Invasion Of The Body Snatchers

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Published on: Jun. 2, 2002

Last revision: Nov. 12, 2010

it penetrates the skin and scales of fish and encysts itself in the skin of the fish, appearing as black specks or spots on the fish's skin. When the fish is eaten by a fish-eating bird, such as a kingfisher, the life cycle starts anew.

Another of these parasites is called yellow grub (such as Clinostomum complanatum). You may occasionally see them in the fins or tail of a fish, but you're more likely to encounter the yellowish, worm-like larvae in the flesh of the fish when you fillet your catch. As an adult, the yellow grub lives in the mouth of fish-eating birds, such as great blue herons. The birds pass the eggs through their feces.

White spot flukes (such as Posthodiplostomum minimum) often encyst themselves in the liver, heart or other internal organs of fish. In some cases more than 50 percent of the liver tissue may be taken over by the parasite.

Cestodes (tapeworms)

Tapeworms-another type of flatworm-live as adults in the intestinal tract of fish and as larvae in the body cavity of fish. Sometimes both stages are found in the same fish. Anglers are more likely to notice the forms in the body cavity because they are more visible when cleaning a fish. You'd have to cut into the intestines to see the adults.

The bass tapeworm (Proteocephalus ambloplitis) uses a small aquatic crustacean called a copepod as the first host in its life cycle. Small fish (particularly sunfish and bass) eat a lot of copepods early in their lives, and many of those probably contain the early stages of the tapeworm.

Parasites in the copepod are activated when ingested by the fish. The parasite larvae then penetrate the intestine and move to the body cavity of the fish. Here, they encyst or migrate through various organs. In heavy infections, bass tapeworm larvae can actually destroy gonads and render a fish incapable of reproducing. As smaller fish are eaten by larger fish, the parasites mature to adults in the intestine of the large fish. The tapeworms produce eggs that pass out of the fish and begin a new life cycle.

Nematodes (roundworms)

Roundworms are generally long, tubular worms ranging in size from the diameter of thread to the diameter of pencil lead. Several species infect fish, but the most noticeable examples are probably large larval forms that are encysted in the body cavity of fish. These worms mature to the adult stage in a suitable fish, bird, turtle or mammal host. Smaller species normally live within the inside of the intestine. Occasionally they extend from the anus of the fish and can be seen by an angler.

Acanthocephalans (thorny-headed worms)

Acanthocephalan adults live inside the intestines of many species of fish. These worms actually do not have a head, but they do have a holdfast organ (called a proboscis) that has rows of hooks that allow these worms to maintain their position inside the intestinal tract. They probably do not cause substantial harm to their fish host except in extreme densities. Fish become infected by eating the intermediate host (usually an aquatic crustacean like an ostracod or amphipod), or by eating smaller fish that have encysted larval stages.

Crustacea (fish lice and anchor worms)

Ectoparasites are those that can be seen on the outside of the body of a fish. Often, these are a type of crustacean, more similar to crayfish than to any of the other parasites we have mentioned. The fish louse (Argulus spp.), a saucer-shaped animal larger than a fish scale, attacks various fish species. It uses two large sucking disks to hang on to the outside of the fish where it digests blood, mucous and epithelial cells.

Anchor worm (Lernaea spp.), another common crustacean parasite, has two pairs of horns at its anterior end that embed (or "anchor") into the host's flesh. The damage to the host scales and skin can be extensive and is often results in secondary infections caused by bacteria and fungi. Adult anchor worm females appear to have long forked tails, but these are actually mature egg sacs.

Both the fish louse and the anchor worm have free-swimming larval stages that can move from one fish to another. Sometimes larval ectoparasites infect small fish and then move to larger fish as they mature.

Parasitic infections in wild fish in reservoirs, ponds, streams, and rivers are usually self-limiting and cause little concern to fisheries managers. In hatcheries, however, parasitic infections can cause havoc because of high fish densities. Controlling infections in such situations often involves removing support for part of the complex life-cycle of the parasites. For example, it's possible to reduce fish parasites in ponds by removing some of the vegetation that snails require

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