Tomalley is the green substance which acts as the liver and pancreas of a lobster. Lobsters have an open circulatory system, which means that their body cavities are bathed by a continuous wash of blood and interstitial fluid, and the tomalley, along with other organs, floats in the body cavity. In addition to being vital to a lobster's well-being, it is also a culinary delicacy; in fact, the term usually specifically refers to the liver and pancreas of a lobster that has been cooked.
Anyone who has ever sliced a whole cooked lobster open has seen tomalley; it's the strange green goo that many people unwittingly throw away. In fact, it has a rich, complex flavor which some consumers find very enjoyable. Some people eat it straight, or use it as a spread for crackers and breads. It can also be mixed into sauces and soups to enhance the flavor, with most cooks sieving the tomalley to break it up, distributing the flavor and smoothing the texture.
Tomalley is also sometimes called “lobster paste,” although consumers should be careful when purchasing plain lobster paste in the store, because a variety of lobster products are labeled with this term. Some recipes may call specifically for lobster paste or tomalley, usually when the recipe already involves a whole lobster, so it assumes that the cook will be able to extract the substance.
Although tomalley is a delicacy, it can unfortunately be hazardous. The liver is designed to filter toxins, and when it cannot filter and express them, it sequesters them. In lobsters, this means that the tomalley can contain high levels of PCBs, lead, and mercury, none of which are particularly good for human health. In addition, lobsters can contain the toxin responsible for paralytic shellfish poisoning (PSP). Cooking does not deactivate this toxin, so people who eat this part of the lobster do run the risk of becoming seriously ill.
In some cases, avoiding tomalley altogether may be advisable; children, for example, should probably not be served it because their bodies and brains are still developing, and pregnant women are at increased risk of seafood-related health problems, as are people with compromised immune systems. People should never eat tomalley from a lobster harvested in a region which is experiencing a red tide, as these algae blooms contain the toxins which cause PSP.