Whether farmed or wild caught, the cheaper the shrimp, the more likely it is to have been treated with chemicals, particularly sodium tripolyphosphate and sodium bisulfite.

Sodium bisulfite is used to keep shrimp shells from undergoing melanosis (also called black spot), which is a darkening of the head and shell after the shrimp are harvested and exposed to oxygen. This reaction is harmless — think of it as the marine equivalent to the browning of apple flesh after you cut it.

The chemical is part of the slushy brine mix used to store the shrimp on most fishing boats before they make it to shore for further processing. It may also be used on farmed shrimp when the processing facility is far from the farm.

Adding sodium bisulfite doesn’t actually keep the shrimp from deteriorating, though; it’s purely for aesthetics. American consumers want their shrimp to be perfectly pink, without any shadows.