Here's why:
* Common nouns refer to general categories of people, places, things, or ideas.
* Proper nouns are specific names of people, places, things, or organizations.
Since "shrimp" refers to a general type of crustacean, it's a common noun.
Examples:
* Common Noun: I love to eat shrimp.
* Proper Noun: My favorite restaurant is The Shrimp Shack.