The turtle can be seen as a metaphor for both the Joads and the migrants in general: the turtle is tough, tenacious, and unstoppable.
Steinbeck writes, "All over the grass at the roadside a land turtle crawled, turning aside for nothing, dragging his high domed shell over the grass." As the turtle tenaciously carries forward with his life, so too do migrants as they protect both their own lives and the lives of their families.