Parasites of one form or another are everywhere on the planet. This is is particularly true in the oceans of the world and therefore it holds true in a saltwater aquarium. One of the most prevalent parasites in the saltwater aquarium world is Cryptocaryon irritans (Cryptocaryon) which is a ciliated, protozoan parasite that causes a disease known as marine "ich" or marine "white spot" disease in both wild and aquacultured marine fishes.
Cryptocryon infects many different species of fish but seems to be more prevalent in certain species.
Crytocaryon irritans has four different stages in its life. Theronts is the stage in which free swimming tomites, which have been released from their cysts which were residing on the ocean floor (or aquarium substrate), attach to the skin or in the gills of the host fish. These tomites burrow into the skin or gills and encase themselves in a protective cyst where they feed on the fish's body fluids and cells. The tomites can survive without a host at this stage for between ten and eleven days. At this point, many of the Top Ich & Other External Fish Parasite Treatments have been found to be effective treatments.
Tomites actively feeding at this point are in the trophont stage. In controlled experiments, the parasite has remained in the fish's gills or just under the skin for 4-5 months at reduced temperatures (12°C (53.6°F), then developed and infected other fish when the water temperature was raised to 27°C (80.6°F).
The tomites, while encased inside the fish's gills or under the skin, are next to impossible to treat effectively with chemicals, however, freshwater dips have been known to "explode" the tomites while "on" the fish's skin before it has the opportunity to burrow into the fish's skin or gills. Hyposalinity has also been shown to be an effective treatment.
The trophont feeds on the body fluids and cells of the fish for about 3–7 days before leaving the host. It has also been found that trophonts will actively leave fish that have died, however, are not immediately able to infect other fish at this point as they require additional time to develop from protomonts (released tomites) to tomonts, just as they would if they had left a live host.
At this point, the tomite moves to the substrate for 2 to 18 hours where it sticks to the surface and encysts, whereupon it becomes a tomont At this point, before it completes encystment (8 to 12 hours), it is open to effective treatment. Once the cyst has completely formed the encased tomont is impervious to treatment.
The encysted tomont divides many times at this point, producing numerous daughter (approximately 100 to 1000) tomites. These tomites are released as theronts, the free-swimming stage at which they can infect other fish. The theronts may take up to 72 days to be released into the water with most being released between 5 and 13 days after formation. In laboratory studies, Yoshinaga and Dickerson (1994) observed, that theronts were released only between the hours of 2:00 am and 9:00 am, even in total darkness; some suggest this strategy increases the chance for theronts to find a host, as many fish may be resting or closer to substrate during this time period.
After release, the theronts actively seek a fish host and can survive at this point for 6 to 8 hours. During this time, the theront is most susceptible to treatment.
Once the theront locates a host it only takes 5 minutes for it to burrow into the skin while during gill invasion, the parasite can become enclosed by a thin layer of cells within 20–30 minutes. At this point, the cycle starts all over again.
Once the parasites have left the host, bacterial infections are quite common at the sites where the parasites have entered and departed the fish's skin and gills.