Hydroid Stolonal Contractions Mediated by Contractile Vacuoles

The growth and development of colonial hydroids is dependent upon a flow of gastrovascular fluid through a common vasculature. Gastrovascular flow is believed to be driven by polyp contractions, stolonal contractions and endodermal ciliation. In the athecate hydroid Hydr•actinia symbiolongicarpus we found that polyp contractions, stolonal contractions and gastrovascular flow were only weakly correlated on a local scale. Stolons isolated from vascular continuity to polyps were found to display contraction frequencies indistinguishable from those observed in intact colonies. Transmission electron micrographs reveal a unique organelle, with characteristics of a contractile vacuole, in the apical stolonal endoderm. The activity of this organelle, inferred from static observations, correlates with the observed pattern of stolonal contractions. This is the first record of a contractile vacuole in an eumetazoan animal. Its description calls for a revision of existing interpretations of the mechanism of stolonal contractions in hydroids.