreme examples: cetaceans have lost the vomeronasal system, and furthermore, toothed whales have lost all of their olfactory nervous systems. Baleen whales can smell in the air, but their olfactory capability is limited. Fully aquatic sea snakes have lost the main olfactory system but they retain the vomeronasal system for sensing underwater. Amphibious species show an intermediate status between terrestrial and aquatic species, implying their importance on understanding the process of aquatic adaptation. The olfactory capabilities of aquatic amniotes are diverse, reflecting their diverse phylogenetic backgrounds and ecology.

中文翻译：

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水生羊膜的嗅觉

羊膜动物起源于陆地，但水生/两栖类群在羊膜动物中独立出现多次。在成为水生动物时，具有不同系统发育背景和身体计划的物种必须适应自己以应对新环境带来的类似问题，这使得羊膜动物的水生适应成为最伟大的自然实验之一。特别是，水生适应性嗅觉的进化引起了极大的兴趣，因为水下嗅觉所需的受体与陆地嗅觉的受体显着不同。在这里，我回顾了水生/两栖羊膜动物的嗅觉能力，尤其是鲸类和海蛇的嗅觉能力。大多数水生/两栖羊膜动物的嗅觉器官、受体基因库和嗅觉能力减弱。值得注意的是，鲸类和海蛇是极端的例子：鲸类失去了犁鼻系统，此外，齿鲸失去了所有的嗅觉神经系统。须鲸可以闻到空气中的气味，但它们的嗅觉能力有限。完全水生的海蛇失去了主要的嗅觉系统，但保留了犁鼻系统，用于在水下进行感知。两栖物种表现出介于陆生和水生物种之间的中间状态，这意味着它们对理解水生适应过程的重要性。水生羊膜动物的嗅觉能力是多种多样的，反映了它们不同的系统发育背景和生态。但它们的嗅觉能力是有限的。完全水生的海蛇失去了主要的嗅觉系统，但保留了犁鼻系统，用于在水下进行感知。两栖物种表现出介于陆生和水生物种之间的中间状态，这意味着它们对理解水生适应过程的重要性。水生羊膜动物的嗅觉能力是多种多样的，反映了它们不同的系统发育背景和生态。但它们的嗅觉能力是有限的。完全水生的海蛇失去了主要的嗅觉系统，但保留了犁鼻系统，用于在水下进行感知。两栖物种表现出介于陆生和水生物种之间的中间状态，这意味着它们对理解水生适应过程的重要性。水生羊膜动物的嗅觉能力是多种多样的，反映了它们不同的系统发育背景和生态。