observed luminescence in a piece of plaice
It's probably bacteria.

Here's a pdf describing the phenomenon, along with an in-depth history of the reported occurrence, which should tell you everything you might want to know.


  These seafood products exhibited luminescence due to the presence of certain bacteria that are capable of emitting light.  Luminescence by bacteria is due to a chemical reaction catalyzed by luciferase, a protein similar to that found in fireflies... Most of these species require salt except for the two nonmarine luminescent bacteria.


There is some link to potential pathogenicity, so I'd recommend not eating it.  That document also requests that you contact the researcher, so why not?  Here is a potentially-relevant paper but 1. is very old, 2. is in Russian, and 3. I have no access to it.  And this supposedly tells you how to isolate them.