Ichthyophthirius multifiliis is a freshwater ciliated protozoan (a "preanimal" that has many threadlike formations on its external epithelium or skin that enable it to move) that infects the skin and gills of freshwater fish. It is commonly known as "whitespot" or "ich" for short and consists of tiny, barely visible specks, the "trophont" phase of the protozoan embedded in a cyst and feeding upon the external mucous of the fish's epithelium. They interfere with gas exchange via the fish's gills therefore this parasitic protozoan will weaken the fish to death if left untreated.
The trophonts are the adult parasitic phase. They break free from the epithelium when mature to form "tomonts" which fall to the bottom of the tank encased in the cyst. They undergo asexual division and will eventually rupture and burst, expelling thousands of microscopic free-swimming theronts which seek a host fish to feed on. These theronts will die after 48hrs or so if a vertebrate host is not found.
What you are referring to is
Cryptocaryon irritans the saltwater "version" of the FW "ich". It is very similar but there are subtle and important differences. The lifecycle is basically the same but a thing to remember is that the encysted stage can last for 4 weeks+ so just because the theronts die within 24-48hrs does not mean you are anyway in the clear. A huge advantage of the SW variant is that it cannot tolerate osmotic changes as well as a vertebrate (i.e. a fish) can. Treating with a treatment of hyposalinity will make life very difficult for these parasites as they sruggle to adjust their internal solutions to become less concentrated. The process of osmosis stipulates that these saltwater protozoans, when placed in a less concentrated solution than exists in their own internal fluid, will begin to have their fluids drawn out of them. Hence, a FW dip may be very beneficial to defeating SW "ich". Of course copper dosage is the only clear-cut cure that will definitely work, but that cant be used in the presence of LR or invertebrates.
Here is a pic of a badly infested Regal tang:
Thousands of these guys do the damage:
Hope this helps,
Ciaran.
