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Acanthurus japonicus ich 01.jpg
Acanthurus japonicus, Powder Brown Tang


Cryptocaryon irritans (commonly also referred to as White Spot, Crypto or Marine Ich) is a cilliated protozoan that parasitizes marine fish. It is one of the most common infectious disorders encountered in the marine hobby.

The name marine "ich" comes from the abbreviated name of a similar, but otherwise unrelated[1] protozoan infection that affects freshwater fish - Ichthyophthirius multifiliis.

The parasite does occur in nature, however the closed nature of aquarium systems means that infections can reach much higher densities in captivity than typically occur in the wild, and unless treated, may well result in the death of affected fish. Because of its prevalence in the aquaculture industry, the parasite has been well studied and is relatively well understood by science (but not always so well understood by many hobbyists)


Cryptocaryon is caused by a protozoan that has a four stage life cycle, only one of which involves fish. However, it must have fish present to complete the life cycle. Below 20°C reproduction of Cryptocaryon halts and it has been found that trophonts and tomonts can survive at 12.0 °C for 5 and 4 months, respectively, and reinfect fish[2].

The life cycle takes approximately 9 to 10 days at 24 to 25oC to complete[3] and is as follows[4]:

  1. Trophonts, parasitic stage, 3 to 7 days with peak at 4 to 5 days. Parasite attaches itself to the fish and burrows its way underneath the skin where it feeds on tissue debris and body fluids.
  2. Mature, disassociation stage, up to 18 hours. Upon maturing, the trophont leaves the host, sinks to the substrate and actively probes it for several hours.
  3. Tomonts, reproductive stage, 3 to 28 days with peak at 4 to 8 days. Mature stage then creates a cyst and starts to reproduce and divides several times.
  4. Theronts, infective stage, 24 to 48 hours. The cyst ruptures and releases the free swimming tomites (up to 200) which differentiate into theronts and then have to find a host within 2 days. Failure to find a suitable fish host ends in death of the theront.

The only stage that is susceptible to chemical treatment is the infective stage involving the tomites / theronts.

  • [1] - figure showing the A - trophont stage, B - frontal, C - lateral


Symptoms are typically presented as raised white cysts on the skin, gills, eyes and/or fins of affected fish. The trophonts are irritating to the fish, so they may also attempt to scratch on rocks. Severe infections will reduce the efficiency of the gills, so the fish will start breathing heavily to get enough oxygen.

See the gallery for a number of image of various infected fish to assist with identification.

Similar Symptoms


Any treatment of Cryptocaryon must take account of the life cycle of the parasite by either interupting the cycle or killing it within its only susceptible phase, tomites / theronts. Unless there is something specific to the treatment, optimally the treatment and observation period should cover at least the longest possible length of the life cycle (which has been recorded to stretch out to 38 days).

No effective treatments (with the exception of the three day transfer method) are considered "reef safe". All are toxic to invertebrates or other organisms common within a display tank (such as algae), and so must be conducted in a hospital tank.


The best cure for Cryptocaryon is prevention in the first place. All new fish introduced to the aquarium should be quarantined to prevent the introduction of this (and other) infections to an established tank.

Given the lifecycle of Cryptocaryon irritans is between 2-3 weeks, it is advisable to quarantine fish for around 4-6 weeks in order to allow time for at least 2 cycles, and ensure that the symptoms have time to become visibly evident if present. In addition to disease prevention, quarantine also gives the hobbyist the opportunity to observe the new fish, train it onto prepared foods in the absence of other competitive fish, and easily isolate it for hospital treatment if necessary.


Know effective treatments are listed below. It is recommended that unless there is some exceptional circumstances requiring some other treatment, one of the following should be used to treat fish that have an Cryptocaryon infection.

  • Hyposalinity - reduce salinity to 12-14 ppt (equivalent to an specific gravity of 1.009) for at least 4 weeks.
  • Chloroquine diphosphate or Quinine hydrochloride - non-toxic to most fish and bacteria, but highly toxic to algae and some invertebrates (anemones, corals etc.). Dosage rate is 10 milligrams per litre[5], every 5 days for at least 4 treatments. Works best in conjunction with hyposalinity (under 14ppt) [5].
  • Three Day Transfer Method - requires two suitable tanks between which fish are transferred, then tank cleaned, every three days for a total of 12 days.
  • Hydrogen Peroxide - bactericide and caution should be exercised, with treatment in a quarantine tank, as correct dosages for individual species is still in the development stages.
  • Copper - copper is toxic to fish as well as invertebrates, and level and duration of addition must be strictly controlled.
  • Formalin Bath - volatile, irritating and an suspected carcinogen. Has moderate antibacterial properties, is irritating to fish gills so water has to be well aerated (some fish are sensitive to treatment) and has algacide properties.


Other reported treatments:

  • UV Steriliser - can kill the free swimming stage but requires large wattage and low flowrate to be effective. Only kills parasites that pass through the UV filter, not those already on the fish or reproducing in the substrate. So will not stop reinfection or other fish becoming infected.
  • Temperature - below 20°C reproduction of Cryptocaryon halts. Impractical however, as tropical fish unlikely to tolerate the decrease in temperature. Good technique for temperate fish species though. However, it has been found that trophonts and tomonts can survive at 12.0 °C for 5 and 4 months, respectively, and reinfect fish[2].
  • Ozone - can kill the free swimming stage and only kills parasites that pass through the filter where the ozone is introduced. It will not kill those already on the fish or reproducing in the substrate. So will not stop reinfection or other fish becoming infected.
  • Nitroimidazole - take care with dosing, as may require more than manufacturer recommends to successfully treat.
  • Garlic -


The following are suggested as treatments, but are actually ineffective so should not be used:

  • Freshwater dip - totally ineffective as smaller theronts are protected from osmotic shock by the epithelial layer of the fish as they are actually under the skin of the fish. Only the parasites that have broken through the epithelial layer are lysed.[6]
  • TriSulfa or Triple Sulfa - for bacterial infections, not protozoans.
  • Malachite Green - for bacterial and fugal infections, not protozoans.
  • Myxazin - for bacterial infections, not protozoans.
  • Cleaner Wrasse - do not feed on Cryptocaryon, their wild diet is the larvae stage of some isopods.




  1. ( Diggles, B.K., Adlard, R.D., Taxonomic affinities of Cryptocaryon irritans and Ichthyophthirius multifiliis inferred from ribosomal RNA sequence data, Diseases of Aquatic Organisms, 22(1), (1995), 39-43.
  2. 2.0 2.1 ( Dan, X.M., Lin, X.T., Yan, Y.X., Teng, N., Tan, Z.L., and Li, A.X., A technique for the preservation of Cryptocaryon irritans at low temperatures, Aquaculture, 297(1-4), (2009), 112-115.
  3. ( Diggles, B., and Lester R. Infections of Cryptocaryon irritans on wild fish from southeast Queensland, Australia, Diseases of Aquatic Organisms, 25 (1996), 159-167.
  4. ( ): Colorni, A., Biology of Cryptocaryon irritans and strategies for its control, Aquaculture, 67(1-2), (1987), 236-237.
  5. 5.0 5.1 (Noga 2010): Noga, E.J., Fish Disease: Diagnosis and Treatment, 2nd ed., John Wiley & Sons, 2010.
  6. ( Porritt, M., Cryptocaryon irritans, Reef Culture Magazine, 1.