Crustacean urodynamics : a study of Cancer pagurus (L.) (Brachyura: Decapoda)
Various aspects of the dynamics of osmoregulation and renal function in the osmoconforming crab Cancer pagurus CD have been studied during consecutive periods of exposure to 100, 80 and 100% s.w. Renal function was investigated using a novel method of directly collecting urine which enabled the quantification of urine excretion rates and the process of micturition. Urine is excreted when the body fluids are hyperosmotic to the external medium or less than 10 mOsm/kg hyposraotic and within this range, excretion rate is linearly related to the magnitude of the osmotic gradient. During expostire to dilute media, excretion rate progressively declines as the body fluids become isosmotic and the rate of osmotic water influx declines. Re-immersion in 100% s.w. following a period of exposure to dilute media causes an immediate inhibition of micturition and a long lasting anuria. The blood volume of Cancer decreases during exposure to dilute media. Post-dilution anuria is an adaptive response to conserve water and maintain internal volume during periods of osmotic water efflux. The-response to changes in the external medium salinity are mediated by peripheral osmoreceptors. Insulin urine/plasma ratios consistently less than unity were found for Cancer. The ratios were lowest during the first period of exposure to 100% s.w. and they progressively increased during the period of exposure to dilute media. A mechanism is suggested to account for the increase in the ratios based upon the decline in blood volumes, an increase in plasma colloid osmotic pressure and a consequent reabsorption of water from the urine. It is argued that primary urine may be formed by an active metabolic process rather than by plasma ultrafiltration. This argument is based on the inulin clearance data and a consideration of haemodynamic parameters. Several cases, of unilateral anuria were found and data from these animals are compared with those from normal crabs.