Ascidians - Halocynthia roretzi
The internet has come through - apparently they are ascidians - specifically Halocynthia roretzi. Ascidians are sea squirts — hermaphrodite marine invertebrates that live in shallow ocean waters, and which are thought to exhibit characteristics of the evolutionary ancestors of chordates and vertebrates.
(Thanks goes to Karen Sanamyan at ascidiacea.com/
Carving Up Tuna
You can see the body is fully of ice - I'm not sure if that means this is fresh fish or frozen, but if it was frozen they would probably be using the band saw.