53.3 Saturday, Jan. 5 Untangling the trees of obligate symbionts: myzostomes and echinoderms SUMMERS, M.M.*; ROUSE, G.W.; Scripps Institution of Oceanography, UCSD; Scripps Institution of Oceanography, UCSD email@example.com
The obligate association of myzostome worms (Myzostomida) with echinoderms, in particular with crinoids, is an ideal system in which the evolution of symbiotic lifestyles and body plans can be investigated using phylogenetic inference. An association that has persisted since before the Jurassic, the body plans of myzostomes vary considerably and are consistent with four prominent symbiotic lifestyles (free-living, gall-forming, cyst-forming, and internal) in which the myzostome steals food from or directly consumes the host. Those living freely are mainly disk-shaped and tend to “mimic” the host by adapting similar colors and/or appendages that resemble the host, traits which are lacking in those that live internally or form cysts and galls. This variety of life histories and dependence on an echinoderm host over long time-scales presents the opportunity to compare the evolutionary histories of myzostomes and their hosts, as well as investigate the evolution of character traits related to this symbiosis. In this study we combine new and previously published sequence and morphological data to present a systematic revision of Myzostomida and their echinoderm hosts, assess congruence between host and symbiont phylogenies, and infer possible evolutionary events leading to the current diversity of myzostome species, lifestyles, and body plans.