Why do crawfish die submerged in chlorinated water?

Chlorine is highly toxic to crawfish because, unlike fish that can breathe through gills, crawfish breathe through open spiracles on the ventral side of their cephalothorax. When submerged in chlorinated water, the chlorine molecules react with the water to form hypochlorous acid (HOCl), a powerful oxidising agent that attacks the gills and respiratory tissues of the crawfish. HOCl can also alter their osmotic balance, leading to dehydration and death.

In addition, chlorine can also damage the crawfish's exoskeleton, making them more susceptible to disease and predation.