Lets apply this to shrimp. Its not prawnographic but in Spain the market sells more of heads-on shrimp that any other offering.
However market research shows that when people eat out in Spain they are not so keen on head sucking. They prefer to do this in the privacy of their own homes. Main reason is risk of getting shrimp all over clothes and fingers.
Heads-on shrimp reflects a quality to discerning seafood markets like that of Spain that there has been no processing of the product and that the quality is of a higher standard.
Waiting for the definitive report.