A rectifying effect by internal structures for passive feeding flows in a concavo-convex productide brachiopod

Abstract. It has been experimentally shown that the shell morphology of concavo-convex productide brachiopods has the potential to generate passive flows for feeding. However, there still remains the problem of how the presence of internal soft parts influences the course and pattern of the flows. To clarify the effect of soft parts, we performed additional experiments of fluid visualisation using a flow tank and a transparent, polyhedral model with supposed soft parts in a postero-median region. Regardless of the ambient flow directions, the experimental results showed that inflows through ear gapes turned into symmetrical vortices inside the model. The soft parts altered the course of internal flows and interfered with the vortices entering the median region. Therefore, stepwise inflows pushed the precedent vortices forward as the rotational speed decreased. As a result, the slower vortices aligned with the brachial ridges on which the lophophore was arranged. Morphologically, productide brachial ridges...