Abstract
BackgroundAt present, much research effort has been devoted to investigate overall (“average”) responses of parasite populations to specific factors, e.g. density-dependence in fecundity or mortality. However, studies on parasite populations usually pay little attention to individual variation (“inequality”) in reproductive success. A previous study on the acanthocephalan Corynosoma cetaceum in franciscana dolphins, Pontoporia blainvillei, revealed no overall intensity-dependent, or microhabitat effects, on mass and fecundity of worms. In this study, we investigated whether the same factors could influence mass inequalities for this species of acanthocephalan.MethodsA total of 10,138 specimens of C. cetaceum were collected from 10 franciscanas accidentally caught in Buenos Aires Province between 1988 and 1990. To investigate mass inequalities, all the specimens were sexed, and females were classified according to their developmental stage and weighed. Additionally, the relationship between biomass and fecundity (estimated as the number of acanthors) was investigated for some females. Inequalities in fecundity and biomass were assessed using standard methods, i.e. the Lorenz curve and the Gini coefficient (G).ResultsWe found a modest, but highly significant linear relationship between mass and fecundity. The G was very low (0.314) compared with that from other helminth species. G values were significantly lower in gravid females, which presumably exhibit a slow rate of growth. Also, G values significantly increased with total intensity, but only for gravid females, and the effect was more predictable considering only the intensity of gravid females.ConclusionsApparently, high intensity infections increase inequality of reproducing females without producing “crowding” effects. Although several processes could generate this pattern, gravid females, at higher intensities, expanded their distribution and occupied gut chambers with contrasting environmental conditions, which might result in greater variability in body size. The observed inequalities are not expected to strongly influence the population genetics of C. cetaceum, but they reveal subtle individual effects beyond an overall population impact.
Highlights
At present, much research effort has been devoted to investigate overall (“average”) responses of parasite populations to specific factors, e.g. density-dependence in fecundity or mortality
Population structure The intensity of C. cetaceum in the 10 franciscanas ranged between 99–2692 individuals (Additional file 2: Table S2), with a total of 10,138 individuals
Values are similar to those found in a helminth species with determinate growth that infect mammals, namely A. lumbricoides
Summary
Much research effort has been devoted to investigate overall (“average”) responses of parasite populations to specific factors, e.g. density-dependence in fecundity or mortality. Much research effort has been devoted to investigate overall (“average”) responses of parasite populations to specific factors, e.g. density-dependence. Keymer et al [15] investigated the combined effect of cystacanth dose and the concentration of a critical nutrient (fructose) on the average length of the acanthocephalan Moniliformis moniliformis infecting rats. These authors discovered significant ‘crowding’ effects, but failed to detect a significant interaction between cystacanth dose and nutrient concentration. Low fructose concentrations disproportionately increased inequalities at higher parasite doses
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