Alternative pathways into the deep sea: patterns in Bivalvia

Relatively few clades have colonized the deep sea. Here, we analyse evolutionary pathways into this harsh environment, as a continuum defined by two potential endmembers-a 'piecemeal model', with exclusively deep-sea species (deep-sea endemics herein) derived from multiple, independent entries and an 'in situ diversification model' with one entry followed by species proliferation. We focus first on two ancient, distantly related subclades in Class Bivalvia, Mytilidae and Lucinidae, each with hundreds of species occurring globally from the intertidal to abyssal plains. Placing bathymetric ranges into newly inferred molecular phylogenies, we find that the deep-sea endemics within Lucinidae derive in piecemeal fashion, estimating up to 16 phylogenetically isolated entries and perhaps one modest in situ diversification. Mytilidae entered the deep sea just four times, with most endemics stemming from the prolific in situ diversification of Bathymodiolinae. We suggest that the contrasting phylogenetic patterns of entry and proliferation in these clades may be determined by differences in ancestral adult feeding modes. Across Bivalvia, we find that under half of extant families have deep-sea endemics, with the piecemeal model occurring more frequently. Taken together with other clades, we suggest that evolutionary pathways to deep-sea endemicity are more often shaped by multiple, independent events than by in situ diversification.