The Role of Podial Secretions in Adhesion in Two Species of Sea Stars (Echinodermata).

Abstract

Individuals of Asterias rubens and Marthasterias glacialis use their podia in locomotion, anchorage, and feeding. Each podium consists of a stem with a disk at its tip. The stem allows the podium to lengthen, flex, and retract, and the disk allows the podium to adhere to the substratum. Adhesion of sea star podia seems to rely on the epidermal secretions of the disk and not on a mechanical sucker-like operation. The disk epidermis is made up of five cell types: nonciliated secretory cells (NCS cells) of two different types (NCS1 and NCS2), both containing granules that are at least partly mucopolysaccharidic in composition; ciliated secretory cells (CS cells) containing small granules of unknown content; nonsecretory ciliated cells (NCS cells); and support cells. The epidermal cells of the podial disk are presumably functioning as a duogland adhesive system that is involved in an adhesive/deadhesive process. The following model is presented. Adhesive secretions are produced by NCS1 and NCS2 cells (both of them have extruded some of their secretory granules in attached podia). These secretions constitute a layer of adhesive material between the podium and the substratum, this layer being the footprint left by the podium after it has become detached from the substratum. Deadhesion, on the other hand, would be due to CS cell secretions. All these secretions would be controlled by stimuli perceived by the two types of ciliated cells (receptor cells), which presumably interact with the secretory cells via the nerve plexus.