Feeding the Masses: Colonial Transport in the Marine Bryozoan Membranipora membranacea

Abstract

The transport system of cheilostome bryozoans is unusual among long-distance transport systems. In these colonial animals, a network of strands (the funicular system) carries nutrients to non-feeding individuals and to the growing edge of the colony. However a complex of cells appears to plug the pores that connect individuals. Focusing on the cheilostome, Membranipora membranacea, we used time lapse movies to test whether there were contractions/dilations of funicular strands, as expected if muscular pumping were to move material through the strands, and to test whether cells or large vesicles moved directionally along the strands, potentially carrying nutrients. Neither contractions/dilations of the funicular strands, nor persistent movement of particles or other features along the strands, were visible in time lapse videos (10 to 120 min at 4 to 10 sec per frame). The only visible movements were rare back-and-forth movements along the strands, or shaking of the strands. We injected materials that differed in molecular/particle size to investigate the specificity of transport at pore plates. Both fluorescein (367 Da; as sodium salt) and fluorescein-dextran (70,000 Da) moved between individuals; however 2.0μm fluorescent polystyrene beads did not. The fact that both fluorescein and fluorescein-dextran were transported suggests that transmembrane channel or transporter proteins are not required for transport; however there may be an upper size limit (<2μm) caused by something other than the pore itself. Our results are consistent with some transport mechanisms (e.g. paracellular diffusion or transcytosis at the pore plate) but inconsistent with others (muscularly-pumped flow along funicular strands, cell crawling, or transmembrane transport via transporter or channel proteins).