Visualisation of urine release in a fight between two male signal crayfish. Both animals had been injected with fluorescein dye, which accumulates in the bladder staining the urine. Urine is released from the nephropores and dispersed by the frontally projecting gill currents. The highest urine concentration is near the antennular chemoreceptors. The antennules are the small forward pointing antennae. Three of the four bilateral antennular flagella are visible in each animal. Crayfish are reversibly blindfolded by black pond liner wrapped around the rostrum and eyestalks and fixed to the dorsal carapace. Black tape on the posterior carapace was used to seal the hole left by fluorescein injection.