Sandra B. Bay and Dr. Richard A. Heckmann, Zoology

Recently, a non-endemic species of fish, Salvelinus namaycush, was introduced into Yellowstone Lake. These lake trout feed on the native cutthroat trout, an important food source for higher predators in the area. During routine examinations of the lake trout, an abnormally high infection of intestinal nematodes was discovered. These intestinal nematodes are also found in the endemic lake trout in nearby Lewis Lake, but at a lower level of infection.

The intestinal nematodes from both lakes were examined to determine if each lake is infected with the same parasite. Fish were collected from both lakes with the assistance of United States National Park Service officials, necropsies were performed, and nematodes were removed from the intestinal tracts. One hundred percent of the fish recovered from Yellowstone lake harbored extremely high levels of infection (100+ nematodes per fish). However, in Lewis Lake, one-sixth of the fish had high levels of infection (50-100 worms per fish) and two-thirds of the fish did not contain any worms. The nematodes were then examined using light optics and electron optics.

The specimens collected for examination by light optics were first fixed in 4% buffered formalin. These samples were then stained using the Semichons Carmine stain and dehydrated in a graded ethanol series. The samples were cleared in xylene. Whole mounts were prepared and mounted on glass slides with permount. The slides were then examined using a light microscope.

Light microscopy yielded little information about the nematodes. There were no obvious morphological differences between the two groups of parasites. Characteristics such as length of the esophagus, the number of spicules, the appearance of the tegument, and the presence and number of papillae were examined and no significant differences were found.

Samples were also prepared for electron optics examination. These samples were fixed in 2% Na+ cacodylate buffered glutaraldehyde, then post-fixed and stained in osmium tetroxide and uranyl acetate. The specimens were then dehydrated in a graded ethanol series and critical point dried in liquid carbon dioxide. The samples were placed on aluminum stubs, sputter coated with gold and examined on a scanning electron microscope at 15 kV.

Several morphological differences were observed using electron microscopy. We observed that the nematodes recovered from Yellowstone lake consistently had approximately 20 more teeth than the nematodes taken from Lewis Lake. The Yellowstone lake Lake Trout had on average 58 teeth on each side the of the mouth, while the Lewis lake Lake Trout had an average of 48 teeth per side. Other differences were also observed between the oral orifices of the parasites. The nematodes taken from Yellowstone lake possessed a sub-dental ridge, similar in appearance to the human gum-line. However, the nematodes found in Lewis Lake did not have any sort of subdental ridge in any of the samples we recovered. There were also consistent differences between the papillae of the two groups of nematodes. On the parasites taken from Yellowstone 5 Lake, the integument curved around the papillae. Integument contour lines were observed encircling the papillae. The area surrounding the papillae was raised, similar to a bagel or donut. However, the papillae on the nematodes taken from Lewis Lake were very different. There were no contour lines surrounding the papillae. The area surrounding them was flat and the papillae appeared to extend directly through the integument.

The papillae and teeth are important characteristics when determining the species of nematodes and could possibly indicate that we are dealing with two separate species of nematodes. However, these morphological differences are not conclusive. At this point, we are uncertain whether our observations represent a new species of nematode or merely morphological variation within a population. Further studies must be done. However, the stark morphological contrasts between populations are of note. DNA analysis and further study is planned.

Papillae on nematodes taken from Lewis Lake Papillae on nematodes taken from Yellowstone Lake

References

1. Bykhovskaya, 1. 1964. Key to parasites of fresh water fish of the U.S.S.R. 656-661.
2. Moravee, F. 1994. Parasitic Nematodes of fresh water fishes of Europe. 1: 182-195.
3. Postek, M., Howard, D. Etal. Scanning Electron Microscopy: a student’s handbook.
4. Yamaguti, S. 1961. Systemae Helminthum. 3: 47-48. 6