Distribution of helminths in the digestive tracts of New Zealand freshwater eels

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Abstract
Pairs of helminths from the alimentary tracts of Anguilla dieffenbachii and A. australis were tested for association. After the variable of host size is eliminated from the data, 3 significant associations were indicated. A positive association between the digeneans Stegodexamene anguillae and Telogaster opisthorchis in both eel hosts is attributed to the use of the same intermediate hosts, resulting in concurrent infections at all stages of the life cycles. Competition in the eel host is reduced by selective site segregation, modified by interactive site segregation. A strong, negative association between the nematodes Spirocamallanus sp. and Paraquimperia sp. in A. dieffenbachii is proposed as the reason for the relative occurrence of each nematode in each host. Paraquimperia infects both hosts, but is rapidly excluded if Spirocamallanus enters the host (usually A. dieffenbachii). As Spirocamallanus infections decline in older hosts, Paraquimperia may establish again. There was no apparent segregation of the nematodes to reduce competition and avoid competitive exclusion. The cause of an apparent negative association between the flukes Stegodexamene anguillae and Coitocaecum anaspidis is unknown, but may be fortuitous.

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