High capacity for extracellular acid-base regulation in the air-breathing fishPangasianodon hypophthalmus

Abstract

The evolution of accessory air-breathing structures is typically associated with reduction of the gills, although branchial ion transport remains pivotal for acid-base and ion regulation. Therefore, airbreathing fishes are believed to have a low capacity for extracellular pH regulation during a respiratory acidosis. In the present study, we investigated acid-base regulation during hypercapnia in the airbreathing fish Pangasianodon hypophthalmus in normoxic and hypoxic water at 28-30°C. Contrary to previous studies, we show that this air-breathing fish has a pronounced ability to regulate extracellular pH (pH e ) during hypercapnia, with complete metabolic compensation of pH e within 72 h of exposure to hypoxic hypercapnia with CO 2 levels above 34 mmHg. The high capacity for pH e regulation relies on a pronounced ability to increase levels of HCO 3 − in the plasma. Our study illustrates the diversity in the physiology of air-breathing fishes, such that generalizations across phylogenies may be difficult.

Ambient CO2, fish behaviour and altered GABAergic neurotransmission: exploring the mechanism of CO2-altered behaviour by taking a hypercapnia dweller down to low CO2 levels

“…By contrast, in the hypercapniaadapted P. hypophthalmus, well-developed brain pH i regulation gabazine dose had worn off. The measurements were made using iStat System's portable clinical analyser, corrected for temperature and species according to Harter et al (2014) and Damsgaard et al (2015). For both figures, N=12 for the non-gabazine treatments and N=6 for the gabazine treatments.…”

“…involving atypical Cl − :HCO 3 − exchange ratios in the blood (Damsgaard et al, 2015) could lead to depolarizing GABA A receptors; these scenarios are discussed in depth below. Consistent with some of our predictions, we observed similar behavioural alterations in normocapnia-acclimated P. hypophthalmus as found previously in normocapnia-acclimated reef fishes when exposed to relatively high-CO 2 environments: increased boldness and activity , and more time spent in the presence of a predator Dixson et al, 2010).…”

“…All measurements were temperature corrected to 27°C according to Harter et al (2014). These values were further corrected according to Damsgaard et al (2015).…”

“…These fishes are probably thriving in their hypercapnic environment, so they are unlikely to display the maladaptive behavioural abnormalities observed in the hypercapnia-exposed marine fish. Moreover, in Vietnam, some fishes like the facultative air-breathing striped catfish Pangasianodon (formerly Pangasius) hypophthalmus (Sauvage 1878) are extensively cultured in ponds with no aeration and low water exchange and hence are hypoxic (Lefevre et al, 2011Damsgaard et al, 2015), with P CO2 of up to 4.5 kPa (Damsgaard et al, 2015). Despite the harsh conditions, P. hypophthalmus -and the entire aquaculture industry surrounding them -continue to grow at a remarkable rate (Phan et al, 2009;De Silva and Phuong, 2011).…”

“…The high-CO 2 group (3.1 kPa) can be regarded as the control group as the fish are likely to have been raised under such conditions (Damsgaard et al, 2015), whereas the low-CO 2 group was acclimated to 0.04 kPa P CO2 for 2 weeks. We used established behavioural tests: an empty tank trial to measure routine activity , a novel object trial to assess responsive behaviour (Jutfelt et al, 2013), a predator trial to mimic predator avoidance Dixson et al, 2010) and a conspecifics trial to assess social behaviour (Munday et al, 2009).…”