Invasion Of The Body Snatchers

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Published on: Jun. 2, 2002

Last revision: Nov. 12, 2010

won't see them without the aid of a microscope. Some parasitic worms are also microscopic and not likely to be seen. The larger parasitic worms often live within the internal organs, so you'll probably never see them, either. Instead, let's focus on life cycles of some of the parasites that Missouri anglers are most likely to encounter.

Trematodes (flukes)

Species in one group of flatworms are often called flukes. Another name for this group is trematode. They are usually short and small, although a few can be seen with the naked eye. You may have noticed small black spots on the outside of the fish you were cleaning for your frypan. Or, as you prepared a fillet, you might have noticed yellow worm-like structures embedded in the flesh of the fish. Perhaps you've noticed a fish liver that was covered with small white flecks. These are all stages in the life cycle of different species of flukes or trematodes.

These are not adult parasites. They are immature forms called larvae. They mature to the adult stage when the fish host they inhabit is eaten by some predator, such as a larger fish, fish-eating bird or carnivorous mammal. They do not mature to adult parasites in humans. If you eat these stages (even raw) they won't hurt you.

The life cycles of the many types (species) of flukes are fairly similar. An adult parasite in a predator animal matures and produces small eggs that pass to the outside of the host. When a microscopic egg reaches a body of water, the small parasite within the egg is either ingested by an aquatic snail or hatches and then finds a snail for a host. One or more additional parasite stages develop within the body of the aquatic snail.

After the parasites develop, they leave the snail and either penetrate the skin of a fish or are eaten by the fish as they swim through the water.

When these larval stages infect a small fish, the parasite larvae can survive as long as the fish does. As the fish gets older, it is exposed to more and more parasites which all co-exist in the fish until a predator eats it. This is why larger fish tend to have larger numbers of parasites.

The "black spot"parasite has the same general life cycle, using snails and fish as intermediate hosts, but after leaving a snail

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