That's a bizarre definition: Cambridge English dictionary: "animals from the sea that can be eaten, especially fish or sea creatures with shells" Merriam Webs

That's a bizarre definition: Cambridge English dictionary: "animals from the sea that can be eaten, especially fish or sea creatures with shells" Merriam Webster: "edible marine fish and shellfish" These are much more reasonable definitions, more line with modern usage. I doubt you will find any dictionaries that mirror your Britania definition.