hint - you've already worked out there is a fishy theme - try thinking of shellfish/crustaceans and applying them to the clues, bearing in mind that the actual answer definition is unclued!
I thought this clue had a slight ambiguity: "King" and "Pawn" are each chessmen, but so (arguably) is "King Pawn" (recall the old descriptive chess notation). But I still was able to figure it out.