The evolution of molluscan photosymbioses: a critical appraisal

Partnerships between animals and photosynthesizing microbes have evolved repeatedly, although their history, adaptations, and ecology remain controversial and little understood. In a critical review of 17 fossil and living clades of shell‐bearing molluscs with photosymbionts (two of them newly inferred), adaptive shell modifications and ecological aspects are discussed in the broader context of photosymbioses in other phyla. Fossil candidates have characteristics that are rare or unknown in living photosymbiotic molluscs, including cementation, porous shell microstructure, and epifaunal habits on carbonate muds. Many ancient photosymbioses may have lived in planktonically more productive environments than are typical of living tropical forms. This may be related to the late appearance (Early Eocene) of the dinoflagellate Symbiodinium, which can thrive under highly oligotrophic conditions. Living photosymbiotic molluscs represent a small and atypical sample of all the photosymbiotic clades that have evolved. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 497–511.