Copulation and sexual congress of Leptorhynchoides thecatus (Acanthocephala)

Pairs of the acanthocephalan Leptorhynchoides thecatus from laboratory infections were observed copulating at 21 days, 5 wk, and 12 wk postinfection (PI) in green sunfish, Lepomis cyanellus. Additionally, copulating pairs of worms from natural infections were observed. In each instance of paired males and females, the male was in 1 pyloric cecum of the fish and the female in another. Each had its posterior end protruding into the intestinal lumen at the point from which the ceca arise. The completely extruded bursa of the male fully enclosed the posterior end of the female in a firm attachment. In 1 instance at 5 wk PI, 2 males were observed in copula. One male had its copulatory bursa completely retracted, and the copulatory bursa of the other male was positioned just as though the mate were a female. Based on these and previous findings, it is concluded that male acanthocephalans mate indiscriminately and often throughout their lives. Habitation of ceca maximizes sexual congress because individuals often are positioned with their posterior ends extending into the intestinal lumen within the small area from which ceca originate. Emigration to find a mate is unnecessary.