As a follow-up to my last post, I've attached a jpg of a chunk of
Chondrus crispus that I successfully 'transplanted' into my aquarium.
This particular specimen was growing on a bivalve lodged between some rocks, off the coast of Maine, in about 6' of water. I couldn't free the bivalve shell, so I broke off the piece that had the Chondrus attached (don't worry, no bivalves were killed in the process

.
I used hot glue (flowing white blobs) to attach the fragment to a larger rock in my aquarium. I've darkened the photo to highlight the new growth at the base (lighter colored thalli).
I've been less successful with Ascophyllum nodosum, never being able to keep it more than 75 days or so, before it was all gone.
RAS