From octopuses to snails, the complicated molluscan family tree has now been mapped in unprecedented detail, researchers report. This includes sequences for 13 new complete genomes from across the phylum. The genome-based phylogeny helps to resolve long-standing evolutionary debates and provides new insights into how the extraordinary diversity of species emerged from a single common ancestor. The phylum Mollusca is highly diverse with myriad morphological, ecological, and behavioral adaptions spanning both terrestrial and aquatic environments. The most well-known groups – bivalves, gastropods, and cephalopods – are accompanied by five lesser-known classes, including worm-like mollusks, chitons, and deep-sea limpets. However, while the eight living molluscan classes are unequivocally descended from a common ancestral group, efforts to determine other evolutionary relationships have been complicated. Phylogenomic analyses have gradually clarified the relationships among these groups, resolving two major clades: Conchifera (including bivalves, gastropods, cephalopods, and others) and Aculifera (comprising worm-mollusks and chitons). However, due to limited genomic resources, previous molecular studies often conflicted with morphology-based classifications and the fossil record, creating persistent uncertainties.

Here, Zeyuan Chen and colleagues present a comprehensive phylogeny of living Mollusca, constructed using genome-wide markers from 77 species encompassing all major clades and multiple representatives from each of the eight extant classes. In addition to previously published data, Chen et al. assembled 13 new genomes spanning key taxonomic groups, including two near-complete assemblies of the enigmatic deep-sea class Monoplacophora. According to the authors, the phylogeny supports a Cambrian origin for Mollusca, followed by a rapid divergence into the major clades Aculifera and Conchifera. Consistent with fossil evidence and morphological hypotheses, the poorly defined Monoplacophora was identified as the sister group to all other Conchifera. Additionally, Cephalopoda was confirmed to be the sister group to a clade comprising Gastropoda and Diasoma (which includes Scaphopoda and Bivalvia), further refining the evolutionary relationships within the phylum. “Throughout the long evolutionary history of mollusks and continuing today, aspects of a flexible genome led to a flexible phenome: Endless forms of mollusks showcase the power of animal evolution,” write the authors.