EFFECTS OF EXTREME PH ON THE PHYSIOLOGY OF THE AUSTRALIAN YABBY CHERAX DESTRUCTOR - ACUTE AND CHRONIC CHANGES IN HEMOLYMPH CARBON-DIOXIDE, ACID-BASE AND IONIC STATUS

Freshwater habitats throughout the world are becoming increasingly thr eatened by the likelihood of acidification, but little consideration h as been given to the importance of severe alkalization. Acute and chro nic fluctuations in haemolymph acid-base status (P-CO2, C-CO2 and pH), [Na+] and [Ca2+] were monitored for up to 504 h (21 days) in the Aust ralian freshwater crayfish Cherax destructor exposed to low- and high- pH water. The importance of carapace [Ca2+] during acid exposure was a ssessed. Crayfish were exposed to pH 7.1, pH 4.5 and pH 8.0 water cont aining calcium at 500 mu mol l(-1) while the effect of a lower calcium concentration (50 mu mol l(-1)) was assessed in pH 4.5 water. Cherax in acid water containing 50 mu mol l(-1) Ca2+ exhibited a significant decrease in CO2 content after 2 h (mean decrease 1.13 mmol l(-1), veno us; 1.57 mmol l(-1), arterial) and large ranges in P-CO2 throughout th e treatment (2.4-7.3 mmHg). The overall acid-base response was a metab olic acidosis compensated by a respiratory alkalosis. The haemolymph N a+ concentration in both control (pH 7.1, 50 mu mol l(-1)) and acid-ex posed animals in lower-Ca2+ water was up to 50% reduced compared with that in animals in pH 7.1, 500 mu mol l(-1) Ca2+ water, Ion regulatory mechanisms, causing a subsequent increase in haemolymph [Na+] after 2 88 h, were implicated as an important component in acid-base homeostas is. Crayfish in acid, low-Ca2+ water also exhibited a 3.2 mmol l(-1) i ncrease in haemolymph [Ca2+] and showed a haemolymph alkalosis compare d with animals in acid water with higher [Ca2+]. At higher water [Ca2] in pH 4.5 water (500 mu mol l(-1) Ca2+), the haemolymph pH of Cherax was only 0.1 unit lower than that of animals in 50 mu mol l(-1) Ca2acid water after 96 h, and both Ca-CO2 and Cv(CO2) were unchanged comp ared with the initial condition. As with low-Ca2+ acid-exposure, the p otential haemolymph acidosis appeared largely to be compensated by res piratory alkalosis. There was a transient 31% reduction in haemolymph [Na+], although osmolality was unchanged (control 411+/-7.29 mosmol kg (-1)). Acid-base equilibrium recovered rapidly, probably in associatio n with changes in ion flux and the re-establishment of normal haemolym ph Na+ concentration. Alkaline-exposed Cherax destructor exhibited a m ixed respiratory alkalosis and metabolic acidosis. Whereas haemolymph [Ca2+] increased by 1.8 mmol l(-1) after only 1 h, haemolymph Na+ leve ls 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 th ose of the European and North American crayfish species studied to dat e.