Symbiotic associations with bacteria are found virtually everywhere in nature, but the steps that lead to the evolution of these vital interactions are relatively unknown. The ecology of a host organism can play a key role in the initiation and maintenance of its partnership over host generations. The symbiotically luminous genus of cardinalfish Siphamia is comprised of 23 species, but little is known of the biology and ecology of this group, including which luminous bacteria they associate with. Most Siphamia sp. are habitat specialists, associating closely with a particular substrate, such as branching coral, crinoids, and sea urchins. At the California Academy of Sciences, I am examining how ecological characteristics of these hosts, including habitat specialization, influence the establishment and maintenance of their symbiotic association with luminous bacteria and how these ecological processes influence the stability of relationship over time.