In most wahoo (ono) you fillet, you will find the giant digenetic trematode (Hirudinella ventricosa), a parasitic, worm-like liver fluke in the flatworm group that inhabit their stomach. These stomach parasites have been observed worldwide in tropical and subtropical waters.
In an examination of wahoo stomachs in the Hawaiian Islands, scientists found a 98 percent infection of these parasites. Size and sex of the host had no significant effect on parasitic infestation.
Wahoo is the preferred host of this parasite, both by being almost always present and by achieving a consistently large size in this host. It is a characteristic parasite of wahoo.
Hirudinella ventricosa are found in the stomachs of ono in Hawaii and around the world.
They are medium to dark brown to pinkish in color and slug-like in texture and appearance. Extended worms are usually about the size and shape of a human thumb, and contracted ones about the size of a walnut. Smaller worms are still massive but capable of contorting into many shapes.
There are usually two or three in each wahoo stomach, but there can be a few more or a few less. Occasionally there are none at all, and very occasionally there are many more, up to 12 or so.
Cut open the stomach of a wahoo, and in addition to examining what the fish has been feeding on, you can check for the presence of trematodes. In most cases, they will still be alive and moving very sluggishly.
Trematodes have two suckers, which you will probably notice on the worm. One near the mouth for latching onto tissue in its host animal, and the other, usually in the middle for feeding, holding and moving about - both on the same side. They are easily seen and are close together on the anterior end of the worm.
The trematode feeds on the wahoo's blood, but the weird thing is that although the fish can get stomach ulcers from it, the fish doesn't seem to be too terribly harmed by it. If the trematodes were found in greater numbers in each fish, the damage to the fish would be much greater. I don't know why there are usually just the pair.
Studies show that the worms cause wounds by penetrating the stomach lining to feed on blood. They absorb a considerable amount of blood.
Researchers could not be certain that the wounds were not caused after the death of the host.
Some mechanism of the parasites or the hosts appears to regulate both the numbers and sizes. The worm does not affect the portion of the fish eaten by humans.