Effects of conspecifics and heterospecifics on individual worm mass in four helminth species parasitic in fish.

Intraspecific and interspecific effects on the growth and body size of helminths are rarely studied in natural situations, yet knowing what determines helminth sizes and thus fecundity is crucial to our understanding of helminth ecology and epidemiology. The determinants of average individual worm mass were investigated in four common species of helminths parasitic in trout, Salmo trutta. In the acanthocephalan Echinorhynchus truttae, there was a negative relationship between the intensity of infection by conspecifics and average individual worm size. However, in the acanthocephalans Pomphorhynchus laevis and Acanthocephalus anguillae and in the cestode Cyathocephalus truncatus, the relationship was positive: individual worms were larger on average when co-occurring with many conspecifics than when co-occurring with very few. In addition, the average mass of individual C. truncatus in a host decreased as the total mass of other helminth species in the same host increased. This interspecific effect involves the whole helminth community, as the combined effect of all other helminth species is a better predictor of reduced mass in C. truncatus than the mass of any other species taken on its own. These results illustrate the importance of considering helminth interactions and helminth growth in a natural setting.