Whenever you’ve caught a fish that looks as if it had already been sprinkled with freshly ground pepper, you’ve encountered a fish disease commonly called “Black Spot.” The cause is Uvulifer amploplites, a type of worm known as a trematode. While the symptomatic spots look mundane, they represent just one life stage in a very complicated parasitic creature.
Adult trematodes inhabit the digestive tracts of fish-eating birds like herons and kingfishers. They produce eggs that pass into the water as the bird defecates. The eggs hatch, liberating larvae called miracidiae that swim about until they find a snail to act as their next host.
The larvae burrow into the snail and form “mother sporocysts,” which then produce many sporocysts. This life stage then burrows into the snail’s liver, where they remain for about 6 weeks. At that point, a further metamorphosis occurs, as tiny cercariae emerge from the liver and return to the water. Here they swim about until they encounter a fish, which they dig into and form the familiar black cysts in the fish’s skin and flesh.
The life cycle is completed when a bird eats an infected fish, the cysts rupture, and the parasite is freed to mature in the bird’s gut and produce more eggs. Though some consider the spots unappetizing properly cooked fish will not result in a problem for humans.
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