 Fish

Fish are aquatic animals; their feeding roles depend on their diet and life stage. Small and large fish in the illustrative food web are generic groups chosen to distinguish two feeding positions, rather than a measured relationship between named species.

Vertebrates minus tetrapods.

This paraphyletic subgroup is easy to form the "acquatic only" (fishes) vs "things that come out of water" (tetrapods). Though mudfishes make that distinction harder.

Which kind of makes sense, why would you want for limbs unless you are going to stay out of water!