0 a relationship between two species in which one gets an advantage from living closely with the other and the other is not affected by it -- 片利共生（共棲）（兩物種之間的共生關係，在此關係中一個受益而另一個不受影響）
According to that hypothesis, proto-digeneans possessing a typical turbellaria-like structure adopted commensalism before becoming tissue parasites in gastropods.
The same is true of commensalisms, which for our purpose can be regarded as asymmetric versions of mutualism.
Mutualisms and commensalisms also fit into this framework.
Commensalism is symbiosis in which one organism benefits and the other is not harmed or helped.
Allowing the barnacle species to distribute themselves throughout global waters is a high fitness advantage of this commensalism.
The symbiotic relationships are mutualism, commensalism, amensalism, and parasitism.
Interactions of these organisms range from obligate mutualism to commensalism to outright antagonism.
An example of commensalism occurs between the hawkfish and fire coral.
