Set a steamer over a medium saucepan of water and steam the potatoes for about 10 minutes, or until tender and easily pierced with a knife. (This can be prepared ahead, covered, and set aside at room temperature.)
Place a large, heavy, nonstick skillet over medium heat and add the clam juice, cream, and saffron. Bring to a gentle simmer. Add the potatoes and season with salt and pepper. Bring the clam juice mixture to a boil, add the crayfish and flounder, reduce the heat, and simmer gently for about 2 minutes, or until the crayfish just start to turn pink, turning the seafood halfway through using tongs. Add the squid and cook for about 1 minute, or until opaque.
Using a slotted spoon, divide the seafood and potatoes among 4 warmed soup bowls. Quickly bring the clam juice mixture to a gentle simmer. Ladle over the seafood. Sprinkle with tomato. Serve immediately.
