Symbiosis is basically where two organisms live together. There are three types of symbiosis:
Mutualistic is where BOTH organisms benefit from the relationship - such as algae and fungi when they become lichen.
Commensalism is where one benefits and the other isn't affected either negatively or positively, kind of like marriage.
Last kind is Parasitic Symbiosis where one benefits and the other is harmed in the process. This includes all parasites such as tape worms, etc.
Ram my guess for your mutualistically symbiotic Nep with a smile is good old bical with the ants?