Wood Chris M, Pelster Bernd, Val Adalberto Luis
Department of Zoology, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada.
Department of Biology, McMaster University, Hamilton, ON, L8S 4K1, Canada.
J Comp Physiol B. 2025 Feb;195(1):39-51. doi: 10.1007/s00360-024-01597-7. Epub 2024 Dec 20.
The pirarucu is one of the very few obligate air-breathing fish, employing a gigantic, highly vascularized air-breathing organ (ABO). Traditionally, the ABO is thought to serve mainly for O uptake (ṀO), with the gills providing the major route for excretion of CO (ṀCO) and N-waste. However, under aquatic hypercapnia, a common occurrence in its natural environment, branchial ṀCO to the water may become impaired. Under these conditions, does the ABO become an important route of ṀCO excretion to the air? We have answered this question by measuring ṀCO and ṀO in both air and water phases, as well as the pattern of air-breathing, in pirarucu under aquatic normocapnia and hypercapnia (3% CO). Indeed, ṀCO to the air phase via the ABO increased 2- to 3-fold during exposure to high water PCO, accounting for 59-71% of the total, with no change in the dominant contribution of the ABO to ṀO (71-75% of the total). These adjustments were quickly reversed upon restoration of aquatic normocapnia. During aquatic hypercapnia, ṀCO via the ABO became more effective over time, and the pattern of air-breathing changed, exhibiting increased frequency and decreased breath volume. Ammonia-N excretion (86-88% of total) dominated over urea-N excretion and tended to increase during exposure to aquatic hypercapnia. We conclude that the ability of the ABO to take on the dominant role in CO excretion when required may have been an important driver in the original evolution of air-breathing, as well as in the functionality of the ABO in modern air-breathing fish.
巨骨舌鱼是极少数必须呼吸空气的鱼类之一，它拥有一个巨大的、血管高度丰富的呼吸空气器官（ABO）。传统观点认为，ABO主要用于摄取氧气（ṀO），鳃则是排出二氧化碳（ṀCO）和含氮废物的主要途径。然而，在其自然环境中常见的水生高碳酸血症情况下，鳃向水中排出ṀCO的功能可能会受损。在这些条件下，ABO会成为向空气中排出ṀCO的重要途径吗？我们通过测量巨骨舌鱼在水生正常碳酸血症和高碳酸血症（3% CO₂）状态下，在空气和水相中ṀCO和ṀO以及呼吸模式，回答了这个问题。实际上，在暴露于高水相PCO₂期间，通过ABO向空气相排出的ṀCO增加了2至3倍，占总量的59 - 71%，而ABO对ṀO的主要贡献（占总量的71 - 75%）没有变化。恢复水生正常碳酸血症后，这些调节作用迅速逆转。在水生高碳酸血症期间，随着时间推移，通过ABO排出的ṀCO变得更加有效，呼吸模式发生变化，表现为频率增加和呼吸量减少。氨氮排泄（占总量的86 - 88%）在尿素氮排泄中占主导地位，并且在暴露于水生高碳酸血症期间有增加的趋势。我们得出结论，ABO在需要时承担排出CO₂主导作用的能力，可能是呼吸空气最初进化过程中的一个重要驱动因素，也是现代呼吸空气鱼类中ABO功能的一个重要驱动因素。
