1. The animal is one foot long.
2. The animal is 49 feet long.
3. There is a krill in the water.
4. There are two krill in the water.
(Are they all grammatical? How can we use 'krill'? Would you let me know the use of 'krill'? Thank you.)
The word "krill" is a collective word -- it has no singular or plural -- or maybe the best way to think of it is that it's always plural!
1 and 2 are fine.
3 and 4 = There are krill in the water.
OR
There are plenty of krill in the water.