Functional bursting by the dracunculoid nematodePhilonema oncorhynchi

Abstract

Using biomedical techniques experimental determinations of the hydrostatic pressure in the pseudocoel of adult femalePhilonema oncorhynchiindicated that the rate of increase in pressure (dP/dT) and absolute pressure values (cm/H2O) shown by bursting worms in distilled water are correlated with the diameter of the nematode. At bursting pressures, wall tension in a wide size range of worms was virtually identical, indicating that the bursting process is independent of muscular contraction. That the generation of the hydrostatic pressure was an osmotic phenomenon was confirmed by measuringdP/dTin prelarvigerous and larvigerous female worms subjected to different concentrations of sodium chloride, ranging from 89 to 800 m-osmol/kg, and also to a variety of solutions of similar osmolarity (155–175 m-osmol/kg),e.g.magnesium sulphate, urea, potassium chloride, sodium chloride and sucrose. The overall rate of uptake was faster in the larger worms but, per unit surface area, small worms had an uptake rate three times that of the large individuals.The prediction that the body wall of femaleP. oncorhynchiis permeable to ions such as Na+ was confirmed using radiolabelled22Na and by bringing about changes in the osmolarity of worms subjected, for 5 min periods, to hyperosmotic solutions of sodium chloride and sucrose. The survival ofP. oncorhynchiin the body cavity of sockeye salmon,Oncorhynchus nerka, is dependent upon the permeable nature of the body wall ofP. oncorhynchiallowing the worm to function as an ‘osmometer', because as the anadromousO. nerkaenters fresh water, the osmolarity of its blood plasma is known to decrease by about 15%. At the time of spawning in Cultus Lake, British Columbia, the body fluids of both femaleP. oncorhynchiandO. nerkaare isosmotic, indicating that the worms are able to equilibrate to the above changes and at the same time preventing premature bursting in the body cavity of its host. However, osmotic invasion of water must occur far quicker than ionic exchange since complete release of larvae does take place when female worms pass out into the redd along with the eggs of the fish and burst.