Just for the tl;dr: "Overall mortalities of bled horseshoe crabs were 17.8% (male = 18.75%; female = 17.2%), while mortalities of unbled crabs were 3.5%." So

Just for the tl;dr: "Overall mortalities of bled horseshoe crabs were 17.8% (male = 18.75%; female = 17.2%), while mortalities of unbled crabs were 3.5%." So it appears it indeed affects the crabs.