Fact 1: Sponges are a source of food for many species of fish.

Fact 2: Sponges are also keystone species.

Given the two facts above, answer the question "What are a source of food for many fish species?" with the following options: 
- layers of fat
 - bird species
 - human species
 - keystone species
 - Organic compounds
 - barnacles
 - gastropod shells
 - species of deer


 

    keystone species     