Intra- and interspecific density-dependent effects on growth in helminth parasites of the cormorant, Phalacrocorax carbo sinensis

The action of intra- and interspecific competition, mediated by density-dependent effects on growth, was investigated among the three helminth species found in the alimentary tract of 104 cormorants, Phalacrocorax carbo sinensis. Intraspecific density-dependent effects on worm sizes were observed in the abundant nematode Contracaecum rudolphii and, to a lesser extent, in the acanthocephalan Southwellina hispida. These were expressed as either negative correlations between mean worm size and intensity of infection, or positive correlations between the extent of variability in worm sizes and intensity of infection. There was also clear evidence of some form of negative interaction between the nematode Syncuaria squamata and the acanthocephalan S. hispida from two fronts. First, there was a strong negative correlation between the intensities of infection of the two species across hosts. Second, sizes of female S. hispida became more variable and smaller on average as the number of S. squamata per host increased. This interspecific density-dependent effect on growth was apparently asymmetrical, as sizes of S. squamata were not influenced by the presence or density of S. hispida. These results illustrate the complex nature of density-dependence in helminth growth, and how its effects can act both within and among species.