Prawns and shrimp are two large families of closely-related crustaceans, shellfish with jointed shells that permit active motion. Like crabs and lobsters, they are part of a group referred to as decapods because they have 10 legs. Although there are differences between shrimp and prawns, the terms are used interchangeably in most of the world.
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Shrimp and prawns are classed as two separate families: Shrimp are classed as Caridea, and prawns belong to the Penaeidae. Each family contains hundreds of individual species scattered throughout the world's waters. There are subtle physical differences between them. Prawns' legs are all the same length, but shrimp tend to have one pair longer than the others. Also, the first three pair of prawn legs have small claws or pincers on them, but shrimp only have them on the first two pair of legs. They reproduce differently as well, with prawns releasing clouds of eggs and shrimp incubating eggs on the female's body.
In practice, the terms shrimp and prawn are applied without regard to marine biology. American fishmongers usually differentiate shrimp and prawns by size, with smaller specimens sold as shrimp and larger ones as prawns. Even this is not consistent, and the same shellfish might be sold in neighboring markets as tiger prawns or tiger shrimp. For culinary purposes, shrimp and prawns are both tasty and can be cooked in the same ways, so it's more important to find a brand or supplier that provides you a consistently good product.
Tiger prawns are one of the most widely sold species. Their shells, as the name suggests, are banded with black stripes. These prawns are imported from Asia, where high-volume aquaculture operations turn out millions of pounds of these tasty shellfish. Thailand is the largest producer and is the source of most tiger prawns sold in America. These will vary widely in quality, depending on the aquaculture operator. The best tiger prawns are sweet in flavor and firm in texture, but they can also be mushy and unpleasant. Canvass your local fishmongers for recommendations before buying in quantity.
Domestic Shrimp and Prawns
Some consumers have reservations about imported Asian shrimp, because of concerns over pollution, sustainable practices and safe food handling. For those who love seafood but would prefer to buy a domestic product, there are several shrimp fisheries in the U.S. There are commercial harvests of wild shrimp on both coasts, and shrimp are widely farmed across the country. Monterey Bay Aquarium's Seafood Watch program considers these farms to be sustainably operated, because they are in areas where the shrimp cannot displace native species if they escape. Both farmed and wild-caught American shrimp earn their "Best Choice" rating.