I'm aiming at a design where a single tendon both closes the claw and flexes the arm, all with a single movement.

I've connected the claw to the end of the arm using two short bars. The centre of claw, where the two claw parts join is connected to the end of the tendon.

Pull the tendon and the claw closes.

By continuing to pull the tendon the arm bends upwards to its tightest curve. As the arm drops again the extended stops on the bottom half of the arm pieces stop the arm from over extending. When the tendon is relaxed the arm falls so that it sticks out straight.

Here's the motion of the arm in action. Notice how the claw closes first then the arm raises.
Next step, fit a pair of arms into a crab body and see if it works!