Cryptocaryon irritans is a species of ciliate protozoa that parasitizes marine fish, and is one of the most common causes of disease in marine aquaria. The symptoms and life-cycle are generally similar to those of Ichthyophthirius in freshwater fish, including white spots, on account of which Cryptocaryon is usually called marine ich. However, Cryptocaryon can spend a much longer time encysted.
Infections can be extremely difficult to treat because of other creatures, such as corals and other invertebrates, which will not survive standard treatments. Ideally fish with Cryptocaryon are quarantined in a hospital tank, where they can be treated with a copper salt or using hyposalinity. The display tank needs to be kept clear of fish for 6-9 weeks, the longer the better. This gives time for the encysted tomonts to release infectious theronts, which die within 24-48 hours when they cannot find a host.
Cryptocaryon irritans was originally classified as Ichthyophthirius marinus, but it is not closely related to the other species. It belongs to the class Prostomatea, but beyond that its placement is still uncertain.
Useful treatments of Cryptocaryon irritans are copper solutions, formalin solutions, and quinine based drugs (such as Chloroquine Phosphate and Quinine Diphosphate).