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Black Spot (Crassiphiala bulboglossa) in Perch and Pickerel

Volume 2, Issue 10
September 2000
Updated November 2002

Sand grain sized (sometimes larger) black spots on the skin of yellow perch (Perca flavenscens) and chain pickerel (Esox niger) are usually caused by a small immature larval trematode parasite named Crassiphiala bulboglossa. “Black grubs,” “Black spot disease,” or “Neascus infection” can be found in many different fish species, and is fairly common and widespread. The unsightly condition may be caused by several different species of parasites.

Life cycle of grub

The digenetic trematode parasite lives in three different host animals during it’s life. The final host for the adult trematode C. bulboglossa is naturally the Belted Kingfisher (Ceryl alcyon). Adult worms live in the bird’s mouth where they produce eggs. The eggs are swallowed, pass through the digestive system unharmed and are released into the water with the kingfisher’s feces. The eggs mature in water and release the ciliated miracidia, which then swim away and penetrate the appropriate molluscs (Helisoma sp.).

Black spots on dace

Figure 2. Black spot trematode in skin of blacknose dace.

These molluscs are the first intermediate hosts of the trematode.

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 with out 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 the final host to complete the trematode’s lifecycle. The worms are hermaphrodites having both male and female organs.

There is no practical treatment or control of this parasite available at this time.

Special points of interest:


C. bulboglossa does not infect humans, although a related species has been found in humans, several other mammals and birds.

Black grubs can be found in many different species of freshwater fishes.

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

Cooking fish kills the parasite.