Museum of Natural and Environmental History, Shizuoka, Oya 5762, Suruga, Shizuoka, 422-8017, Japan.
Wildlife Research Center, Kyoto University, 2-24 Tanaka Sekiden-cho, Sakyo, Kyoto, 606-8203, Japan.
Cell Tissue Res. 2021 Jan;383(1):353-365. doi: 10.1007/s00441-020-03382-8. Epub 2021 Jan 6.
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.
羊膜动物起源于陆地，但水生/两栖类群体在羊膜动物中多次独立出现。在适应水生环境的过程中，具有不同系统发育背景和身体结构的物种必须适应新环境带来的相似问题，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，水生适应过程中嗅觉的进化非常有趣，因为水下嗅觉所需的受体与陆地嗅觉所需的受体有很大的不同。在这里，我回顾了水生/两栖羊膜动物的嗅觉能力，特别是那些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都有所降低。值得注意的是，鲸类和海蛇表现出了极端的例子：鲸类已经失去了鼻甲骨系统，而且齿鲸已经失去了所有的嗅觉神经系统。须鲸可以在空气中闻到气味，但它们的嗅觉能力有限。完全水生的海蛇已经失去了主嗅觉系统，但它们仍然保留着用于感知水下的犁鼻器系统。两栖物种表现出介于陆地和水生物种之间的中间状态，这暗示了它们在理解水生适应过程中的重要性。水生羊膜动物的嗅觉能力多种多样，反映了它们多样化的系统发育背景和生态。
