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  • Title: Gills and air-breathing organ in O2 uptake, CO2 excretion, N-waste excretion, and ionoregulation in small and large pirarucu (Arapaima gigas).
    Author: Pelster B, Wood CM, Braz-Mota S, Val AL.
    Journal: J Comp Physiol B; 2020 Sep; 190(5):569-583. PubMed ID: 32529591.
    Abstract:
    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.
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