Additional cases of predation on horsehair worms (Phylum Nematomorpha), with a recent record for Missouri.

Abstract: We recovered horsehair worms (Gordius difficilis and G. robustus) from the digestive tracts of two fish species, longear sunfish (Lepomis megalotis) and brown trout (Salmo trutta), from seven streams in Missouri and Minnesota. Horsehair worms occurred in more than one fish at one site, and in two streams in more than one year. Horsehair worms have not been recently documented from Missouri, and they have not been previously reported from Fillmore County, Minnesota.

Key Words: Gordius difficilis, Gordius robustus, Phylum Nematomorpha, Lepomis megalotis, Salmo trutta, Centrarchidae, Salmonidae, Minnesota, Missouri

Introduction

Horsehair worms (Phylum Nematomorpha) are most commonly encountered during their free-living adult phase, when they occur in a variety of freshwater habitats as elongate, slender, slow-moving worms (Poinar 2001, Smith 2001). Sometimes they are found in tangled aggregations of many individuals. Adult horsehair worms do not feed, but immature individuals are internal parasites of terrestrial or aquatic invertebrates, depending on the species.

Horsehair worms as a group are widely distributed but understudied. Many details of the geographic ranges and life histories of individual species remain to be elucidated (Poinar 2001; Smith 2001). Recently, Cochran et al. (1999) and Poinar (2001) reported several cases of predation on adult horsehair worms, and Cochran et al. (1999) summarized previous anecdotal accounts in the literature. In addition, Bolek and Coggins (2002) removed three Gordius difficilis from the intestines of two green frogs (Rana clamitans), but they did not indicate whether they thought that the worms were preyed upon directly or had been present initially in other invertebrates consumed by the frogs. Most observations of predation on horsehair worms involved their consumption by freshwater fish, but horsehair worms have been reported in the diets of only a small proportion of fish populations and typically make up at most a trace component of the diet. Moreover, most individual fish offered horsehair worms in laboratory feeding tri als ignored or rejected them (Cochran etal. 1999).

The purpose of this note is to report additional cases of predation by fish on horsehair worms. These provide new insight into the potential importance of predation as a source of mortality in this group.

Methods

Evidence for predation on horsehair worms was obtained opportunistically subsequent to fish collections by seining or angling. Horsehair worms encountered as prey were preserved in 70% ethanol and archived in the invertebrate collection at the Milwaukee Public Museum. All fish lengths reported below are total lengths.

Identification of horsehair worms to species can be difficult (Poinar 2001, Smith 2001). Cochran et al. (1999) identified horsehair worms from several sites in Wisconsin and Minnesota as Gordius robustus, the only species in the genus previously reported from the Midwest (Chandler 1985, Watermolen and Haen 1994), on the basis of the presence in males of a bibbed posterior and a post-cloacal crescent. Subsequently it was shown that another species of Gordius also inhabits the Midwest. Gordius difficilis was originally described as G. aquaticus difficilis from a single North Carolina male by Montgomery (1898), but he later synonymized the species with G. robustus (1907). Smith (1994) used a series of 16 specimens from Massachusetts to resurrect G. difficilis. Hanelt and Janovy (2000) and Bolek and Coggins (2002) reported G. difficilis from Nebraska and Wisconsin, respectively, and examination of specimens from the sites in Wisconsin and Minnesota where Cochran et al. (1999) reported G. robustus has revealed th at they were G. difficilis (Bolek, personal communication; Cochran, in review). For this study, G. difficilis was distinguished from G. robustus by its lack of a dark ring behind the calotte (the anterior tip of the body), its paler white or off-white coloration (G. robustus tends to be reddish brown), its more slender body, and, in males, the presence of a parabolic fringe of hairlike processes anterior to the cloaca.

Results

We found horsehair worms in the digestive tracts of two fish species at seven locations in Missouri and Minnesota:

(1) A longear sunfish (Lepomis megalotis), 13.2 cm long, was collected on 7 March 2000 by seining in the North Fork White River at Highway 14/181, Douglas County, Missouri. It had approximately 6 cm of a horsehair worm protruding from its anus; a smaller separate segment that included the anterior end of a worm was found in the colon. Unfortunately, the diagnostic posterior end of the worm was unavailable, but the worm was similar in color (reddish brown), relative thickness, and overall appearance to a female horsehair worm with an unlobed posterior discovered entwined with an elongate egg mass on the previous day in the Strawberry River at Highway 289, Izard County, Arkansas. The latter worm is tentatively identified as a G. robustus, a species previously known from Arkansas (Chandler 1985).

