In the pirarucu (Arapaima gigas), gill surface area and thus gas exchange capacity of the gills are reduced with proceeding development. It, therefore, is expected that A. gigas, starting as a water breather, progressively turns into an obligate air-breathing fish using an air-breathing organ (ABO) for gas exchange. We assessed the air-breathing activity, O2 and CO2 exchange into air and water, ammonia-N and urea-N excretion, ion flux rates, and activities of ion transport ATPases in large versus small pirarucu. We found that even very young A. gigas (4–6 g, 2–3 weeks post-hatch) with extensive gills are air-breathers (18.1 breaths*h−1) and cover most (63%) of their O2 requirements from the air whereas 600–700-g animals (about 3–4 months post-hatch), with reduced gills, obtain 75% of their O2 from the air (10.8 breaths*h−1). Accordingly, the reduction in gill surface area hardly affected O2 uptake, but development had a significant effect on aerial CO2 excretion, which was very low (3%) in small fish and increased to 12% in larger fish, yielding a hyper-allometric scaling coefficient (1.12) in contrast to 0.82–0.84 for aquatic and total CO2 excretion. Mass-specific ammonia excretion decreased in approximate proportion to mass-specific O2 consumption as the fish grew, but urea-N excretion dropped from 18% (at 4–6 g) to 8% (at 600–700 g) of total N-excretion; scaling coefficients for all these parameters were 0.70–0.80. Mass-specific sodium influx and efflux rates, as well as potassium net loss rates, departed from this pattern, being greater in larger fish; hyper-allometric scaling coefficients were > 1.0. Gill V-type H+ ATPase activities were greater than Na+, K+-ATPase activities, but levels were generally low and comparable in large and small fish, and similar activities were detected in the ABO. A. gigas is a carnivorous fish throughout its lifecycle, and, despite fasting, protein oxidation accounted for the major portion (61–82%) of aerobic metabolism in both large and small animals. ABO PO2 and PCO2 (measured in 600–700-g fish) were quite variable, and aerial hypoxia resulted in lower ABO PO2 values. Under normoxic conditions, a positive correlation between breath volume and ABP PO2 was detected, and on average with a single breath more than 50% of the ABO volume was exchanged. ABO PCO2 values were in the range of 1.95–3.89 kPa, close to previously recorded blood PCO2 levels. Aerial hypoxia (PO2 down to 12.65 kPa) did not increase either air-breathing frequency or breath volume.

中文翻译：

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鳃和呼吸器官在 O2 吸收、CO2 排泄、N 废物排泄和小型和大型 pirarucu (Arapaima gigas) 中的离子调节

在巨骨舌鱼 (Arapaima gigas) 中，鳃表面积和鳃的气体交换能力随着发育的进行而减少。因此，预计 A. gigas 从水中呼吸器开始，逐渐变成使用空气呼吸器官 (ABO) 进行气体交换的专性呼吸空气鱼。我们评估了空气呼吸活动、O2 和 CO2 向空气和水的交换、氨氮和尿素氮的排泄、离子通量率以及大型和小型 pirarucu 中离子传输 ATP 酶的活性。我们发现，即使是非常年轻的 A. gigas（4-6 克，孵化后 2-3 周）具有广泛的鳃，也是空气呼吸器（18.1 次呼吸*h-1），并且可以满足其大部分 (63%) 的 O2 需求而 600-700-g 的动物（孵化后约 3-4 个月），鳃减少，从空气中获得 75% 的氧气（10.8 次呼吸*h-1）。因此，鳃表面积的减少几乎不影响 O2 的吸收，但发育对空气中 CO2 的排泄有显着影响，小鱼的 CO2 排泄量非常低（3%），而大鱼的排泄量增加到 12%，产生了超异速生长比例系数。 1.12) 与 0.82–0.84 的水生和总 CO2 排泄量形成对比。随着鱼的生长，质量比氨排泄量与质量比 O2 消耗量大致成比例下降，但尿素-N 排泄量从总 N-的 18%（4-6 g）下降到 8%（600-700 g）排泄; 所有这些参数的比例系数为 0.70–0.80。特定质量的钠流入和流出率，以及钾的净损失率，偏离了这种模式，在较大的鱼中更大；超异速生长比例系数 > 1.0。Gill V 型 H+ ATPase 活性大于 Na+，K+-ATPase 活性，但在大鱼和小鱼中水平普遍较低且相当，在 ABO 中也检测到类似的活性。A. gigas 在其整个生命周期中都是一种肉食性鱼类，尽管禁食，蛋白质氧化仍占大型和小型动物有氧代谢的主要部分（61-82%）。ABO PO2 和 PCO2（在 600-700 克鱼中测量）变化很大，空气缺氧导致 ABO PO2 值较低。在含氧量正常的情况下，检测到呼吸量与 ABP PO2 之间呈正相关，平均单次呼吸交换了超过 50% 的 ABO 量。ABO PCO2 值在 1.95–3.89 kPa 范围内，接近之前记录的血液 PCO2 水平。空气缺氧（PO2 降至 12.65 kPa）并没有增加呼吸频率或呼吸量。但在大鱼和小鱼中水平普遍较低且具有可比性，在 ABO 中也检测到了类似的活动。A. gigas 在其整个生命周期中都是一种肉食性鱼类，尽管禁食，蛋白质氧化仍占大型和小型动物有氧代谢的主要部分（61-82%）。ABO PO2 和 PCO2（在 600-700 克鱼中测量）变化很大，空气缺氧导致 ABO PO2 值较低。在含氧量正常的情况下，检测到呼吸量与 ABP PO2 之间呈正相关，平均单次呼吸交换了超过 50% 的 ABO 量。ABO PCO2 值在 1.95–3.89 kPa 范围内，接近之前记录的血液 PCO2 水平。空气缺氧（PO2 降至 12.65 kPa）并没有增加呼吸频率或呼吸量。但在大鱼和小鱼中水平普遍较低且具有可比性，在 ABO 中也检测到了类似的活动。A. gigas 在其整个生命周期中都是一种肉食性鱼类，尽管禁食，蛋白质氧化仍占大型和小型动物有氧代谢的主要部分（61-82%）。ABO PO2 和 PCO2（在 600-700 克鱼中测量）变化很大，空气缺氧导致 ABO PO2 值较低。在含氧量正常的情况下，检测到呼吸量与 ABP PO2 之间呈正相关，平均单次呼吸交换了超过 50% 的 ABO 量。ABO PCO2 值在 1.95–3.89 kPa 范围内，接近之前记录的血液 PCO2 水平。空气缺氧（PO2 降至 12.65 kPa）并没有增加呼吸频率或呼吸量。