Fish Parasites

2023 March 19

One of Rosie’s friends invited her out ice-fishing on February 20, 2023, and she came back with about a dozen bluegills that Sandy cooked up for dinner[1].

While she was filleting the fish, we noticed that there were black specks about the size of a pinhead scattered through the fishes’ flesh. Pretty much all of them had these specks, one of which is circled in the next picture.

These don’t show a lot of detail, even in a close-up image. Of course, it doesn’t help that we were looking at it through a thin sheet of muscle, but when we attempted to cut one out completely it just kind of turned to mush.

Anyway, these are really common, and we have seen them pretty regularly in fish for years. They are cysts of certain trematode worms in the genus Apophallus, and are commonly known as “black spot”. There is a similar parasite that is also commonly seen around here, where the cysts are yellow-white. And these are known, unsurprisingly, as “yellow grub”.

The Michigan State University Extension Service has a page that is intended primarily to reassure fishermen (what_are_those_spots_in_my_fish?) where they talk about both of these parasites. The immediate upshot is that these are harmless to humans, as mammals are not part of their life cycle. And even if they were able to infect humans, they are killed immediately when the fish are cooked.

The normal life cycle goes like this:

  1. Eggs hatch into a free-swimming larval stage until they find a snail to infest.
  2. The larvae grow through several development stages, and also clone themselves so that a single larva infesting a snail can multiply into hundreds or thousands of larvae.
  3. The larvae leave the snail and swim off looking for fish to infect. They burrow through the fish’s skin and into the muscle, where they form a cyst that does not seem to much affect the health or well-being of the fish. There they wait, until
  4. A kingfisher eats the fish. The cysts then convert into mature worms, infest the throat of the kingfisher, and commence laying eggs that wash out of the kingfisher’s mouth when it eats more fish or drinks. The eggs then hatch out and look for snails, bringing us back to Step 1.

The Yellow Grub parasites have a similar lifecycle, except that instead of their final host being kingfishers, they infest herons and bitterns.

While these particular parasites don’t infest humans or other mammals, there are some fish_parasites that can, so it is generally advisable to cook fish (particularly wild-caught fish) just to be sure.


[1] They were delicious.

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