<P> Tomalley (from the Carib word tumale, meaning a sauce of lobster liver), crab fat, or lobster paste is the soft, green substance found in the body cavity of lobsters, that fulfills the functions of both the liver and the pancreas . Tomalley corresponds to the hepatopancreas in other arthropods . It is considered a delicacy, and may be eaten alone but is often added to sauces for flavour and as a thickening agent . The term lobster paste or lobster pâté can also be used to indicate a mixture of tomalley and lobster roe . Lobster bisque, lobster stock, and lobster consommé are made using lobster bodies (heads), often including the lobster liver . </P> <P> The hepatopancreas of a crab is also called tomalley, or crab "fat"; in crabs the tomalley is yellow or yellow - green in color . In Maryland and on the Delmarva Peninsula, the hepatopancreas of the blue crab is called the "muster" or "mustard", probably because of the yellow color, which is not the bright yellow of regular prepared yellow mustard, but closer to one of the brown mustards, such as Dijon mustard . Particularly when eating steamed or boiled crabs, it is considered a delicacy . </P>

What is the green and red stuff in a lobster