Would these be the famous Green Lipped Mussels from New Zealand? (Perna canaliculus)
They are used in many supplements for joint pain, arthritis and such.
I have given Mack canned mussels (they were water-pack) and he loved them.
I would not feed any seafood raw though, because of parasites or bacteria, particularly if you're personally gathering them from the wild. Farm raised mussels for human consumpsion might be OK. Quickly steam or boil them and I'll bet he loves them.
In fact....I'll be right over - I love mussels. I had to get past my childhood memories of sitting on the rocks, collecting mussels and using them for bait. I felt the same way about squid (ick! BAIT!) until I learned to love calamari.