Interspecific and Intraspecific Competition among Pinworms in the Hindgut of Periplaneta americana

Abstract

To determine the relative importance of inter- and intraspecific factors in regulating number and fecundity of pinworms in the hindgut of Periplaneta americana, repeated samples of roaches, each consisting of an adult male, an adult female, a late instar, and an early instar, were collected from each of 2 colonies, 1 colony (mixed) infected with Thelastoma sp., Hammerschmidtiella diesingi, and Leidynema appendiculatum and the other (isolate) infected with only the last-named species. Roaches were dissected, volume of hindgut estimated, and the number of female worms of each species was recorded. Female worms were dissected and the number of uterine eggs was counted. Gut volume was a significant predictor of numbers of all species except H. diesingi, numbers of which were correlated positively with those of its 2 interspecific associates. There was little evidence of interspecific competition within the host. In the mixed colony, L. appendiculatum occurred with Thelastoma sp. less often than expected, but this was the only negative interspecific interaction observed. Fecundity was not significantly correlated with numbers of interspecifics but was strongly and negatively cor- related with numbers of intraspecifics in all species. Leidynema appendiculatum occurred at about the same intensity in isolate and mixed colonies. These results suggest that the number of each species is independently regulated. Typically this occurs through nonoverlapping niches, but all evidence points to there being broad niche overlap in the species. Unpublished findings suggest that L. appendiculatum and Thelastoma sp. limit their own numbers by intraspecific interference, a phenomenon reported in several other pinworms. Reduced fecundity is presumably a sublethal manifestation of this interference. We propose that intraspecific interference holds numbers of each species below carrying capacity and permits their coexistence despite broad niche overlap. Prevalence was below 50% in the mixed colony and was 100% in the isolate colony. This suggests that the community in the mixed colony is the result of a 2-stage process where interspecific effects play an important role in establishment of a species but where intraspecific effects dominate the in-host environment.