User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-5638823-20121019201635/@comment-5043481-20121021205449

It's not purple, it's blue. This means it has copper based blood instead of your heamoglobin, which is iron based. Copper compunds are blue, iron ones are red. Both carry oxygen.

Actual crabs and things do hove blue blood, like this.