Kashrut
Among the numerous laws that form "kashrut" are prohibitions on the consumption of certain animals (such as pork and shellfish), mixtures of meat and milk, and the commandment to slaughter mammals and birds according to a process known as "shechita". There are also laws regarding agricultural produce that might impact the suitability of food for consumption.
Shellfish Association of Great Britain
Table below: all values = per 100g serving
Contrary to the "old wives tale" that certain shellfish (i.e. prawns) raise cholesterol we now understand that the amount of saturated fat in the diet has a greater effect in raising blood cholesterol than the amount of cholesterol in the diet. So while dietary cholesterol is present in prawns, crabs and lobsters, as well as in squid and octopus, they contain very little saturated fat and for most people do not cause a rise in the level of cholesterol in the blood. Shellfish such as cockles, mussels, oysters, scallops and clams are very low in cholesterol, about half as much as chicken, and contain much less cholesterol than red meats.