Madreporite Function and Fluid Volume Relationships in Sea Urchins

An effort was made to demonstrate an influx of seawater through the madreporites of sea urchins and to evaluate how such an influx, along with osmotic differences and other factors, could contribute to fluid homeostasis. Fluorescent microbeads placed in the medium of Strongylocentrotus droebachiensis were taken up into the pore canals and stone canal and distributed (in small numbers) to the distal tube feet, confirming a slow bulk inflow of seawater through the madreporite, where it is partially purified. Probably none of this fluid stream is diverted to the perivisceral coelom (as it is in asteroids), since experiments with Strongylocentrotus pallidus showed no significant movement of a soluble fluorescent tracer into that compartment. The osmotic concentrations of the perivisceral coelomic fluids in these two species, and in Strongylocentrotus franciscanus, were higher than that of ambient seawater by 2.66 +/- 0.39 mosmol kg-1 (mean +/- SE). That small hyperosmoticity, along with the net hydrostatic pressure differences induced by the flexing peristome, probably stabilizes body fluid volume. Likewise, fluid in the tube feet of S. franciscanus is elevated by 7.94 +/- 1.04 mosmol kg-1 above seawater, which should contribute to their inflation. Blockage of the madreporite does not lead to an obviously reduced activity of the tube feet, but over the long term, an influx through the madreporite is necessary. Specimens of S. droebachiensis with an obstructed madreporite, fed ad libitum, had significantly (P < 0.006) reduced gut contents vs controls after 28 days, indicating impaired movement or feeding; and the body weights (i.e., volumes) of unfed specimens were significantly (P < 0.013) more reduced after 21 days. Compared to starfish, the rigid test of sea urchins reduces the need for an influx of seawater through the madreporite, but some small admission is still essential.