1. A specialist or expert in biology or one who studies biology.
2. A scientist who studies living organisms.
3. A scientist devoted to the study of organisms and their relationships to their environments and the underlying mechanisms that govern how those organisms work.

An individual who specializes in the branch of the living organisms that inhabit the sea: Jim's uncle was a marine biologist who spent much of his life either in the water engrossed in rare fish or in front of his computer reading about the newest discoveries in ocean life.