Dynamics of the Parasite Assemblage of Pimephales promelas in Nebraska

Present theory states that in species-poor assemblages interactions among species are negligible and assemblage structure is a result of host/parasite interactions. A corollary to this theory is that parasite species assemblages should behave as accumulations of multiple-kind items drawn from a population in which only the relative probability of drawing each kind varies. This study was designed to test this prediction by comparing descriptor values, including diversity indices, species density values, equitability, and prevalence to those obtained from data produced by a Monte Carlo simulation model. The parasite assemblage consisted of 7 parasite species or types found in the fathead minnow, Pimephales promelas. The data presented indicate that all of the assemblage descriptors are positively intercorrelated, as in the Monte Carlo simulations. Also, the field data are described best by a model in which a single population structuring mechanism operates, suggesting that aggregating mechanisms are environmental in origin and act on all parasite populations more or less uniformly. In addition, the P. promelas assemblage structure appears to be a function of current year streamflow, suggesting that abiotic factors influence parasite assemblage structure through effects on individual parasite species' probability of infection. A major question in parasite ecology concerns the interactive or noninteractive nature of parasite species assemblages (Holmes, 1961; Hair and Holmes, 1975; Price, 1980; Bush and Holmes, 1986; Hoberg, 1986; Kennedy et al., 1986). Proponents of the role of interaction in structuring parasite communities tend to study rich intestinal faunas and interpret their results in terms of interactions described for free-living communities. Those working with species-poor assemblages, however, tend to find little evidence of interactions in such assemblages (Kennedy et al., 1986; Janovy and Hardin, 1988). In speciespoor assemblages, interactions among species are considered negligible and assemblage structure is attributed to host/parasite interactions, in a manner consistent with the prediction of Price (1980, 1984). A corollary to this theory is that parasite species assemblages should behave as accumulations of multiple-kind items drawn from a population in which only the relative probability of drawing each kind varies. Janovy and Hardin (1987, 1988) studied a relatively impoverished assemblage of fish parasite species. The study described an assemblage of 7 parasites in the plains killifish Fundulus zebrinus. The samples were collected over a 5-yr period from a braided river in which streamflow fluctuated. The results from these 2 studies suggest that the abiotic conditions, primarily previous year's streamflow, influenced the probabilities of infection with the different parasite species. It has been proposed further that the F. zebrinus system behaves as a Monte Carlo model in which parasites are drawn at random from a supraassemblage (i.e., environment) of infective stages (Janovy et al., 1990). The present study is an attempt to test the predictions of Janovy and Hardin (1988) and Janovy et al. (1990) by extending the work to a different host species. The host species, the minnow Pimephales promelas, was chosen for study because it has a geographical distribution much broader, and an ecological distribution narrower, than that of plains killifish (Pflieger, 1975). The parasite system studied consisted of 7 parasite species or types that exhibited a variety of life cycles. These parasites were Trichodina sp. (Ciliophora: Peritrichia), gill surface; Myxosoma funduli Kudo, 1918 (Myxozoa: Myxosporea), gill tissue; Dactylogyrus bychowski Mizelle, 1937 (Monogenea), gill surface; Gyrodactylus hoffmani Welborn and Rogers, 1967 (Monogenea), body surface, mainly fins; Gyrodactylus sp. (Monogenea), gill surface; Neascus sp. (Digenea: Posthodiplostomum), eye; Neascus sp. (Digenea: Posthodiplostomum), body cavity. The eye and the body cavity of minnows present 2 very different habitats, therefore it is possible that the 2 Neascus forms have different effects on host biology. Because of this possibility, the 2 larval Received 28 May 1991; revised 6 May 1992; accepted 22 May 1992. * Present address: Department of Medical Microbiology and Immunology, University of WisconsinMadison, Madison, Wisconsin 53706. t Present address: Department of Veterinary Science, University of Wisconsin-Madison, Madison, Wisconsin 53706.