Effects of extreme pH on the physiology of the Australian 'yabby' Cherax destructor: acute and chronic changes in haemolymph carbon dioxide, acid-base and ionic status

Abstract

Freshwater habitats throughout the world are becoming increasingly threatened by the likelihood of acidification, but little consideration has been given to the importance of severe alkalization. Acute and chronic fluctuations in haemolymph acid­base status (PCO2, CCO2 and pH), [Na+] and [Ca2+] were monitored for up to 504 h (21 days) in the Australian freshwater crayfish Cherax destructor exposed to low- and high-pH water. The importance of carapace [Ca2+] during acid exposure was assessed. Crayfish were exposed to pH 7.1, pH 4.5 and pH 8.0 water containing calcium at 500 µmol l-1 while the effect of a lower calcium concentration (50 µmol l-1) was assessed in pH 4.5 water. Cherax in acid water containing 50 µmol l-1 Ca2+ exhibited a significant decrease in CO2 content after 2 h (mean decrease 1.13 mmol l-1, venous; 1.57 mmol l-1, arterial) and large ranges in PCO2 throughout the treatment (2.4­7.3 mmHg). The overall acid­base response was a metabolic acidosis compensated by a respiratory alkalosis. The haemolymph Na+ concentration in both control (pH 7.1, 50 µmol l-1) and acid-exposed animals in lower-Ca2+ water was up to 50 % reduced compared with that in animals in pH 7.1, 500 µmol l-1 Ca2+ water. Ion regulatory mechanisms, causing a subsequent increase in haemolymph [Na+] after 288 h, were implicated as an important component in acid­base homeostasis. Crayfish in acid, low-Ca2+ water also exhibited a 3.2 mmol l-1 increase in haemolymph [Ca2+] and showed a haemolymph alkalosis compared with animals in acid water with higher [Ca2+]. At higher water [Ca2+] in pH 4.5 water (500 µmol l-1 Ca2+), the haemolymph pH of Cherax was only 0.1 unit lower than that of animals in 50 µmol l-1 Ca2+ acid water after 96 h, and both CaCO2 and CvCO2 were unchanged compared with the initial condition. As with low-Ca2+ acid-exposure, the potential haemolymph acidosis appeared largely to be compensated by respiratory alkalosis. There was a transient 31 % reduction in haemolymph [Na+], although osmolality was unchanged (control 411±7.29 mosmol kg-1). Acid­base equilibrium recovered rapidly, probably in association with changes in ion flux and the re-establishment of normal haemolymph Na+ concentration. Alkaline-exposed Cherax destructor exhibited a mixed respiratory alkalosis and metabolic acidosis. Whereas haemolymph [Ca2+] increased by 1.8 mmol l-1 after only 1 h, haemolymph Na+ levels increased by 36 % after 2 h, possibly as part of a net H+ loss from the haemolymph. Increased HCO3-/Cl- exchange could contribute to the 4.3 mmol l-1 decrease in haemolymph CO2 level after 0.5 h of alkaline exposure. The responses of Cherax to extreme pH are different from those of the European and North American crayfish species studied to date.