The reason shellfish is not considered Kosher or clean is because they cannot fully process the dirt that they intake which then makes them full of metal and other things. When those metals and excess vitamins are taken into the body, over time can become bad for the person.

Is shellfish considered unclean?

All crustaceans and mollusk shellfish have no scales and are therefore unclean. These include shrimp/prawns, lobster, scallops, mussels, oysters, squid, octopus, crabs and other shellfish) is not clean.

Why you should not eat shellfish?

Shellfish may contain varying levels of heavy metals that can build up in your body and cause health issues. In addition, shellfish can cause foodborne illness and allergic reactions.