Urine Formation in Crustaceans

Urine formation in crustaceans involves filtration, re-absorption and secretion. In the end-sac of crayfish antennary gland chloride is reabsorbed from the isosmotic ultrafiltrate during its passage through the tubule so that the final urine excreted is hypoosmotic. Urine concentration is known to vary in crustaceans depending on whether they are freshwater or marine.

It is very dilute in the freshwater crustaceans, but it is close to the blood osmolarity in the crustaceans of brackish water. This may be due to the absence of salt-reabsorbing segment in the tubule of the marine forms. The hydrostatic pressure of the blood provides the driving force for filtration.