While Black "Ich" is not all that common, it is not uncommon either and appears from time to time, particularly on tangs and surgeonfish.
Black "Ich" is caused by one or more genus of flatworm (Phylum Platyhelminthes) belonging to the class Turbellaria. Most cases of black "Ich" appear to be caused by Paravortex sp. (Noga, 2000) however, Cannon and Lester (1988) found turbellaria of the genus Ichthyophaga on a parrotfish (Scarus ribulatus) in addition to Paravortex sp. on dusky flathead (Platycephalus fuscus).
Noga (2000) describes the life cycle of the parasite as follows. Juvenile turbellaria attach to the host fish and grow from around 77 to 450 μm. After around 6 days, they drop off the host and fall to the substrate where their eggs and larvae develop internally for 3 to 4 days. The adult's body wall ruptures and up to 160 juveniles escape and can immediately infect any susceptible hosts. The life cycle takes around 10 days at 24.5°C (76°F).
The most obvious sign of black "Ich" are numerous dark spots covering the body of the infected fish. The dark spots are not the turbellaria but rather a reaction of the skin to the parasite which causes hypermelanisation (Noga, 2000).
Photograph of yellow tang with black "Ich" infection.
It should be noted that anemonefish may develop black spots on their skin through contact with corals, corallimorphs, zoanthids and anemones with which they are not familiar. These spots tend to be larger and less uniform than those from turbellaria.
Noga (2002) recommends formalin baths for the treatment of parasitic turbellaria. The formalin baths should be repeated every three days until there are no signs of infection. With this regime subsequent baths will kill parasites that were not on the fish during previous baths.
Some hobbyists have had success with the use of freshwater dips. These should also be repeated at 3 day intervals.
References
Cannon L.R.G. and Lester R.J.G. 1988. Two turbellarians parasitic in fish. Dis. Aquat. Org. 5:12-22.
Noga E.J. 2000. Fish Disease: Diagnosis and Treatment. Iowa State University Press, . 367pp.
Last updated: May 8, 2004