Meeting Abstract

78.9  Friday, Jan. 6  Sponsorship required for permanent residency in sediment LI, Jingchun; University of Michigan, Ann Arbor

The importance of biotic associations in the evolution of marine taxa is implicitly understood. However, relative to terrestrial systems, meaningful hypotheses and tests of specific mechanisms are underdeveloped. The marine bivalve superfamily Galeommatoidea represents an ideal system to address this deficit: it is a megadiverse clade and embodies a clear lifestyle dichotomy in that members are either free-living or have obligatory commensal associations with invertebrate hosts. I have performed a meta-analysis to determine if this lifestyle dichotomy is correlated with specific ecologies. Galeommatoidea has significant diversity in the two primary benthic habitats: soft- and hard-bottoms. Known free-living species are restricted to hard-bottom habitats, typically hidden within crevices of rocks/coral heads/encrusting epifauna. In contrast, commensalism is almost exclusively associated with infaunal sediment habitats. Preliminary gene trees show that evolutionary transitions from hard-bottom to sediment occur through the formation of a biotic association with a burrowing infaunal host. The details of the biotic association vary among taxa: it may involve direct attachment to the host, or else clustering around its tube/burrow, but all commensals locate within the oxygenated envelope produced by their host’s bioirrigation. Sediment-dwelling bivalves are exposed to intense predation pressure that drops markedly with depth of burial. Commensal galeommatoideans attain depth refuges many times their body lengths, independent of siphonal investment, by virtue of their host’s bioirrigation. The formation of commensal associations with infaunal bioirrigators may well be a prerequisite for the colonization of sediments by Galeommatoidea as well as a key factor in their exceptionally high diversity.