Habitat-associated evolutionary rates in deep-sea invertebrates

In environments where resources occur as ephemeral patches within a matrix of resource scarcity, patch characteristics can strongly affect organisms' life-history traits. Depending on patch longevity, life histories may evolve to match the availability of resources, potentially resulting in deterministic patterns of evolution. In the deep sea, organic falls, hydrothermal vents, and cold seeps create patches of high productivity that persist over different lengths of time, ranging from decades to millennia. Using publicly available data for mitochondrial CO1, we examined the relationship between habitat type and rates of molecular evolution among mussels (Mytilidae: Bathymodiolinae) and tubeworms (Siboglinidae), two clades that have diversified in these deep-sea environments. We observed an inverse relationship between habitat longevity and rates of mitochondrial sequence evolution, supporting the hypothesis that resource longevity structures life-history variation. Contrary to expectation, animal size does not correlate with rates of molecular evolution after accounting for phylogeny. Within both clades, species that specialize on organic falls exhibit faster substitution rates than related species at vents and seeps. The similar patterns recovered in two deeply diverged phyla suggest that the proposed relationship between resource longevity and rates of molecular evolution may be widespread.