The symbiotically luminous cardinalfish genus Siphamia is comprised of 25 described species. However, little is known of the biology and ecology of this fascinating group of coral reef fishes, including which luminous bacteria they harbor in their gut-associated light organs. Most Siphamia sp. are habitat specialists, associating closely with a particular substrate, such as branching coral or sea urchins. We are investigating various ecological drivers of speciation in the host, including the fish’s association with luminous bacteria, throughout it’s broad Indo-Pacific distribution.