The pair have a Philosophical discussion on whether to intervene if one of the animals/aliens being studied is in danger. One of them is for intervening and the other against altering what would have happened had they not been there.
At the end of the story one of the Naturalists is in grave peril and is rescued by another visiting Alien. It had been studying them and chose to intervene when necessary.
I read this sometime in the 1970's-1980's but can recall no other details.
As JohnRennie stated: The predator lives underground and catches prey by sending tentacles up though the ground. The biologists are watching it catch a deer-like animal when the man gets too close and is also grabbed by a tentacle.