ABSTRACT
Starfish possess spacious, fluid-filled body cavities, but are often described as being both isosmotic and lacking excretory organs; seemingly, a physiological paradox. Recent work provides the answer. It indicates that seawater enters the madreporite, but is more important for perivisceral coelomic fluid balance than supplying ambulacral fluid to the tubefeet. It supplements a largely overlooked low-level hyperosmoticity as a way to maintain fluid homeostasis. A high molecular weight fluorescent tracer, fluorescein isothiocyanate dextran (FID), allowed madreporic uptake to be quantified in the Florida species, Echinaster graminicola. Additional work has now been carried out on 5 Pacific forms. After 24 hours of exposure to FID in their media, Pisaster ochraceus, Leptasterias hexactis, Henricia leviuscula, and Luidia foliolata all showed significant levels of it in their perivisceral and ambulacral fluids. In Pycnopodia helianthoides it was only detected in the latter. Loss of perivisceral fluid usually accelerates madreporic replacement. Comparison of the rates of madreporic uptake to the levels of hyperosmoticity maintained reveals different patterns of adaptation by the several species. Pisaster, for example, is rather impermeable, and relies almost wholly on madreporic uptake for fluid balance. Pycnopodia is more permeable, and depends heavily on osmotic uptake. Luidia makes good use of both mechanisms.
