Solute Secretion by the Tube Foot Epithelium in the Starfish Asterias Forbesi

The K+ and Cl− levels and osmotic concentration of the tube feet fluid in intact starfish (Asterias forbesi) are higher than that in the external sea water or perivisceral (coelomic) fluid. The increase in K+ in the fluid in the tube feet lumen is brought about by the active secretion of K+ by the tube feet epithelium. No exchange of material from the external environment with the more distal portions of the water vascular system across the madreporite was observed in this study. It is suggested that the fluid volume in the tube feet is maintained by active K+ secretion, by the tube feet epithelium, which creates an osmotic driving force for water into the lumen of the tube feet.