I am allergic to shellfish which are oddly not even a type of fish.
they aren't. Then what the heck are they?
Many types of shellfish are actually crustaceans which are closer to insects. There are other types as well.
Insects you say? oh my. I wonder if a bug tastes as good as a lobster.
many species of clams, mussels, oysters, winkles, and scallops. Some crustaceans that are commonly eaten are shrimp, lobsters, crayfish, and crabs. Echinoderms are not as frequently harvested for food as molluscs and crustaceans; however, sea urchin roe is quite popular in many parts of the world, where the live delicacy is harder to transport.
Terminology.
The term “shellfish” is used both broadly, specifically and broadly. In common parlance, as in "having shellfish for dinner", it can
consumed, even if it can be substantiated that they meet all four criteria. The only exception to this is the turkey. There was a time when certain authorities considered the signs sufficient, so Jews started eating this bird without a "masorah" because it possesses all the signs ("simanim") in Hebrew.
Fish must have fins and scales to be kosher. Shellfish and other non-fish water animals fauna are not kosher. (See kosher species of fish.) Insects are not kosher, except for