A = The trophozoites in the host's skin.
B = Trophont leaving the host.
C = The mature trophont with hundreds of maturing tomites.
D = The releasing of tomites that penetrate the skin of the host fish.
A = The cycle continues all over again.
Most fish keepers overlook the fact that there are a number of species of Ich -- how many not yet determined. Each species varies to some extent in its life history. There is considerable confusion in the literature on the subject. The visible stages of Ich are carried out on or in the surface of the host fishes' skin and fins. This first stage or stages of the life cycle which are the most visible to observers are the white spot cysts that are called trophozoites and are highly resistant to drug therapy. In sufficient numbers they harm the fishes to which they are attached to the extent that they can kill them.
Trophozoites mature into trophonts and leave the host, normally for most known species of Ich proceeding to the bottom of the aquarium or pond. These mature trophonts release from 200 to 1,000 "young" for freshwater species called tomites, and up to 250 or more tomites for tropical marine species. These tomites move about looking for a fish host, which they must find, depending upon which species they are, within 4 or 5 hours up to 2 to 3 days at 75° F (24° C) -- the time involved depending upon the Ich species. If they do not find a new host, most or all will die. A few may survive by attaching to the surface of an invertebrate or plant, but this happens only exceptionally. Cooler water temperatures will lengthen the time that the trophonts have to find a host. It is during this free swimming stage that the Ich are most vulnerable to treatment.
It is important to note that these intermediate stages may also attach themselves to plants, or onto flat surfaces of aquatic invertebrates (snails, coral, anemones, etc.), and be accidentally introduced into an aquarium or pond along with new plants or invertebrates. Once the tomite attaches to the host, it matures, forms the white spot cyst becoming trophozoites, and the cycle continues anew.
HTH
Mike
