Freshwater shrimp-sponge association from an ancient lake.

Shrimp-sponge associations occur frequently in marine ecosystems, serving as model systems for the evolution of eusociality. Here, we describe the first known instance of such association in freshwater from an ancient lake in Indonesia. The shrimp Caridina spongicola forms an exclusive and probably commensal association with a yet undescribed spongillinid sponge. Phylogenetic and ecological data suggest a comparatively recent origin of both taxa. Climatic fluctuations may have facilitated speciation and occasional hybridization of the shrimp species, which is derived from a rock-dwelling ancestor. Their extremely localized occurrence in an increasingly disturbed area makes both taxa a conservation priority.