Coordinated contractions effectively expel water from the aquiferous system of a freshwater sponge

Abstract

SUMMARY In response to mechanical stimuli the freshwater sponge Ephydatia muelleri (Demospongiae, Haplosclerida, Spongillidae) carries out a series of peristaltic-like contractions that is effective in expelling clumps of waste material from the aquiferous system. Rates of contraction depend on the region of tissue they are propagating through: 0.3–1 μm s–1 in the peripheral canals, 1–4 μm s–1 in central canals, and 6–122 μm s–1 in the osculum. Faster events include twitches of the entire sponge choanosome and contraction of the sheet-like apical pinacoderm that forms the outer surface of the animal. Contraction events are temporally and spatially coordinated. Constriction of the tip of the osculum leads to dilation of excurrent canals; fields of ostia in the apical pinacoderm close in unison just prior to contraction of the choanosome, apical pinacoderm and osculum. Relaxation returns the osculum, canals and the apical pinacoderm to their normal state, and three such coordinated `inflation–contraction'responses typically follow a single stimulus. Cells in the mesohyl arrest crawling as a wave of contraction passes, suggesting an extracellular signal may pass between cells. Bundles of actin filaments traverse endopinacocytes of the apical pinacoderm. Actin-dense plaques join actin bundles in adjacent pinacocytes to form continuous tracts spanning the whole sponge. The orchestrated and highly repeatable series of contractions illustrates that cellular sponges are capable of coordinated behavioural responses even in the absence of neurons and true muscle. Propagation of the events through the pinacocytes also illustrates the presence of a functional epithelium in cellular sponges. These results suggest that control over a hydrostatic skeleton evolved prior to the origin of nerves and true muscle.