Individual variability in reproductive success of Triaenophorus crassus Forel (Cestoda: Pseudophyllidea), with comments on use of the Lorenz curve and Gini coefficient

Abstract

Maturity, mass, and lifetime fecundity were detemined in individual Triaenophorus crassus from spawning pike (Esox lucius) from Falcon Lake, Manitoba. Only 33% of worms contained eggs, and only 73% of these could release their eggs (up to 4 400 000 per worm). Inequalities in mass and fecundity were quantified using Lorenz curves and Gini coefficients calculated for worms pooled across hosts, and separately for worms in each host. Inequalities in mass were correlated with, but always lower than, corresponding inequalities in fecundity, although inequalities in fecundity and mass were more similar at high infection intensities. Consequently, inequalities in worm mass did not provide an accurate estimate of inequalities in reproductive success but could be used as an indirect index for comparative purposes. These inequalities can potentially modify parasite transmission. Ten percent of worms accounted for 85% of all parasite fecundity and, as a result of a contagious distribution of worms among pike and a lack of detectable intensity-dependent effects on parasite fecundity, 55% of worm eggs came from 10% of spawning pike. Worm numbers and distribution among hosts, maturity, and inequalities in mass and fecundity were consistent over 3–4 consecutive years, indicating that these inequalities are a normal component of the life cycle. The concentration of parasite fecundity in relatively few worms or hosts suggests that the population dynamics of T. crassus may be affected by short-term changes in parasite genotype, host activity patterns, or mortality of the few heavily infected hosts.