Seawater inflow through the madreporite and internal body regions of a starfish (Leptasterias hexactis) as demonstrated with fluorescent microbeads



Since the tube feet of starfish can continue to function by osmotic inflation after being separated from the madreporite, doubt has been raised in recent years that this organ is significant for keeping fluid balance. To clarify this question, small specimens of Leptasterias hexactis were exposed to a suspension of fine fluorescent beads for 48 hr and then studied as cryostatic sections under UV light. Abundant beads were found in tube feet and ampullae, clearly showing that madreporite uptake does help sustain their activity under normal circumstances. Even larger accumulations of beads were located in Tiedemann's bodies, indicating that they filter high-pressure water from the stone canal to generate perivisceral coelomic fluid. Since beads were found only in the upper axial sinus, near madreporitic pores and an opening to the stone canal, perihemal fluid is probably drawn up the axial sinus into the low-pressure end of the stone canal where it is mixed with incoming seawater and recirculated. In addition to contributing to tube feet inflation, these flows probably form part of a previously unrecognized pattern of fluid replenishment, purification, and circulation in starfish involving the stone canal, Tiedemann's bodies, perivisceral coelom, and distal perihemal spaces and return via the axial sinus.