Can Orthodox Jewish people eat shellfish?
Kashrut
Among the numerous laws that form kashrut are prohibitions on the consumption of certain animals (such as pork and shellfish), mixtures of meat and milk, and the commandment to slaughter mammals and birds according to a process known as shechita. There are also laws regarding agricultural produce that might impact the suitability of food for consumption.
Onycha
There is some doubt as to whether the onycha of the Old Testament was actually the operculum of a sea snail. H.J. Abrahams says, "The widely held mollusk hypothesis becomes quite perplexing if one considers that the mollusk was counted among the unclean animals in the Bible (Chapters Leviticus 11:9 and 12)." Sea creatures such as the mollusk were an “abomination” and even their carcasses were to be considered an “abomination” and anyone simply touching them became unclean. Rabbeinu Bachyei insisted that only kosher species may be used for the mishkan. The Gemara states that “only items that one may eat may be used for the work of Heaven.” Nachmanides, Torah scholar and famed Jewish theologian, emphasized that the commandment concerning unclean animals pertained also to temple services. James Strong and J. McClintoch write that “it seems improbable that any such substance could have been one of the constituent spices of the most holy perfume; not only because we know of none bearing any powerful and agreeable odor, but specially because all marine creatures that were not finned and scaled fishes were unclean, and as such could not have been touched by the priests or used in the sanctuary.” There is also some doubt that a mollusk would have been referred to as a sweet spice. Bahr states that “the odor of the burned shells is not pleasant.” Although the word onycha has been interpreted as meaning "nail" it is pointed out that nail or claw is actually an extended connotation of onyx, derived from the translucent and sometimes veined appearance of the gemstone onyx which antiquity often describes as a black stone. Coincidentally onycha is the Greek word which was chosen to replace the original Hebrew word which was shecheleth. One of the Hebrew words that "shecheleth" seems to be related to, שחלים, "sh'chalim", refers to a large variety of plants. An ancient Ugaritic text lists onycha among types of vegetables, implying that onycha was a vegetable also. The Talmud specifically states that although onycha ("shecheleth") is not from a tree, it does grow from the ground and that it is a plant (Kerithoth 6b). Condor writes “"Shecheleth", Exod. xxx. 34; [is] rendered by the Septuagint, "onycha", and by the Arabic version, ladana . . . The root of the Hebrew word means to drop or distil, and "shecheleth" would seem, therefore, to mean some exudation.”