Although using Artemia cysts appears to be simple, some procedures are critical for successfully obtaining Artemia nauplii at the quantities needed in larval crustacean production. These include cyst disinfection or decapsulation prior to incubation, and hatching under optimal conditions.
A major problem in the early rearing of prawns is the susceptibility of the larvae to microbial infections. It is recognised that live food can be an important source of potentially pathogenic bacteria, which are easily transferred through the food chain to the predator larvae in commercial hatcheries (Verdonck et al. 1994). Vibrio sp. constitute the main bacterial flora in Artemia cyst hatching media. Most Vibrio spp. are opportunistic bacteria which can cause disease/mortality outbreaks in larval rearing, especially when animals are stressed or not reared under optimal condi tions. Artemia cyst shells are loaded with bacteria, protozoa, fungi, and even contaminated with organic impurities. Bacterial contamination in the hatching medium can reach numbers ofmorethan 107CFU/ml (CFU = colony-forming units). At high cyst densities and high incubation temperatures during hatching, bacterial development (e.g. on the released glycerol) can be considerable and hatching solutions may become turbid, which may also result in reduced hatching yields. Therefore, unless commercially disinfected cysts are used, a disinfection procedure consisting of soaking cysts for 30 minutes in a 2000 mg/L hypochlorite solution (Sorgeloos et al. 1986) should be routinely applied. This treatment, however, will not kill all germs present in the alveolar and cortical layer of the outer shell. A more thorough removal can be obtained with a combination of specially designed commercial disinfectant agents or by using commercially available pre-disinfected cysts.
Complete disinfection can be achieved through cyst decapsulation. Decapsulation is a chemical process whereby the hard shell which encysts the dormant Artemia embryo is completely removed. This is typically performed by a short-term exposure to a hypochlorite solution in an alkaline environment and subsequent quenching with thiosulphate (Bruggeman et al. 1980; Sorgeloos et al. 1986). Decapsu-lated cysts can either be directly hatched into nauplii, or they can be stored for later hatching after being dehydrated in saturated brine or used as such for direct feeding. Dehydrated decapsulated cysts are conveniently stored in the refrigerator at 0 to 4°C for a few days without any decrease in hatching rate. If storage for prolonged periods is needed (up to a few months), the decapsulated cysts are best transferred into a saturated brine solution.
Besides being completely disinfected, decapsulated cysts offer a number of advantages compared to the non-decapsulated ones:
• There is no further need to separate cyst shells from the hatched nauplii, and consequently no cyst shells are introduced in the culture tanks.
• Energy content and individual weight of nauplii that are hatched out of decapsulated cysts are higher (30-55%, depending on strain) than regular freshly-hatched larvae, because energy is required to break out of the shell. In fact, the hatching output of cysts, that have relatively low energy content, may be effectively improved by decapsulation (Vanhaecke et al. 1983).
Hatching small quantities of Artemia cysts is basically very simple but hatching kilograms of cysts, as is a daily practice in large hatcheries, does require proper attention to several critical parameters: aeration, temperature, salinity, pH, cyst density and illumination. Even under optimal hatching conditions, hatching results may fluctuate as a
result of differences in cyst quality or strain inherent characteristics.
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