Invertebrate Biology 122(1): 12-18. zyxwvutsr 0 2003 American Microscopical Society, Inc. zyxwvutsrq Spanning the gap: experimental determination of paratenic host specificity of horsehair worms (Nematomorpha: Gordiida) Ben Hanelt” and John Janovy Jr. School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588-01 18 USA Abstract. Details of the life cycle of freshwater nematomorphs (gordiids) remain unclear. Free- living aquatic larval gordiids must make a critical transition from an epibenthic aquatic envi- ronment to terrestrial hosts. In order to identify potential hosts capable of bridging this eco- logical gap, the specificity of paratenic hosts of three common species of North American gordiids was investigated. All three species were characterized by an identical infection pattern: low host specificity. Gordiids were able to encyst within annelids, mollusks, crustaceans, insects and a vertebrate. Three species of putative host (a turbellarian, a water mite, and a mosquito larva) were not infected with any of the gordiid species. Internal defense reactions (IDR) and feeding behaviors are implicated as preventing infection in these species. Several of the other host species produced either an IDR or an immune reaction to the cysts, although reactions to the cysts were highly variable between species. In most species, IDR did not cause noticeable harm to the encysted larvae. It is proposed that although many species are easily infected with gordiid cysts, most do not act as natural paratenic hosts. For some of these host groups, es- pecially snails, a role as reservoir host is suggested. Of all hosts included in this study, aquatic insects were identified as the hosts likely responsible for spanning the ecological gap and acting as true hosts for gordiids. zyxwvutsr Additional key words: Nematomorpha, Gordian worms, horsehair worm The life cycle of freshwater nematomorphs or gor- diids remains one of the unsolved mysteries in inver- tebrate biology (Bodenheimer 1923; Schmidt-Rhaesa 2001). Gordiids are parasites as juveniles in terrestrial insects such as orthopterans and coleopterans. Once mature, the worms exit their hosts and assume a free- living existence in freshwater environments, including puddles, streams, rivers and lakes. In these aquatic en- vironments, worms produce egg strings in large num- bers. Hatching larvae are non-mobile (Poinar zyxwvu & Doel- man 1974), and usually settle to the bottom of the water column. The critical link in the life cycle is the transition from the epibenthic aquatic environment to their terrestrial hosts, but how the larval gordiids make this transition has been little studied. Direct oral transmission of the larvae to the defini- tive host has been demonstrated by injecting gordiid larvae into the abdomen of flies, and immediately feeding these to mantid definitive hosts (Inoue 1962). Author €or correspondence. Current address: School of Bi- ological Sciences, 220 Life Sciences Bldg., Louisiana State University, Baton Rouge, LA 70803 USA. E-mail: bhanelt@lsu.edu However, it is unlikely that many terrestrial insects come into direct contact with gordiid larvae. Other hosts must be responsible for bridging the gap between the free-living aquatic and parasitic terrestrial environ- ments of gordiids. In this paper, it is assumed that this “first host” serves only in the transmission of parasites and is not required for development, and will be re- ferred to as the paratenic host. Paratenic hosts are dc- fined as those that bridge a trophic gap between inter- mediate and definitive hosts, but within which no development occurs (Baer 195 1). The discovery that gordiids penetrate and encyst within aquatic animals was first noted in aquatic in- sects, and led to the idea that cysts were important in the transmission of gordiids to their definitive hosts. Over a century ago, it was proposed that gordiid larvae encysted within aquatic insects were transferred to the terrestrial definitive host by adult flies (Meissner 1856), and this proposal has been supported by em- pirical studies (Inoue 1962). The importance of the cyst stage has, however, not been further investigated. Early laboratory and field work on the significance of cysts in the life cycle of various species of European gordiids showed that they could infect molluscs, leech-