Prerequisites for parasitism in rhabditid nematodes.

To evaluate their potential for survival in a vertebrate host, dauer larvae from 7 species of rhabditid nematodes were subjected to in vitro conditions designed to emulate those of a vertebrate digestive tract. Dauer larvae from 3 of the 7 species, selected for their ability to survive elevated temperatures and low pH, and representing differing types of phoretic associations with invertebrate hosts, were fed to frogs to examine their ability to survive passage through a vertebrate digestive system. The degree of invasiveness of the phoretic association that dauer larvae had with their invertebrate hosts did not correspond to patterns of in vitro survivorship for any of the experimental conditions. When consumed with a prey item, dauer larvae from all 3 species were recovered from frogs 72 hr postexposure, and no differences for in vivo survivorship were observed among the 3 species. The contention that invasiveness or facultative parasitism within an invertebrate host is a beneficial or necessary step toward vertebrate parasitism by rhabditid nematodes was not supported by the survivorship data.