Dune: I think you mean that I must
assign a default shader and set 'Reflectivity image' to
spec.jpg and 'Opacity image' to
opa.jpg -- if so, I get a bad result: the object loses its skin and I'm back to a clay/grey fish.
Klas: I loaded your clip file, but I'm not exactly sure where the nodes are supposed to connect.

Thanks for your patience with me. I am learning
a lot from you all.