The optimal conditions for hatching Artemia are:
- temperature above 25 o C (77 o F), with 28 o C (82 o F) being optimum;
- salinity of 5 ppt (1.030 density);
- heavy, continuous aeration;
- constant illumination (example: two 40- watt fluorescent bulbs for a series of four 1-liter hatching cones); and
- a pH of about 8. Stocking density is set by adding no more than 5 grams of cysts per liter of water.
Good circulation is needed to keep the cysts in suspension. A container that is V-shaped or cone-shaped is best (2-liter bottles work well; glue a valve on the bottle cap and invert it). The best container is a separation column, found in any lab supply, although it is more expensive. Unhatched cysts, empty shells and hatched nauplii can be easily removed separately.
The hatching percentage and density are usually a function of water quality, circulation, and the origin of the cysts. Preparation and use of Artemia There are seven tasks involved in feeding Artemia to larvae.
- Determine the weight of Artemia cysts required to feed the larvae in a tank of known volume.
- Hydrate and decapsulate cysts (decapsulation is optional, but recommended).
- Incubate cysts.
- Separate cysts from shells and debris (not necessary if cysts were decapsulated).
- Count the hatched Artemia.
- Calculate the number of Artemia remaining in the rearing tank from the previous feeding.
- Calculate the number of Artemia nauplii required by the larvae and transfer them to the rearing tank.
Be careful with step number 6, as remaining nauplii may have little nutritional value and may need to be flushed out of the system. Details of each of the tasks will be discussed in the
following small scale example. Materials and equipment needed are:
- Artemia cysts
- two 250-ml (8.5-fluid ounce) beakers
- distilled water
- household bleach
- sodium hydroxide (NaOH)
- 1-liter Imhoff cone or settling column
- low-pressure air supply (aquarium pump)
- seawater or equivalent (salinity of 5 to 32 ppt)
- siphon tube (approximately 4 feet long) or a valve at the bottom of the cone
- 1-ml pipet
- 10-ml pipet
Determine the amount of Artemia cysts required:
Artemia nauplii are maintained in the larval culture tank at densities of 0.5 to 2 per ml for most species of finfish and up to 6 per ml in the more advanced larval shrimp stages. To estimate the amount of Artemia required one must consider both the volume of the tank and the expected number of Artemia the larvae will consume.
Based on the stage or the age of the larvae, estimate a daily Artemia requirement per ml. This feeding rate can be adjusted slightly, depending on the stocking density (number of target larvae per liter) and the rate at which the Artemia are consumed. The total requirement is then calculated by multiplying the predicted requirement per ml by the total volume of the rearing tanks.
Each gram of cysts contains approximately 200,000 to 300,000 cysts. Artemia generally have at least a 50 percent hatch. Experience with your specific brand will allow you to adjust these figures.
As an example, say that you find your target larvae are at a stage that requires two Artemia nauplii per ml. You wish to feed ten rearing tanks, each 16 liters (541 fluid ounces) in volume. Your requirement for hatched nauplii is therefore: 2 x 10 x 16,000 = 320,000 nauplii. If you assume a 50 percent hatch rate, you require 320,000 x 2 = 640,000 cysts. If 1 gram contains an average of 250,000 cysts, your required weight of cysts is 640,000 รท 250,000 = 2.56 grams.
In the form of a formula:
Weight of Artemia cysts required= Total volume of all rearing tanks (in ml) X No. of Artemia required per ml Percentage hatch rate X No. of cysts per gram
Hydrate and decapsulate cysts:
Decapsulating, or removing the shell from Artemia cysts, serves several functions. The process disinfects the cysts, makes nauplii more digestible if they are unhatched when eaten, helps speed up the hatching process, improves hatchability, and makes it easier for the nauplii to emerge.
If the nauplii have not put as much energy into emerging, they should be a better food source. Separating nauplii from their shells is desirable because shells are indigestible and can lodge in the gut of larvae, causing fatal obstructions. The shells are a known source of bacterial and viral contamination. The decapsulated cysts can be fed to smaller fish larvae than the fully developed nauplii.
Decapsulation is accomplished in three steps: re-hydrating the cysts; decapsulation; and washing and deactivating the residual chlorine.
Re-hydrating:
Dry cysts have a dimple in their shell, which makes it hard to remove the complete inner membrane. For this reason, the cysts are first hydrated into a spherical shape. The cysts should be re-hydrated in soft or distilled freshwater or low salinity water (less than 10 ppt) at 25 o C for 60 to 90 minutes.
The lower the temperature, the longer it takes to rehydrate them. But, no matter what the temperature, never leave the hydrated cysts longer than 2 hours or some of them may not survive the decapsulation procedure.
Hydration should be done in a container similar to the one used for hatching regular cysts, for the same reasons of circulation and aeration. Cysts should be filtered on a 100- to 125-micrometer collection screen and rinsed, but this step may be skipped if you do not have the screen.
Decapsulating Method :
It is best to decapsulate the hydrated cysts immediately, but they can be refrigerated for several hours if necessary. During the hydrating process, prepare the chlorine solution as described below.
To decapsulate, first measure 10 ml of seawater (5 ppt salinity) per gram of cysts. Place the water and a plastic-coated stirrer into a glass pie dish. Center the dish on a magnetic stir plate and turn it on.
Slowly add sodium hydroxide (NaOH) to the seawater at a rate of 150 mg per gram of cysts and allow it to dissolve. Use bleach to wash the cysts from the screen and into the dish containing seawater and NaOH.
The bleach should be added at a rate of 5 ml per gram of cysts. With the addition of the bleach, the reaction (decapsulation) begins. Ice can then be placed in the solution to keep the temperature below 30 o C (86 o F). Slowly turn up the speed of the stirrer and run it as fast as possible, without splashing.
Watch the solution carefully. A white foam layer will develop and the solution will change from brown to gray or sometimes a light orange, which should take approximately 6 minutes. When no more color change is seen, the decapsulation process is complete. Immediately drain the pie dish through a 100- to 120-micrometer sieve, over the sink, and wash the cysts thoroughly with tap water or seawater.
The washing should continue for about 10 minutes until no chlorine smell can be detected. Scrape the cysts into a beaker and pour in enough 0.1 N HCl to wash the cysts, for no more than 30 seconds. This brings the pH to the neutral range. Pour the cysts into the sieve again and wash with water for 3 minutes. The cysts are now ready for incubation.
Great care should be taken with the chemicals. They are caustic and can cause injury, particularly to the eyes. Always wear protective eyeware.
Author:
Granvil D. Treece
