Summary Ever since Mivart asked Darwin to explain a bird's use for half a wing, biologists have been challenged to explain extraordinary evolutionary change mechanistically. Here, we investigate the enigmatic evolutionary origins of Janthinidae, a family of marine snails that raft passively in the neuston, a vast oceanic surface habitat, by constructing floats of mucus bubbles. We present the first molecular phylogeny including Janthinidae, which confirms that janthinids are derived from Epitoniidae (wentletraps) — benthic predators and parasites of sea anemones and corals. Our data support the hypothesis that floats and rafting evolved via modified epitoniid egg masses rather than by juvenile droguing. Our phylogeny also reveals sequential modifications of float formation and function among janthinid lineages. We interpret these changes as sequential adaptations to a neustonic existence, a conclusion supported by the positive association of derived janthinid traits with ecological prevalence.
