Parsha Mitzvot: Shemini: Mitzvah 164 – Concept 187
“Do not make yourselves abominable by means of any teeming thing; do not contaminate yourselves through them lest you become contaminated through them (Vayikra 11:43).” We are forbidden to eat creatures that live in the water, other than fish (Rambam, Hilchot Ma’achalot Assurot – The Laws of Forbidden Foods).
Why does the verse use a double term, “do not contaminate yourselves through them lest you become contaminated through them”? This teaches us that there are two stages of impurity that develop from eating these creatures; The first in the natural impurity that comes from violating a Torah law, and from the impurity of the creature. The second stage; “lest you become contaminated through them,” describes how the person is now more susceptible to further impure influences. (Rabbi Avraham of Sochachtchov – Ne’ot Deshe)