River crabs can't be eaten when they are dead. When a crab dies, its body will produce a chemical called histidine, which will decompose to produce the toxic substance histamine. The longer the crab dies, the more poisonous substances in the body, even if cooked toxin is not easy to be destroyed, and this toxin will allow people to produce an allergic reaction, which in turn causes poisoning, so the river crab can not be eaten after death.

River crabs:

River crabs, square crabs mitten crab arthropods. River crabs often burrow in rivers, rivers, lakes and marshes of the mud bank, night activities. Its head and thorax combined with the cephalothoracic armor is square, hard, the front of the body with a pair of eyes, the side with two pairs of very sharp crab teeth; chelicerae surface full of downy hair, chelicerae after four pairs of footsteps, laterally flattened and longer, the abdominal limbs have been degraded, the abdomen of the female is rounded, the abdomen of males for the triangle. It feeds on fish, shrimp, animal carcasses and grains, and often migrates to the estuary out to sea to spawn in the fall of each year.