I know I might lose some people on this one. :) And I couldn't really find a good link on the info. But I've been doing a bit of research on Kosher laws and the health benefits of them. Even though we aren't morally bound anymore to the law of Moses, (See Acts 10:9-13 & Rom 6:14 for why we are no longer obligated to follow the Jewish law.) there are a lot of common sense guidelines that I think can help us out. The seafood that the Old Testament forbids is anything that does not have scales & fins. If you look at these animals, most of them are bottom feeders. They take in the excrement of other creatures. Because of this, they are less clean, less healthy, and contain more toxins than a fish higher up in the food chain. Most nutrition experts agree it's best to consume these foods rarely, if at all.
Non-kosher seafood includes: lobster, shrimp, catfish, squid, octopus, clams, oysters, crab, scallops, crawfish etc. You can eat most regular fish that have fins & scales.
More on kosher foods:
I personally have decided not to consume these or only on the really rare occasion. It helps that I don't like most of them! But I do love crab & crawfish! :) My decision is not based on religion but simply on nutrition.