Substrate specificity analysis of PmproPO1 and PmproPO2 was achieved by examining the ability of the PmproPO1 and PmproPO2 gene-silenced shrimp to hydrolyze various PO substrates. At 48 h after the second dsRNA injection (or 150 mM NaCl only for the control), hemolymph was collected from the gene silenced and control shrimp and 400 lg of this was separately mixed with the PO substrates of: the monophenol L-tyrosine, the diphenols L-DOPA or dopamine, and the catechol compound DHI, all in 10 mM Tris–HCl buffer (pH 8.0). The reaction mixture was incubated at room temperature (RT) for 30 min before an equal volume of 10% (v/v) acetic acid was added. The increase in the absorbance at 490 nm (A490) was subsequently monitored using a SpectraMax M5 microplate reader (Molecular Devices). The protein concentration was determined by the Bradford assay (Bio-Rad). The addition of reaction buffer instead of shrimp hemolymph was used as the negative control. The activity of the PO enzyme was defined as DA490/mg total protein/min. The experiment was performed in triplicate with three samples from three pooled shrimp/group.