The bioluminescent symbiosis between the sea urchin cardinalfish, Siphamia tubifer, and the luminous bacterium, Photobacterium mandapamensis, is highly specific - the fish seem to only associate with members of a sub-clade of the bacterial symbiont. We are investigating key ecological factors that help to promote the specificity and stability of this association between host generations, including the possibility of local symbiont enrichment by a local host fish population.