The smaller partner has commonly been called a symbiont or symbiote by authors.
symbiont (sĭm'bē-ŏnt', -bī-) or symbiote|
An organism in a symbiotic relationship. In cases in which a distinction is made between two interacting organisms, the symbiont is the smaller of the two and is always a beneficiary in the relationship, while the larger organism is the host and may or may not derive a benefit. See also host, parasite.