A few years ago, I suffered a food poisoning from eating grilled squid. From then on I've never enjoyed squid. I have eaten fried calamari very rarely because I find it easier to swallow but otherwise I am not a big fan of shellfish in general. Though it is just a personal preference that I don't eat it as much, I find it interesting that shellfish was regarded as "unclean" in the Levitical food laws.
"Of all the creatures living in the water of the seas and the streams you may eat any that have fins and scales. But all creatures in the seas or streams that do not have fins and scales—whether among all the swarming things or among all the other living creatures in the water—you are to regard as unclean. And since you are to regard them as unclean, you must not eat their meat; you must regard their carcasses as unclean. Anything living in the water that does not have fins and scales is to be regarded as unclean by you." - Leviticus 11:9-12.
One of the reasons shellfish was prohibited in those days was because it was simply unhygienic to consume them. They did not have proper ways to kill germs and store them such as refrigerators (you can read here for a more in-depth explanation). Although I believe that this food law is not applicable to us anymore, as Jesus explained in Acts 10: 9-16, maybe there is a valid point why God declared shellfish to be off limits to the people. Shellfish are like water filter. They purify the water in the ocean by sucking in algae and plankton, possibly including toxins and contaminants in the water. These toxins accumulate in the shellfish. And when you eat a bad shellfish, the toxins go into your body. That is partly the reason why shellfish is a common allergen and now I understand why I got shellfish poisoning.
There is nothing wrong about eating shellfish (as we are under the New Covenant). If cooked properly, they are delicious and are be a good source of protein, vitamins, and essential fatty acids. Shrimp and oysters are particularly rich in Vitamin D. Oysters are excellent source of zinc (essential for male reproductive health). I still eat prawns and lobsters when they are served in restaurants and bring our guests to try our local delicacy, the Singapore chili crabs, but I don't typically cook with them at home. It's just my personal preference.
Have you experienced food poisoning from eating shellfish?