Ontogeny and morphometrics of the gill and swim bladder of air-breathing striped catfish Pangasianodon hypophthalmus

Abstract

The air-breathing fish Pangasianodon hypophthalmus has been shown to have highly plastic branchial surfaces whose area (SA) increases with temperature and aquatic hypoxia. This modulation occurs through development of inter-lamellar cell mass (ILCM). Paradoxically, in conditions where this fish has been shown capable of covering its entire aerobic scope from the water phase, it has been shown to have a very small branchial SA. To address this paradox, we measured the SA, harmonic mean diffusion (τh) and calculated the anatomic diffusion factor (ADF) of the branchial and swim bladder surfaces in fish ranging from 3 to 1900 g at 27ᵒC in normoxia. Since the lamellae were distinguishable from the ILCM, we measured the actual SA as well as the potential SA if ILCM were lost. As a result of low τh, P. hypophthalmus has a high capacity for branchial oxygen uptake with or without ILCM. The actual and potential gill ADF being 361 and 1002 cm2µm−1kg−1 respectively for 100g fish. The ADF of the swim bladder was similarly 308 cm2µm−1kg−1. By swimming the fish to exhaustion at different temperatures we showed that modulation of this SA is rapid, indicating that the apparent paradox between previous studies is eliminated. Regression analysis of log-log plots of respiratory SA in relation to body mass show that the gills scale with mass similarly to active water breathing fish, whereas the swim bladder SA scales similarly to the mammalian lung. This fish presents a combination of respiratory surfaces not previously seen in air-breathing fish.