posted by rfvv .
1. Blue whales eat only krill.
2. There is a krill in the sea.
3. There are many krills in the sea.
4. There are much krill in the sea.
5. There are many krill in the sea.
(Which ones are grammatical? Would you check them? Thank you.)
Krill is both singular and plural.
1 and 5 are correct.