A fisherman from Hawaii wrote the following:
This Fat juicy worm lives inside of every Wahoo that I catch. Inside the stomach. Right at the junction to the intestines. Sometimes there are two.
What is this parasite?
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It is Giant Stomach Worm (Hirudinella Ventricosa).
It is a platyhelmith parasitic to Wahoo fish, feeding on its blood and found inside the stomach. It makes 98% of Wahoo's parasitic infections.
It is a fleshy worm that varies in size and shape; with extended worms the size of a mans finger and contracted ones about the size of a walnut.
It requires two hosts to complete its lifecycle; a snail and a wahoo fish.
A picture from National Geographic Society
Source: lahaina news
Note: A youtube video shows this worm moving.