To be a stickler for terminology, shrimp don't have blood or a closed circulatory system as in vertebrates; instead, a fluid called hemolymph sloshes about within the body cavity. The oxygen-carrying molecule in their hemolymph is hemocyanin, a respiratory protein which binds oxygen using
two copper atoms.
According to
this paper, decapod crustaceans (like shrimp) need around 31.3 to 38.1 μg (1 μg = 1/1,000,000 of a gram) of copper per gram of body mass "to meet the requirements of both copper-associated enzymes and the copper bearing respiratory pigment haemocyanin".
The line between need and toxicity
can be fine (
source), but copper is an
essential trace metal required for copper-dependent enzymes in energy production, pigmentation, and so on.
We should distinguish this dietary requirement from the effects of the cupric ion (Cu+2). This is copper's major toxic species -- doing much damage to crustacean gills by inhibiting osmoregulation -- and results when
copper sulfate medications are added to water (
source).
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