Mid-Atlantic people call it “mustard” and it’s also known as tomaley, and it’s found in all kinds of crabs, as well as lobster (which contains more of the green color). Many sources say that mustard is simply crab fat, but this is not the case. It is actually an internal organ called the hepatopancreas. The liver and pancreas serve a similar function to crustaceans as they do to humans.
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The reason many people confuse crab mustard for fat is because the hepatopancreas is a fatty organ, and this is especially true in Chesapeake Bay blue crabs because of the extra fat they prepare for hibernation. It touches the butter that makes Maryland crabs so tender and delicious. The way the crabs are cooked also makes a difference. Marylanders steam their crabs instead of boiling them, which helps them retain more of the creaminess. Many people consider the mustard to be the most flavorful part of the crab, and that’s why, and Maryland blue crabs generally have such passionate fans. However, the role of the hepatopancreas in the crab’s body raises some concerns about eating.
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