Commercial Fish
and Shellfish Technologies
(Graphic above links to Virginia Tech SeaGrant Web page)
Amyloodinium ocellatum (abbr. A. ocellatum) is a marine dinoflagellate. While most marine dinoflagellates (small protozoan organisms) exist as free living members of the planktonic community, some such as A. ocellatum live at least a portion of their life cycle as parasitic organisms.
Amyloodinium ocellatum exists in its parasitic stage as a trophont (feeding form), Figure 1. This trophont is attached via anchor-like roots and can infest the gills, fins and body of the host fish. When the trophont has matured (average size 80 - 100+ microns), it falls free of the host and forms a tomont, Figure 2. This tomont, an encysted stage which falls to the bottom of the tank, subdivides internally and can form as many as 200+ infective stages. These infective stages excyst (hatch) as dinospores (commonly called swarmers), Figure 3, and are the infective stage. These swarmers actively swim through the water column searching for a new host. The duration of this life cycle is temperature dependent, as is trophont size and tomont fecundity, and can range from seven to as many as twenty days.
Figure 1. Trophont |
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Figure 3. Dinospore |
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Amyloodinium ocellatum can cause extensive fish mortality in recirculating aquaculture production systems. A. ocellatum's fecundity rate, wide range of environmental tolerances, and resistance of the tomont stage to chemotheraputents make early identification of this parasite a high priority in brackish and marine culture systems. Once diagnosed, a quick response is essential to prevent rapid loss of fish stocks.
There are three methods by which A. ocellatum can be controlled: chemical treatment, flushing, and filtration.
CHEMICAL
TREATMENT:
No FDA approved chemicals are available as theraputents against this parasite
in food fish production. However, research has identified several chemicals
shown to have reasonable success in its control. Benzalkonium chloride and
ionic copper are the most effective at dislodging trophonts from affected
fish. These chemicals do not however kill the encysted tomont stage. In order
to clear fish of detrimental levels of this parasite, repetitive doses are
required, and this only keeps the parasite under control. Benzalkonium chloride
is illegal for any use in food fish production, and ionic copper is only registered
with the EPA for use as an algaecide.
FLUSHING:
Flushing of production systems is another means of minimizing infestation
levels of A. ocellatum. This is effective by physically removing the
encysted tomont stage before it has had the opportunity to excyst. It is important
when implementing treatment flushing that the water being removed from the
system be withdrawn
from the tank bottom where tomonts accumulate. When flushing, a minimum of six tank volumes per day should be exchanged. This may need to be increased depending on water temperature, density of fish in the production unit, salinity, and the degree of parasite infestation.
FILTRATION:
The best method to date in the control of A. ocellatum infestations
in intensive aquaculture production systems is filtration. This physically
removes the tomont stage from the production system while allowing for minimal
water exchange. Filtration on a commercial scale in intensive recirculating
aquaculture can be accomplished via microscreen, drum or bead filters. While
microscreen/drum filters generally have a direct filtrate stream exiting the
system, bead filters do not. As such, it is imperative that the bead filters
be backwashed a minimum of one time per day. System volume should be filtered
at least once per hour, and down to at least fifty microns. In research or
hatchery environments, cartridge or diatomaceous earth filters may also be
economically employed to control this parasitic disease.
Authors: Michael H. Schwarz, Stephen A. Smith
This
FACT SHEET was published by: (the three links immediately
following leave the Aquaculture Information Center website).
Virginia Tech
Virginia Sea Grant College
Program
Virginia
Cooperative Extension
For
further information contact:
Michael H. Schwarz
Virginia Seafood Agricultural Research and Extension Center
102 South King Street
Hampton, VA 23669
E-mail: mschwarz@vt.edu
Back to Aquaculture Information Center Homepage Updated 05/31/2002
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