Their Day in the Sun: molecular phylogenetics and origin of photosymbiosis in the ‘other’ group of photosymbiotic marine bivalves (Cardiidae: Fraginae)

Abstract

The subfamily Fraginae (Cardiidae) is a morphologically diverse group of small-bodied marine clams inhabiting shallow seas worldwide. Like the exclusively photosymbiotic giant clams (Cardiidae: Tridacninae), some fragines are known to host zooxanthellae photosymbionts. However, surveys to widely determine photosymbiotic status and the lack of a comprehensive phylogeny have hindered attempts to track the evolution of photosymbiosis in the group. Worldwide sampling of all fragine genera and subgenera with phylogenetic reconstructions based on four gene regions [nuclear (28S) and mtDNA (16S, cytochrome oxidase I, cytochrome b)] does not support a monophyletic Fraginae. Sampled taxa form four restructured clades: (1) the ‘Fragum’ group, (2) the ‘Trigoniocardia’ and ‘Ctenocardia’ groups, (3) the ‘Parvicardium’ group and (4) the ‘Papillicardium’ group. Maximum likelihood analyses strongly support a clade of European cardiids uniting species from three subfamilies. Live examination of > 50% of species reveals that less than half of derived genera and subgenera host photosymbionts, supporting a single and relatively late origin of photosymbiosis in the Fraginae. The evolutionary implications for a small and little modified earliest diverging photosymbiotic lineage are discussed. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 97, 448–465.