Topneck clams are a species of hard-shell clams found on the East Coast of the United States, where hard-shell clams are sold by size. Topneck clams are about 2 inches wide. Though each size range has a different name and preferred preparation method, the species of clam is the same -- the hard, or northern, quahog clam.
Topnecks are best served on the half-shell, steamed or grilled, according to Grub Street. The smallest clams, called littlenecks, measure only an inch across and are popular in raw bars. The next biggest size is the cherrystone, commonly used in pasta sauces or as a cooked appetizer. The topneck is next in size. Larger clams are called chowders, often measuring as much as 4 inches across. Chowders are tough old clams and are usually simmered in soups.
Occasionally names are given to intermediate sizes, says Barnegat Bay Shellfish. Clams even smaller than littlenecks can be called pasta necks. It may not be legal to capture and eat clams that small in some areas. Middlenecks are in between littlenecks and topnecks in size.
Juvenile clams start out about the size of a grain of sand and gradually get larger as they get older. Chowder clams might be 20 years old. One quahog clam was found to be 507 years old, according to The Independent.