The structure, ultrastructure, and function of the protonephridial system of Asplanchna priodonta (Rotifera)

Asplanchna priodonta is a common, predaceous rotifer described from freshwater lakes and ponds worldwide. As with most freshwater organisms, the cells, tissues, and body fluids of A. priodonta are of a higher osmotic concentration than the surrounding medium. In order to maintain its internal environment at a higher concentration, the rotifer must osmoregulate and maintain specific ion balances. The organ thought to be used by A. priodonta in the maintenance of a constant internal environment is the protonephridium. A protonephridium consists minimally of a proximal terminal organ. a protonephridial tubule, and a distal pore (nephridiopore) which opens to the exterior. Historically, excretory and osmoregulatory roles have been attributed to protonephridia on the basis of morphological observations. Because the terminal cell is in a fluid-filled body cavity, is internally flagellated, and provides an apparently semipermeable barrier between the body cavity and tubule lumen, it is believed to pump and ultrafilter body fluid as it enters the tubule. Ultrafiltration presumably occurs as a result of a pressure difference between the extracellular space (the pseudocoelomic cavity in rotifers) and the lumen of the protonephridial system. The origin of the pressure difference has been speculated to be the beating of the internal flagella, located within the lumen of the terminal organs (flame bulbs in A. priodonta). An alternative source of the pressure differential is contraction of body wall musculature, resulting in an elevation of extracellular fluid pressure in the body cavity. It is possible that both sources contribute to the pressure difference which drives ultrafiltration, but there is a paucity of experimental evidence to support either explanation. Lack of experimental evidence can be attributed to the fact that protonephridia are small, fragile organs from which it is difficult to obtain reliable physiological information. This investigation was undertaken to provide additional physiological and morphological data pertaining to protonephridial function. Transmission electron microscopy (TEM) was used to characterize the morphology of the terminal organs, protonephridial tubule, and contractile bladder of the ubiquitous freshwater rotifer, A. priodonta. The effects of varying the osmotic concentration of culture medium on the rate of terminal organ flagellar beat and the rate of contractile bladder discharge were determined. Laser ablation was used to determine the effects of the destruction of individual terminal cells on the overall function of the system. Observations were made on individual specimens of A. priodonta with heavy bacterial loads in their pseudocoelomic fluid in an attempt to visualize convective flow in the regions surrounding the weirs of the protonephridial terminal organs. Additional observations were made on uninfected individuals in which tracer particles had been injected into the pseudocoelomic fluid and on free protonephridia that were dissected from individual rotifers. Tracers were placed in Ringer's solution containing the free protonephridia, again, in an attempt to observe directed flow in the vicinities of the terminal organ weirs.