Introduction
Nematomorphans have a pseudocoelom (a body cavity incompletely lined with tissue of mesoderm origin). This type of body cavity is also found in nematodes, gastrotrichs and rotifers. Nematomorphs are commonly called horsehair worms or gordian worms (Plate 10.1). As the name horsehair would indicate, adult worms are slender and very long, 30 cm or more in length in some North American species. Sometimes, numerous specimens are collected tangled together; hence the name gordian worms, referring to the Gordian knot of Greek mythology. Except for a few atypical marine nematomorphs, they are freshwater in distribution. Although fairly abundant, there have been very few studies of North American horsehair worms and no specific studies of Alberta's fauna. One species, Gordius attoni Redlich, has been described from northern Saskatchewan and extending beyond the Arctic circle west into the Yukon (Redlich 1980). This species is undoubtedly one of perhaps several species found in Alberta.
Life Cycle
Adult horsehair worms are free-living in both standing and running water; whereas the larvae are parasitic in arthropods, especially grasshoppers and crickets. Adults overwinter in water. Male horsehair worms are much more active than females, and males are often seen in a serpentine swimming action near the substratum. Mating occurs in spring; females will then release fertilized eggs, which are extruded as long slender gelatinous strings. Eggs hatch into microscopic larval horsehair worms that do not resemble the adults Fig 8.1).
The larva will somehow enter an arthropod, usually a nonaquatic arthropod, such as a grasshopper or cricket. Some workers suggest the arthropod will eat the larval horsehair worm, while others maintain the larval horsehair worm will penetrate into the arthropod. Also, for some species of nematomorphs, the larva might be eaten by an aquatic arthropod in which it will encyst, but develops no further unless the aquatic arthropod containing the larva is eaten by the correct intermediate host, such as a grasshopper or cricket (Poinar and Doelman 1974). Within the hemocoel of the correct intermediate host, the larva will molt and eventually take on the appearance of the adult horsehair worm. A large horsehair worm takes up most of the available internal space of the arthropod. Usually in autumn the adult worm will emerge from the arthropod. Perhaps the worm will emerge and then the arthropod will die, or it might be that the arthropod dies and then the worm emerges. Horsehair worms have been considered as biological control agents of grasshoppers. But apparently the inability to culture the worms in large numbers is a major drawback for this. Adult free-living horsehair worms have a nonfunctional gut, and they do not feed. The immature worm in the hemocoel apparently absorbs nutrients through the cuticle.
Collecting, Identifying, Preserving
Horsehair worms occur sporadically in lakes and streams of various sizes. As indicated, they often occur tangled together in large numbers. In some years, almost every aquatic habitat of a particular area might have horsehair worms, but the next year perhaps none of these aquatic habitats of that area will have nematomorphs. This might be correlated with the abundance of the arthropod hosts. In southern Alberta, the cricket Anabrus often is a good source of horsehair worms. Pond-net sampling of the substratum and associated aquatic vegetation is a suitable method to collect the adults. The adults can be preserved directly in about 80% ethanol. To separate some nematomorphs, it is necessary to determine whether areoles (see NEMATOMORPHA pictorial key ) are prominent in the cuticle. A small piece of cuticle should be cut out of the worm and examined under the microscope. Soaking the cuticle in a strong detergent or glycerin for a few hours aids in clearing the cuticle.
Alberta's Fauna and Pictorial Key
The pictorial key is only to the family level and the key's diagnostic features are modified from keys of Chitwood (1959) and Pennak (1978). Gordius appears to be the most common genus of horsehair worm in Alberta, at least in central Alberta.
Some Taxa Not Reported From Alberta
Another group of "worm-shaped" freshwater invertebrates are members of the phylum Nemertea, which like triclads and microturbellarians are acoelomate. Nemertines, sometimes called proboscis worms or ribbon worms, are mainly marine. There are only a few freshwater nemertines in North America. Prostoma graecense (Bohmig) (= P. rubrum Leidy) and the closely related P. eilhardi (Montgomery) are both sporadically distributed in North America. A third species, P. canadensis (Gibson and Moore), has been reported from Parry Sound, Lake Huron, Ontario (Moore and Gibson 1985). Freshwater nemertines are found mainly in standing water with lots of vegetation. Mature worms are long, to about 30 mm. They can be readily separated from other "worms" (oligochaetes, nematodes, and nematomorphs) by the nemertine's long proboscis that is housed within the body, but when extended (during feeding) can be longer than the body. Another distinguishing feature is three pairs of eyes on the rounded anterior end of the worm. A recent review of the freshwater nemertine fauna of the world is Moore and Gibson (1985).
Pictorial Keys