(2) Horsehair worms were found in three of five brown trout (Satmo trutta) collected by angling on 20 June 1999 in Gilmore Creek on the St. Mary's University campus, Winona County, Minnesota. Worms occurred in two of the three smallest trout (20-25 cm long), with three worms in one fish and one in the other. The largest trout (39 cm) contained a single worm in addition to a house mouse (Cochran and Cochran 1999). The only worm saved for preservation was a female G. difficilis. A trout containing horsehair worms was collected at this site in 1997 (Cochran et al. 1999).

(3) A female G. difficilis was found in one of four brown trout (28-30 cm long) collected by angling on 20 July 2000 in Garvin Brook, Winona County, Minnesota. The fish that contained the worm was 29 cm long. This sample of trout was collected approximately 2 km downstream from a site where a trout that contained horsehair worms was collected in 1995 (Cochran et al. 1999).

(4) A male G. difficilis was found in one of two brown trout (26-28 cm long) collected by angling on 18 June 2000 in an unnamed tributary to Diamond Creek, Fillmore County, Minnesota.

(5) Two male G. difficilis were found in one of three brown trout (20-35 cm long) collected by angling on 19 June 2000 in Gribben Creek along County Road 23, Fillmore County, Minnesota. The trout that contained the worms was 26 cm long and was one of two fish that contained slugs and insects of terrestrial origin. The latter were apparently washed into the creek by rain within the previous 24 hours.

(6) A male G. difficilis was found in one of ten brown trout (23-25 cm long) collected by angling on 24 June 2001 in Beaver Creek (Whitewater River basin), Winona County, Minnesota.

(7) Anglers D. Hoffman and N. Sitzman provided us with a horsehair worm removed from a trout collected on 4 September 2002 in the South Branch Whitewater River, Winona County, Minnesota, along with a living worm found at the same site. The posterior end of the worm from the trout was missing and the intact specimen was a female. However, both specimens displayed traits associated with G. robustus: reddish brown color, relatively large diameter, and a dark ring on the callotte.

Discussion

As in the present account, many previously reported cases of predation on horsehair worms involved fishes of the families Centrarchidae and Salmonidae (Cochran et al. 1999). Both sunfishes and trout tend to fall within the size range of predatory fishes that are small enough to retain invertebrates in their diets but large enough to consume horsehair worms. In addition, trout often occur naturally or are stocked into cold spring-fed streams. Recent collections by one of the authors (PAC) suggest that G. difficilis occurs frequently in cold spring runs or other spring-fed habitats.

Our results, along with those of McLennan and MacMillan (1984), suggest that predation on horsehair worms may occur in at least some habitats at frequencies greater than what is implied by the primarily anecdotal reports summarized by Cochran et al. (1999). Even though our individual sample sizes are small, all four samples of brown trout from southeastern Minnesota that we examined during the summer of 2000 included at least one fish that contained a horsehair worm. Moreover, we have found horsehair worms in more than one fish at the same locality, in fish from more than one locality in the same drainage collected within a short time, and in fish from the same streams (Gilmore Creek and Garvin Brook) collected in different years (Cochran et al. 1999 and this study). Our data provide additional support for the suggestion that predation by fish is a potentially important source of horsehair worm mortality even though horsehair worms are not known to be an important component of the diet of any fish species (Co chran et al. 1999).

At least some of the horsehair worms we recovered from trout came from fish that were collected shortly after significant rainfall. Increased discharge after rains may carry horsehair worms from springs and spring runs into streams where they are more exposed to trout at a time when the trout are predisposed to feed on relatively unfamiliar or atypical prey (e.g., terrestrial items).

Finally, our observations have provided new information on the geographical distribution of horsehair worms. In his review of horsehair worm distribution in the United States, Chandler (1985) included no records of any species of horsehair worm from Missouri. Riley (1877), however, reported G. robustus in locusts collected in Missouri, but he did not provide specific locality data. In Minnesota, no horsehair worms have been formally reported from Fillmore County, although Johnson et al. (1949) mentioned that "occasional hairworms (Gordiaceae)" were collected during their biological survey of the Root River basin, a drainage system that includes Diamond and Gribben creeks.

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