They also eat a wide variety of insects. Freeze-dried shrimp can be bought in some pet stores and these would provide some of the same nutritional value as the crayfish of their natural diet. They will also eat a small abount of vegetation, especially as juveniles. In the wilds, turtles gnaw on carrion bones and eat egg shells from a variety of aquatic birds to get the large amount of calcium required by turtles. You can replicate this by providing cuttlebone at all times.
Some keepers attach it to the side while others let it float in the water.
Some of us scrape off the brittle backing, while others feel the chance of injury is very small. Bartlett, R. D. 1998. Notes on Ringed and Other Map Turtles. A Complete Pet Owner's Manual. Reptile Hobbyist, Vol. 4, No.3, 36-44.