MARA stands for "mandibular anterior repositioning appliance" and is one of many orthodontic appliances that have been developed over the years to encourage growth of the lower jaw. You CAN have the MARA in place without braces, but it likely won't have the effect you desire. The MARA forces you to position your lower jaw forward in order to bite. In younger, growing patients, there is some evidence that unloading your jaw joint like this encourages earlier growth of the lower jaw...whether it actually results in more overall growth of the jaw is debatable. In adult patients, by far the largest effect of the MARA is its tendency to push the top teeth back, with only a few examples present in the literature showing any sort of skeletal change. Without braces, the MARA will primarily tip these top teeth back, creating spaces between your back teeth in the top jaw and having little to no effect on your top front teeth at all. In the lower arch, the MARA would mostly cause your bottom front teeth to flare forward, or worse...become crowded. Many believe these changes to be unstable without further treatment, meaning that you could expect your teeth to revert back to their pre-MARA positions once the appliance is removed. Unfortunately, much of the research suggests that the use of the MARA in adults is of questionable benefit, and likely would be of no long-term benefit without additional orthodontic treatment.
I'm a board-certified orthodontist. D.M.D., M.S.