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Yellow Grub (Clinostomum complanatum)

Volume 2, Issue 6
September 2000
Updated November 2002

Clinostomum complanatum (Syn. C. marginatum) is a fairly common, unsightly yellow grub found on many different fish species. It can be located in the flesh, gills, operculum and just under the skin of virtually any species of North American freshwater fish. Infected fish exhibit large yellowish nodules. The yellow grub has a complicated lifecycle involving several developmental stages each infecting a different animal.

Life cycle of grub

The final host for the digenetic trematode C. complanatum is the Great Blue Heron (Ardea herodias). Adult worms live in the heron’s mouth where they produce eggs. The eggs are swallowed by the heron, pass through it’s digestive system unharmed and are released into the water with the heron’s feces. The eggs mature in water and release the ciliated miracidia, which then swim away and penetrate the appropriate molluscs (snails or less often clams), often only those of a certain genus. These molluscs are the first intermediate hosts of the trematode.

Yellow Grub

Fig. 2 Yellow grubs encysted in skin of yellow perch.


In the molluscs, the miracidium grows to become a redia (a saclike animal with a pharynx and a gut), which may produce either cercariae or sporocysts (sac-like animals without a pharynx or gut). The cercariae actively penetrate and migrate into the tissues of the second intermediate host, which is most often a fish. When a cercaria penetrates and migrates into the tissues of a fish, it causes obvious mechanical damage and hemorrhaging. The damage caused by one cercaria is negligible, but in greater numbers they may kill the fish. After the cercaria has localized and transformed into a metacercaria, little subsequent damage occurs, unless enough metacercariae accumulate that their collective mass interferes with the fish’s metabolism. The infected fish must be eaten by a blue heron to complete the trematode’s lifecycle. When the fish is eaten and digested by the heron, the metacercaria emerges from the fish flesh and migrates to the heron’s mouth where it matures to the adult egg producing worm. This completes its lifecycle. The worms are hermaphrodites having both male and female organs.

There is no practical treatment or control of this parasite.

Special points of interest:


C. complanatum does not infect humans. Although a related species has been found in humans and other animals in Asia.

C. complanatum can be found in many different species of freshwater fishes.

C. complanatum can live for 4 years in a fish.

Cooking fish kills the parasite.