The first photo (left) shows the underside of a fresh Canadian snow crab. Note the leech egg cases as well as live leeches located on the third leg. The second photo is a close up of the leech egg cases on the leg of a cooked Canadian snow crab.
Certain marine leeches deposit egg cases onto the outer surfaces of large crabs, such as snow crabs, in order to facilitate dispersal and protection for the young leeches. Neither the leeches or egg cases penetrate the shell surfaces. The egg cases can occasionally survive cooking intact as seen in photo 2, however the cooking and freezing process to which snow crabs shipped to the US are typically exposed destroys the delicate eggs inside.
Although unattractive to the eye, any such egg cases observed on cooked crabs pose no known public health risks to consumers and do not affect the health, quality, or taste of the crab meat.