Circulation of fluids in the gastrovascular system of a stoloniferan octocoral

Cilia-based transport systems characterize sponges and placozoans. Cilia are employed in cnidarian gastrovascular systems as well, but typically function in concert with muscular contractions. Previous reports suggest that anthozoans may be an exception to this pattern, utilizing only cilia in their gastrovascular systems. With an inverted microscope and digital image analysis, we used stoloniferan octocoral colonies growing on microscope cover glass to quantitatively describe the movement of fluids in this system for the first time. Flow in stolons (diameter ≈300 μm) is simultaneously bidirectional, with average velocities of 100-200 μm/s in each direction. Velocities are maximal immediately adjacent to the stolon wall and decrease to a minimum in the center of the stolon. Flow velocity is unaffected by stolonal contractions, suggesting that muscular peristalsis is not a factor in propelling the flow. Stolon intersections (diameter ≈500 μm) occur below polyps and serve as traffic roundabouts with unidirectional, circular flow. Such cilia-driven transport may be the plesiomorphic state for the gastrovascular system of cnidarians.