Water animals must have scales and fins to be considered kosher.
(Deuteronomy ch.14) Fish have to have scales and fins. The kosher diet excludes shellfish.
Jewish dietary rules are based on the Torah. Within the Torah there are a series of core rules specifying what is considered fit and not fit for eating.
The conversation rules that that they had in place to make seafood a sustainable resource was based on three component framework.
The ion that is in the seafood. For example, if you eat crab legs, you might feel sleepy and drowsy. That's because if the iron intake you have from eating the crabs!
If u have diarrhea after eating it.
I am aware of no reason you can't take most if not all medications after eating seafood.
The Torah
Because the Torah has a section detailing what foods are and are not suitable for eating. Jews follow these rules as part of our path to living a righteous life.
The risks for eating a lot of seafood fluctuate. If you love seafood, eat a variety of types from a variety of sources and there should be no problem. There is more danger in eating just one kind of seafood only that can be more prone to contamination than others or eating seafood from a single source. For several years, I took a tuna sandwich to work for lunch every single day. I love tuna sandwiches. My choice of brands was limited because I only like the kind that comes unsalted. That was until blood tests showed elevated mercury levels, not dangerously high but it was recommended that I eat much less tuna.
The Maoris had the principles of Māori Economic Development conservation in place to make seafood a sustainable resource.
GOD
The law of Kosher dictates that the only seafood they can eat is fish. So that rules out shellfish like clams and lobster. They go further to say that the fish must have a backbone, fins and scales.