I haven't heard of these for years,used to love them roasted over an open fire with the smaller mussels...
My uncle once pulled up one of those chip shop size pickled onion size jars with an enormous clabby inside, his most unusual catch.
I used to be friendly with the head diver at the Marine Research Station on Millport...... He would bring a big bag of them in periodically. A group of us would then meet up in a local hotel. where we would have them on hot buttered garlic bread with a cold pint ..... MAGIC.....