So what do these folks who willy nilly ignore taxonomy and gill structure use to decide if a shrimp is a prawn or a prawn is a shrimp? For the most part (as in the United States and also in Southeast Asia) the word prawn is used to describe both shrimps and/or prawns if they are large enough (typically if each is larger and about 1 oz in weight). For this reason, you may see shrimp used to describe smaller shrimp or prawns if there are more than 16 to a lb, and prawns to describe both shrimps and prawns if they are 16 or fewer prawns to a lb.
It is far more useful if one is a chef looking for a 'jumbo prawn' to cook to expect that asking for a prawn will yield something large than asking for something with dendritic gill structures!