Tolerance of chronic hypercapnia by the European eelAnguilla anguilla

Abstract

SUMMARYEuropean eels were exposed for 6 weeks to water CO2 partial pressures (PCO2) from ambient (approx. 0.8 mmHg), through 15±1 mmHg and 30±1 mmHg to 45±1 mmHg in water with a total hardness of 240 mg l–1 as CaCO3, pH 8.2, at 23±1°C. Arterial plasma PCO2 equilibrated at approximately 2 mmHg above water PCO2 in all groups, and plasma bicarbonate accumulated up to 72 mmol l–1 in the group at a water PCO2 of 45 mmHg. This was associated with an equimolar loss of plasma Cl–, which declined to 71 mmol l–1 at the highest water PCO2. Despite this, extracellular acid–base compensation was incomplete; all hypercapnic groups tolerated chronic extracellular acidoses and reductions in arterial blood O2 content (CaO2), of progressive severity with increasing PCO2. All hypercapnic eels, however, regulated the intracellular pH of heart and white muscle to the same levels as normocapnic animals. Hypercapnia had no effect on such indicators of stress as plasma catecholamine or cortisol levels, plasma osmolality or standard metabolic rate. Furthermore, although CaO2 was reduced by approximately 50% at the highest PCO2, there was no effect of hypercapnia on the eels' tolerance of hypoxia, aerobic metabolic scope or sustained swimming performance. The results indicate that, at the levels tested, chronic hypercapnia was not a physiological stress for the eel, which can tolerate extracellular acidosis and extremely low Cl–levels while compensating tissue intracellular pH, and which can meet the O2 requirements of routine and active metabolism despite profound hypoxaemia.

The contribution of air breathing to aerobic scope and exercise performance in the banded knifefishGymnotus carapoL.

“…The data from the present study comprise the first report of U crit performance in a fish with bimodal respiration, but A. calva, L. oculatus and M. cyprinoides are all capable of sustained exercise at speeds greater than 2BLs -1 (Farmer and Jackson, 1998;Seymour et al, 2007), which was the U crit measured for G. carapo. The normoxic U crit of G. carapo is similar to that of unimodal water-breathing species with 'anguilliform' locomotion, such as Anguilla anguilla (McKenzie et al, 2003), but G. carapo is significantly less athletic than 'subcarangiform' swimmers of similar size at similar temperatures, such as Leuciscus cephalus, Cyprinus carpio or Dicentrarchus labrax (Claireaux et al, 2006;McKenzie et al, 2007b). The relatively limited aerobic swimming performance of the knifefish is consistent with the species' ecology: it inhabits slow-moving turbid waters, where it uses its electrical sense to hunt for prey (Graham, 1997) and it is reported to bury in the mud if it is exposed to sudden rapid currents (Graham, 1997).…”

“…The relative partitioning of M tO2 between water and air at IMR was also similar to a previous report, which found that approximately 35% of M tO2 was derived from air in normoxia (Liem et al, 1984), very similar to the present study, rising to approximately 75% in hypoxia at ~10kPa (Liem et al, 1984), compared with 85% at ~4kPa in the present study. The AMR and net AS of the knifefish were similar to that of the anguilliform swimmer A. anguilla but less than that of subcarangiform swimmers (L. cephalus, C. carpio and D. labrax) of similar size at similar temperatures (McKenzie et al, 2003;McKenzie et al, 2007b;Claireaux et al, 2006).…”

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

“…The ability to regulate pH e during hypercapnia in P. hypophthalmus is also highly developed compared with several water-breathing fishes, such as the common carp Cyprinus carpio, which recovered 50% of the initial pH e disturbance after 48 h exposure to 8 mmHg CO 2 (Claiborne and Heisler, 1984) and the white sturgeon Acipenser transmontanus, which only compensated 35% of pH e after exposure to 28 mmHg for 72 h (Crocker and Cech, 1998). The European eel exhibits a medium capacity for acid-base regulation, compensating 75% of its disturbance after 6 weeks at a P CO2 of 15 mmHg (McKenzie et al, 2003). P. hypophthalmus thus shows an acid-base regulatory capacity during a respiratory acidosis in line with active water-breathers such as cod and rainbow trout, which compensate the full pH e disturbance within 24 h of exposure to 7.5 and 15 mmHg, respectively (Eddy et al, 1977;.…”

Introducing a novel mechanism to control heart rate in the ancestral pacific hagfish

“…, while arterial blood oxygen saturation is halved (McKenzie, 2003). How eels regulate heart rate during hypoxic hypercapnia is unknown.…”