Asymmetric reproductive output by the monogenean Pseudodiplorchis americanus.

Pseudodiplorchis americanus accumulates its entire reproductive output in utero during hibernation of its host, Scaphiopus couchii. Infective larvae are released only during the toads' brief entry into water for breeding. In one host population studied prior to transmission, in S.E. Arizona, adult worms contained 0-326 infective stages; 19% of 178 worms produced 51% of the total larvae. Reproductive output was positively correlated with body length which was considered to reflect worm age; first-, second- and third-year worms produced means of 5, 43 and 96 larvae, respectively. Within second- and third-year cohorts, there was a density-dependent reduction in offspring numbers. Under controlled laboratory conditions, the mean rate of offspring accumulation during the first 6 months of development was significantly reduced only in burdens in excess of those found naturally (36-68 worms). In older parasites, a density-dependent reduction in the mean accumulation rate occurred in burdens of 20-32 worms, where 88% of parasites produced offspring at less than half the maximum rate. The asymmetry in individual reproductive output increased with competition and there were significant reductions in the median rates of offspring accumulation at smaller burdens (17-25 and 5-14 worms, in experimental and natural infections, respectively) than evident from decreases in the mean. Thus, individual variation is a more sensitive measure of competition than is the mean.