A scientist who studies animal life is a zoologist. There is no particular name for a scientist that specialises in platypuses.
The name for this type of scientist is a teuthologist
The type of scientist that studies the environment is an enviromentologist.
what scientist studies hydrogen and oxygen
A botanist.
An Entomologist
paleontologist
Sedimentologist.
A zoologist.
Meteorologists
a mineralogist
Cosmologist
microbiologist