Stress-induced failure of osmoregulation in the parasitic nematode Pseudoterranova decipiens: indirect evidence for hormonal regulation.

When third-stage larvae of Pseudoterranova decipiens maintained at 5 degrees C are placed in either 40% artificial sea water (ASW, iso-osmotic) or 15% ASW (hypo-osmotic) and weighed once at 0 h and again at 24 h, they neither lose nor gain weight, and the osmotic pressure (OP) of their pseudocoelomic fluid (PCF) remains unchanged. In contrast, when worms are weighed six additional times during the 24 h interval, those maintained in isoosmotic conditions lose weight, while those maintained in hypo-osmotic conditions gain weight. Worms which had been exposed to hypo-osmotic conditions and weighed at various times between 0 and 24 h exhibited an increase in weight which was correlated with the number of weighings. Worms exposed to hypo-osmotic conditions and weighed three additional times between 0 and 24 h also gained weight, and the OP of the PCF decreased such that worms experiencing the greatest increase in weight suffered the greatest dilution of the PCF. In worms ligatured at the head or tail or at the head and tail, and then exposed to either 15% or 40% ASW, the effect of multiple weighings is exaggerated in a complex way. The presence of a ligature on the tail in worms immersed in an iso-osmotic medium leads to an increase in weight and to a very marked additional increase in weight in worms immersed in a hypo-osmotic medium. The presence of a head ligature in worms in an iso-osmotic medium leads to a decrease in weight and to a smaller weight gain in a hypo-osmotic medium.(ABSTRACT TRUNCATED AT 250 WORDS)