JAECKLE, W.B.; BALSER, E.J.*; Ill. Wesleyan U., Bloomington: Nephridial function in larvae of asteroid echinoderms

Evidence for a nephridium in larvae of echinoderms was first provided 20 years ago. Ruppert and Balser (1986) presented morphological evidence that this larval kidney is formed by the axocoel, the pore canal, and when present, a pulsatile vesicle or heart. This nephridium functions as follows: blastocoelic fluid is forced across a filter (basal lamina of a podocytic epithelium) into the lumen of the left axocoel by negative pressure created by the coelomic cilia or by positive pressure created by the heart. Organic materials within this primary urine were thought to be reclaimed by the cells that line the axocoel and pore canal prior to discharge from the larval body via the dorsal pore. Tests of this hypothesis are largely lacking and we present here the results of specific experiments designed to evaluate the functional predictions of the �larval nephridium hypothesis�. We exposed larvae of asteroid echinoderms collected from surface waters of Bransfield Strait, Antarctica to the iron-containing protein ferritin (2 mg / ml, 0C) for from 2 to 36 h. After each incubation period, the larvae were fixed and the presence of the iron label was detected using the �Prussian Blue� reaction. The label associated with ferritin was initially found in cells of the larval stomach, intestine, and esophagus. With increased incubation time, cells of only those coeloms that are continuous with the left larval axocoel accumulated the label. In addition, the cells of the pore canal itself became heavily labeled as did free mesenchyme cells within the blastocoel. At no time did the cells of the larval epidermis contain any reaction product. Taken together our results provide experimental evidence that offers support for the larval nephridium hypothesis. This work was supported by NSF grant NSF/OPP 03�38218 to K. Halanych and R. Scheltema.