The enzyme in a clownfish' scales making them immune to the sea anemone's venom is what type of relationship?

I've found a few relationships that say mutualism and in 'Biology:Understanding Life' book, that says commensalism. So i'm confused.

Commensalism usually refers to two or more species that share living space or food source, where one gains some kind of benefit without affecting the fitness of the other. Mutualism refers to a beneficial relationship that two or more species get from living with each other, where both species gain benefits. Mutualism can be separated into facultative (optional) or obligate (necessary), and is sometimes alternatively called symbiosis. The question you must ask your self about the example of clown fish and anemones is: do both parties benefit, or only one? Does the fish simply gain a safe place to live or is there also a benefit to the anemone?

There isn't actually a sharp line dividing the two, and sometimes a case could be made for both! Have a look at the wikipedia article on mutualism, it outlines the issues fairly well.