Osmotic and ionic regulation in the North American crayfish Pacifastacus leniusculus (dana) Public Deposited

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  • Osmotic and ionic regulation was studied in the North American crayfish Pacifastacus leniusculus (Dana), stepwise acclimated to increasing salinities. Stepwise acclimation involved transferring crayfish at 48 hour intervals from freshwater to 100% seawater in 20% increments. In experiments where 70% seawater was used, animals were moved directly from 40 to 70% seawater. The salinity of the 100% seawater used in this study was 30, 67 parts per thousand (%₀). Two approaches to the study of osmotic and ionic regulation were used: in one, animals were sacrificed at the end of 48 hours in the test salinity, and in the other crayfish were left in the test salinity for prolonged periods of time (up to 32 days). In the latter, animals were sacrificed at various times throughout the experiment. In both approaches the osmotic and ionic concentrations of blood and tissues were studied. Blood samples were analyzed for osmotic, sodium, potassium, chloride and calcium concentrations. In some experiments, abdominal muscle was digested with peroxide and analyzed for sodium, potassium and chloride. In two experiments one after 48 hours in 0, 40 and 70% seawater and one after eight days in 40 and 70% seawater amino nitrogen was estimated on TCA extracts. Stepwise acclimated crayfish survived for 22 days in 60 and 70% seawater but a majority died within seven days in 90% and 18 days in 80% seawater. When these crayfish are subjected to a hyperosmotic stress, they maintain the osmotic concentration of their blood below that of the medium (hyposmotic regulation) for at least 48 hours. These crayfish showed little dehydration with salinity stress as evidenced by muscle and whole animal water content. Blood volumes remained constant except in 100% seawater where a significant increase was noted. Ionic concentrations in abdominal muscle, especially sodium and chloride, increase in the higher salinities but amino nitrogen does not. The difference between the measured osmotically active constituents of the abdominal muscle and the osmotic concentration of blood increases in the higher salinities. Since water content remained essentially unchanged, this suggests the increased intracellular concentration by some material not analyzed in this study. Crayfish subjected to 70% seawater for periods longer than 48 hours reach a steady state within the first four days, the osmotic concentration of the blood becoming isosmotic or slightly hyperosmotic to the medium after eight days. Blood sodium and chloride are regulated below the concentration of the medium up to 27 days for sodium and 32 days for chloride. Ionic concentrations in the abdominal muscle show an increase with time. Concentrations of amino nitrogen are highly variable and do not show a consistent pattern of change. Blood and tissue osmotic constituents of animals kept in 40% seawater for eight days remained essentially unchanged. Excretion of salt and water by the crayfish kidney was studied. Urine was analyzed for osmotic, sodium and chloride concentrations. Urine volumes were estimated by plugging the nephropore and weighing the crayfish eight hours later. Filtration rates were estimated using the appearance of inulin-C¹⁴ in the bath. The urine osmotic and ionic concentrations increase when crayfish are stepwise acclimated to increasing salinities but are always hyposmotic to the blood. U/B ratios for inulin are greater than one indicating water reabsorption, and remain unchanged when the animal is stressed to increasing salinities. Estimations of urine volumes by the weight-gain method indicate that even though the volume is reduced the crayfish still produces some urine when subjected to a hyperosmotic stress. The experiments with inulin indicate that filtration is markedly reduced in 40 and 70% seawater.
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