Parasites

June 2009

“For most creatures being parasitized is a way of life” writes Annie Dillard in A Pilgrim at Tinker Creek. She later notes that an estimated ten percent of the world’s animal species are parasites. Lake Champlain is not immune.

Parasites, by definition, have an extended relationship with their host during which time they feed from the host, but in most cases they do not kill the host. Under this definition, mosquitoes or black flies do not count as parasites because they have a fleeting relationship with their food source. Of course, the exact line between an “extended relationship” and a “fleeting relationship” is fuzzy.

Most studies of parasites in freshwater systems focus on those that affect fish. Wildlife ecologist David Marcogliese estimates there are over 700 known parasites of Canadian fish, not including bacteria and viruses. The numbers for Lake Champlain are probably not much lower. These range from the very large and well-known sea lamprey to more cryptic mussels, worms, and fungi. 

Mussels are perhaps the most benign parasites in this grouping but they do use fish as a means of travelling. If mussels only colonized new habitats by crawling from one to another, their mobility would be severely diminished. Instead, many mussels produce larvae (called glochidia) that attach to the gills of fish and hitchhike. Different mussel species vary in the degree of specificity of host fish for their glochidia. Some depend upon a single species while others are generalists. Only rarely are glochidia abundant enough to cause real harm to their hosts.

Many different types of worms can act as aquatic parasites including segmented worms, and flatworms. Leeches, segmented worms related to earthworms, sit near the border between an “extended” and “fleeting” relationship with their hosts. They act like smaller version of sea lamprey, attaching to the outside of a host for a period of time to live off bodily fluids. Of course, fish also feed on leeches, making them popular bait. 

Flatworms, including tapeworms and flukes, have a much more complicated life cycle often depending upon three or more hosts within a lifetime. Most of these parasites live inside their hosts. Different species or life stages will migrate to different organs including the liver, intestines, eyes, or under the skin depending on the species.

Once while wading in a slow-moving stream, I watched an undulating string about two feet in length moving with some purpose toward me. It was a horsehair worm a group that lives as adults in the water, but parasitizes terrestrial insects. They got their name because they would often be found in horse troughs after insects also fell in, and it was thought the hairs of the horses had come alive. These creatures are harmless to people, pets and livestock, but are deadly parasites of crickets, grasshoppers, and some beetles. The eggs of the worms get into the insect through as yet unknown means. Once there, they hatch and grow until they are ready to reenter the water. At that time they induce thirst in their victims causing them to return to the water where the worms can complete their lifecycle.

Fungi present a threat to many fish species. Natural history writer Adrian Forsyth notes “one sees many a bluegill drifting around … barely alive, enveloped in a hideous, cloudy white shroud of Saprolegnia fungi.” Additionally, male bluegills expend tremendous energy waving their tails over nests to prevent the growth of egg-eating fungi. Aquatic fungi were the subject of the first ever definitive report of a fungal disease in any vertebrate according to biologist Nicholas Money. He cites William Arderon’s 1748 description of “minute tubes, filled with a brownish liquor” radiating from a dead fish. Today the woes of fungal infection are well known to anyone who has suffered from athlete’s foot, and chytrid fungi are thought to be a leading factor in global declines of amphibian populations. 

Minnesota Sea Grant offers the following advice to prevent devastating infections in fish: “Anglers can help prevent infections by taking extra care when returning fish to the water. To protect the mucous coating, remove the hook while the fish is still in the water or wet your hands before handling the fish. Release the fish gently and as quickly as possible after the hook is removed.”