Frank Lyko, a biologist at the German Cancer Research Center, studies the six-inch-long marbled crayfish. Finding specimens is easy: Dr. Lyko can buy the crayfish at pet stores in Germany, or he can head with colleagues to a nearby lake. Wait till dark, switch on head lamps, and wander into the shallows. The marbled crayfish will emerge from hiding and begin swarming around your ankles. “It’s extremely impressive,” said Dr. Lyko. “Three of us once caught 150 animals within one hour, just with our hands.”