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Amyloodinium ocellatum (Marine Velvet)

Posted 12/18/2010 at 10:42 AM by snorvich
Updated 03/03/2014 at 10:11 AM by snorvich

What is Oodinium?

Velvet or Coral Fish Disease is caused from an infestation of the dinoflagellate Amyloodinium ocellatum. The life cycle of Velvet is very similar to the life cycle of marine ich. This organism is parasitic on fish at one stage in its life cycle; during that stage it is visible to the naked eye. The total life cycle for this parasite approximates 3 weeks; but for total safety after a tank is infected two life cycles should be waited (six weeks). The lifecycle is very similar to cryptocaryon irritans but the effect on fish is much more severe and is one of the causes of wiping out an entire tank's population of fish.

Due to the fact that this organism is able to reproduce so rapidly, when an Oodinum outbreak occurs in an aquarium and it is not immediately diagnosed and treated, in a closed saltwater system it can reach overwhelming and disastrous numbers in a very short period of time. This parasite is one of the most common causes of a tank wipeout, or an abrupt loss of all the fish in a saltwater aquarium. Often times by the time you notice the problem it is really too late.

The Life Cycle of Amyloodinium ocellatum
• Free-swimming cells called dinospores are released from a mature cyst and go in search of a host fish. Typically these cells can survive seven to eight days without a host, but in lower tank temperatures at around 75-80 degrees, some strains may last up to 30+ days. Raising the temperature will speed up the lifecycle but it also reduces dissolved oxygen in your tank water. For fish with this parasite in their gills, this is an unfortunate treatment.
• Once a host is found, typically heading for the soft tissue inside the gills first, the dinospores lose their swimming capabilities and become non-motile parasitic trophozoites. At this stage they turn parasitic, as each attaches to the host fish by sending out a filament for feeding.
• After deriving nutrition for 3 days to a week the trophozoites become mature and drop off into the substrate, may remain hidden in the mucus membrane, or sometimes be deeply embedded in the tissue of a host fish, where at this point each forms a type of hard shell covering.
• Inside each encrusted cyst the cells, now called tomonts, reproduce internally by non-sexual division. Upon reaching maturity in about five days, each cyst ruptures and releases hundreds of new free-swimming dinospores to start the cycle all over again, but in much large numbers. This means that over time the effect is multiplied.

Most similarly symptomatic to Brooklynella, Oodinium organisms primarily attack the gills first. At the onset of this infestation fish often scrape up against objects in the aquarium, lethargy sets in, and rapid respiration develops, which is the result of excess mucus in the gills due to the invasion of the parasites. This is typically noticed as fish staying at the surface of the water, or remaining in a position where a steady flow of water is present in the aquarium such as near overflows or powerheads.

As the disease progresses outwards from the gills, the cysts then become visible on the fins and body. Although these cysts may appear as very tiny white dots smaller than the size of a grain of salt, similar to but unlike the first sign of Saltwater Ich or White Spot Disease, what sets Oodinium apart from other parasites is that at this point the fish have the appearance of being coated with what looks like a whitish or tan to golden colored, velvet-like film, thus the name Velvet Disease.

Now in the advanced stage of the disease the production of gill and body mucus increases, the fish becomes listless, refuses to eat, and it's not unusual for a secondary infection to develop. For fish that reach this end stage of the disease, it's typically too late. They usually do not respond to treatment, and most often will die.
Most Effective Treatments for Oodinium
• Remove all fish from the main aquarium, give them a freshwater dip. Prepare a freshwater dip. For this dip, adjust pH (so as reduce more osmotic stress than need be) and add Methylene Blue (at double in tank strength), use a specific gravity of 1.001 for the saltwater fish. This dip should be no less than 3 minutes and no more than 5 minutes to be effective. This is very effective in removing Oodinium directly from the fish (including gills). Do not be alarmed if the fish ‘lays down’ and acts dead, this is a common initial reaction and the fish will usually perk up a minute or two into the dip. How this works is that the cell membrane of the Oodinium cyst cannot withstand the change in osmotic pressure as well as the fish and will burst, that is why the minimum three minutes is a must. This dip is more effective for Oodinium than Cryptocaryon even though this is recommended for both due to the fact that the Oodinium Cyst does not imbed nearly as deep as the Cryptocaryon cyst does, allowing for a much more likely rupture of the cell membrane due to osmotic pressure.

Following a fresh water dip use a formalin bath, and then place them into a QT with vigorous aeration provided. To address complications from secondary infections, also treat the fish with an appropriate antibiotic or anti-bacterial medication. Continue treating the fish in the QT until the oodinium appears to be gone, and then keep treating for another week after that.
• Unfortunately, Oodinium can withstand a broad salinity range (from 3 to 45 ppt) so Hyposalinity is not an effective treatment.
• Treatment with copper is often the recommended course of action, however keeping copper exactly at the proper level is very difficult and infeasible for most aquarists which is why I prefer a freshwater dip followed by a formalin bath.

Preventing Reinfection
Reinfection will occur no matter how effectively the fish have been treated if the organisms are not eradicated from the main aquarium. Because they require a fish host to survive, this can be accomplished by keeping the tank devoid of any fish for at least six weeks. For fish-only aquariums the tank temperature can be elevated to 85 to 90 degrees to speed up the life cycle of the organisms, which will help to eliminate all cysts and dinospores in three weeks.
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