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.
They might taste similar since they are closely related, common ones that are eaten are shrimp, lobsters, crabs and crayfish. Crayfish remind me of insects a lot.
Maybe I should try that. What else do they have in common with insects?
They are invertebrates. They are both in the main class of the phylum Arthropoda so they are both arthropods which have an exoskeleton. 
I never knew that. You are just full of facts.
shellfish are low on the food chain and eat a diet composed primarily of phytoplankton and zooplankton. Many varieties of shellfish, and crustaceans in particular, are actually closely related to insects and arachnids; crustaceans make up one of the main subphyla of the phylum Arthropoda. Molluscs include cephalopods (squids, octopuses, cuttlefish) and bivalves (clams, oysters), as well as gastropods (aquatic species such as whelks and winkles; land species such as snails and slugs).
Molluscs used as a food source by humans include
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