Amyloodinium ocellatum – a.k.a Oodinium a.k.a. Marine Velvet Disease

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If someone were to ask me what the most deadly marine disease was, one of the first things that come to mind would be Marine Velvet. Unfortunately it is not uncommon, and the dinoflagellate infestation can frequently be found in newly imported fish. The A. ocellatum parasite responsible for premature fish loss is actually an algal protozoan and closely related to the dinoflagellates that cause red tides.

ocellatum can completely wipe out an entire aquarium in the right conditions: poor nutrition, low water quality, improper life support equipment, and other stressors.

Life cycle of “Oodinium”:

Life cycle is typically 6-12 (but as long as 28) days depending on temperature.

  1. Trophont stage- the only time you can see the parasite, nonmotile and hosting a fish absorbing nutrients for reproduction
  2. Encysted stage- a.k.a tomont or palmella, the parasite divides while still on the fish. Incubation period can be 3-6 days.
  3. Dinospore stage- newly hatched parasites emerge from cysts and are free swimming, looking for new hosts


Marine Velvet is not easy to spot.

  • TIP: make sure to look at thin, transparent areas of your fish and try to view these areas at an angle to best see affected areas.
  • First signs of infection include rapid respiration (the gills are typically attacked first).
  • A classic infestation has been described as a dusting of powdered sugar or a foggy or faded area on the fish’s body and can be accompanied by cloudy eyes and fins. Fins may appear clamped and fish may stop eating. You may see the fish flashing or scraping its body against décor or the substrate.
  • Severe infestations look velvety in texture, thus the name. In these instances you will also see sloughing off of the protective slime coat.
  • Death can occur in as little as 12 hours, without any outward appearance if the gills are severely damaged by the parasite. It has even been known to colonize the guts or esophagi of many fishes, making it difficult to spot and control.


  • Prevention! Especially in reef tanks, as most medications can kill coral and live rock! Have the right equipment to house your fish, proper UV sterilizer, feed a nutritious diet, quarantine new fish, be educated and ask questions when selecting fish for your tank, etc. This disease can happen so fast, that sometimes aquarists loose their whole tank in a day or two and never saw anything wrong.
  • Repeated fresh water baths dosed with quick cure or formalin may dislodge some of the trophants on the fish (about 10-15 minutes). Fish must be caught quickly or stress of netting can cause disease to worsen.
  • Treatment with copper sulfate (i.e. Cupramine) for 21-30 days.
  • Chloroquine phosphate treatment could be used.
  • Reef safe medications, though much weaker include Rally or Hypercure.
  • Antiparasitic foods and garlic additives may also help if the fish is eating.
  • Monitored hyposalinity may enhance effectiveness of treatments (1.010-1.013).
  • Increasing temperature will increase reproduction rate of parasite and shorten life cycle for decreased treatment time, however, the fish’s metabolic rate and demand for oxygen also increases.

If you think you might be noticing signs of Oodinium in your home aquarium, stop in to see one of our marine certified aquarists to diagnose and discuss your situation. We are experienced with this pathogen and would be happy to help you prevent and eradicate it!

Cool side note: A quick and immediate drop in salinity has dramatically increased effectiveness in controlling “Oodinium”. The parasite has semipermeable membranes and cannot control osmosis. They begin to absorb water as the cells try to equalize osmotic pressure with the change. Eventually, most strains of Amyloodinium pop like fish bags filled with too much air (in this case water).