The Effects of Aerial Exposure on the Distribution of Body Water and Ions in the Freshwater Crayfish Austropotamobius Pallipes (Lereboullet)

Abstract

Submerged crayfish had a total body water content of 76% body mass. Haemolymph volume was around 30% and was dependent on mass, the relationship being described by the regression equation y (volume) = 3·98+0·177x (mass). The inulin space was greater than the copper space in abdominal muscle, suggesting the existence of an interstitial component of the extracellular fluid into which haemo-cyanin does not penetrate. Crayfish exposed to water-saturated air (100% relative humidity, RH) did not lose mass and haemolymph levels of K+ and Na+ were unchanged from the submerged values. When exposed to air (70-80% RH) crayfish progressively dehydrated at the rate of 0·38 % body massh-1 (0·5 % body water h-1) and died after 72 h when 27 % of initial mass had been lost. After a 10 % reduction in mass, haemolymph volume was significantly reduced to about 75 % of its submerged value in 10-g animals and to 65 % in 60-g animals, the decrease in volume being approximately equivalent to the mass lost. After 48 h in air (19% reduction in mass) haemolymph volume was further reduced to 24% of the submerged volume. The total water content of the abdominal muscles showed a small but significant reduction when animals were dehydrated which could be accounted for by a reduction in the extracellular fluid volume, including the interstitial space. After 48 h dehydration in air the osmolarity of the haemolymph increased by 36%, [K+] by 47 % and [Cl-] by 57 % above the submerged levels whilst [Na+] showed an insignificant decrease of 10%. In the abdominal muscle [K+] increased by 15 %, [Na+] increased by 23 % and [Cl-] decreased by 27 %. [Caz+] levels in the haemolymph doubled after 24 h in air, an increase which was independent of the degree of dehydration. The increase is thus a result of emersion and not dehydration and may be due to a mobilization of CaCO3 from the exoskeleton to buffer a respiratory acidosis, as it corresponds to a doubling in [HCO3- + CO32-]. A substantial loss of ions from the haemolymph during dehydration cannot be fully accounted for by uptake into the tissues or by urinary losses, but a 500 mmol 1-1 increase in the Na+ concentration of the proventricular contents suggests that the foregut may play an important role in ionoregulation.