The shell of a barnacle that had been living on a calico scallop, Argopecten gibbus. The interaction between these two species represents the form of symbiosis known as commensalism where one member of the relationship benefits and the other is not affected either positively or negatively. In this case the barnacle benefits and the scallop is not affected. In some cases, however, a heavy load of barnacles could begin to harm a host in which case the interaction may be more parasitic.