The Function of the Madreporite in Body Fluid Volume Maintenance by an Intertidal Starfish, Pisaster ochraceus

The madreporite has been viewed as superfluous and unnecessary because starfish can keep their tube feet inflated by osmotic mechanisms alone. Recent evidence has suggested, however, that the madreporite may be significant in the replenishment of general body fluid. This hypothesis has been tested. The madreporite openings of an intertidal starfish, Pisaster ochraceus, were obstructed with cement, and the animals were used in controlled experiments to compare weight (volume) changes under stable conditions, in response to air drying and recovery, and during adaptations to hyper- and hypoosmotic environments. Over a period of days, normal animals showed positive and negative volume fluctuations of up to about 20% (in part related to posture). Animals with obstructed madreporites generally did not gain weight and were significantly less able to maintain body volume or recover from fluid losses resulting from the stresses applied. The madreporite seemed to contribute little to the initial osmotic responses, but it did participate in subsequent volume readjustments in a hyperosmotic medium that had induced fluid losses. Obstruction of the madreporite did not impede tube foot activity, but may have caused some diversion of general body fluid to the ambulacral system. Rates of seawater uptake through the madreporite of 2.2-2.6 {mu}l g-1 h-1 were calculated from observed maximum mean differences in weight changes.