SANTAGATA, S.; Smithsonian Marine Station, Fort Pierce, FL and Friday Harbor Laboratories, University of Washington: The significance of neuromuscular anatomy and metamorphosis in the larval and juvenile body plans of phoronids and brachiopods.
Molecular, developmental, and anatomical evidence support the monophyly of phoronids and brachiopods. However, discrepancies remain regarding the homology of larval neuromuscular characters and their fate at metamorphosis. Despite differences in life history traits, phoronid larvae and the brachiopod larvae of Glottidia pyramidata (linguliform) and Terebratalia transversa (rhynchonelliform) each exhibit two functional neuromuscular signaling centers at metamorphic competence. In phoronids, the larval and presumptive juvenile neuromuscular signaling centers develop separately and have discrete functional roles during metamorphic remodeling. Glottidia pyramidata larvae integrate these neuromuscular centers earlier in development than do phoronid larvae and gradually remodel feeding structures of the planktonic larva into the benthic juvenile form. Competent larvae of Terebratalia transversa have several unique anatomical features consistent with the switch to a non-feeding larval form but still possess two neuromuscular signaling centers with similar functional roles to that of phoronid and Glottidia larvae. Metamorphosis of Terebratalia larvae results in several juvenile neuromuscular features reminiscent of the morphology of Glottidia larvae. Juvenile anatomical features of phoronids and brachiopods diverge morphologically and functionally as the adult ontogenetic program of each juvenile form progresses. Overall, the larval and juvenile neuromuscular anatomy of phoronids and brachiopods share a ground plan that has been modified in developmental timing through heterochrony and functionally modified by adult habitat diversification.