They might be nature's greatest opportunists. Name a metabolic system or process, and they have it in use within seconds. They might decide to look like plants while persisting on oxidizing the metals they extract from the rock beneath them, or like racoons while relying on photovoltaic fur. They might eat your sarium krellide Duracells when you look the other way, or plug themselves to your wall socket, burn your brandy, borrow Dr. Bashir's imaging radioisotopes or tap into the local subspace field by materializing a warp coil in their tummy.
Since they seem to be able to shunt some of their mass, uh, elsewhere (body weight, so that they can be light as rats, or foreign objects like communicators, so that they can be transparent like drinking glasses or fog and
be light enough for the part), they might also have good reserves of mass and energy there, to be tapped whenever other opportunities don't present themselves.