Data and code from: Alternative pathways into the deep sea: Patterns in Bivalvia

Relatively few clades have colonized the deep sea. Here, we analyze
evolutionary pathways into this harsh environment, as a continuum defined
by two potential end-members—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 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
slightly more often. Taken together with other clades, we suggest that
evolutionary pathways to deep-sea endemicity are as often shaped by
multiple, independent events as by in-situ diversification.