I think in the same way when measuring the serum antioxidant enzymes as in the research below, but there should be a step before that is hemognaizer and filtration and then take the filtrate and use it as a sample..
Article Some Antioxidant Enzymes in Workers Exposed to Pollutants
There are many articles on measuring the activities of antioxidant enzymes in amphipods. But there are also many confounding factors that must be taken into account to get a relevant result (sex, size, parasites, ...).
I work mainly with freshwater amphipods. Feel free to read my articles and contact me if you have any questions.