More than 50 different strains from all continents exist with differences in specific characteristics such as hatching rate, nauplius size, viability, optimal temperature and salinity range requirements.
For morphology see practical work sheets and Fig. 1
Dry eggs or cysts (0.2 – 0.3 mm) hatch in seawater into free-swimming naupliae (0.45 mm) in a period of 24–36 hrs.
Depending on feeding conditions, the naupliae will grow out to fullgrown adults (max. 12 mm) in 1 to 3 weeks time.
Males and females are easily distinguishable and the female will produce offspring after copulation with a male. (only in zygogenetic strains). Prior to copulation the male will ride the female (riding position) for a period of a few hours to several days, by holding her with his graspers. At a certain moment the male will curve its body and insert its paired penis in the uterus aperture at the bottom of the broodpouch upon which the eggs are fertilized.
The offspring will be either in the form of cysts or directly free-swimming naupliae (see reproduction).
In the case of cyst offspring, the female deposits the cysts but they will only become hatchable after being dried.
During the drying process (dehydration) the spherical shape of the deposited eggs changes into the shape of a crunched ping-pong ball (dented).
Once in the dried state the cysts can be stored for later use.
Some strains are parthenogenetic (only females) but most are zygogenetic (males and females).
Two modes of reproduction exist in Artemia:
Fig. 1 LIFE CYCLE OF THE BRINE SHRIMP
(ARTEMIA SALINA)
Oviparous reproduction; After copulation fertilized eggs are surrounded in the broodpouch of the female with a tough brown shell. The egg is then called a cyst. The cysts are released by the female in the water where they will not hatch untill they have been completely dehydrated (in nature by floating ashore and sun-drying). The embryo inside each cyst is then in a state of metabolic dormancy and will not further develop until hydrated again (water absorption). In nature this mostly happens when washed back to the pond water after rainfall. Once sufficiently hydrated the embryo further develops into the instar I larva (nauplius) which will hatch out of the cyst shell.
Ovoviviparous reproduction: After fertilization the eggs are not surrounded by a shell but instead immediately develop further into naupliae in the broodpouch of the female. These naupliae are then released in the water as free-swimming naupliae.
The mode of reproduction is controlled by environmental factors such as oxygen content and/or fluctuation thereof, type of food and reproduction history of the female. In practice the following assumptions are valid:
| Oviparous | Ovoviviparous |
| - low O2-content | - High oxygen content |
| (such as in high salinity) | (such as in low salinity) |
| - Strong O2-fluctuations | - Minor O2-fluctuations |
| - Fe-rich food | - Fe-low food |
| (such as green algae) | (such as organic debris) |
mode of reproduction is determined once the eggs have descended into the broodpouch and are fertilized.
In one batch of eggs produced in a female all eggs are either cysts or ovoviviparous eggs. In other words only one reproduction mode occurs per batch.
Adult Artemia can live for several months (in good conditions) and the female produces a new batch of eggs every 5 days.
Per batch or reproductive cycle 50–200 cysts or naupliae are produced but in oviparous reproduction the number of offspring is generally lower than in ovoviviparous reproduction.
Dehydrated cysts of most strains measure between 200 and 270 μm, and weigh 3.5 μg on average.
Dry cysts are very resistant to extreme conditions. Up to 80°C, hatching efficiency is not affected. Hydrated cysts are killed by temperatures lower than 0°C and higher than 40°C.
Cysts are very hygroscopic and absorb even water from the atmosphere. When stored, water content should be lower than 0.09 g H2O/g cyst or 10% (no metabolism). At 0.3 g H2O/g cyst or 25% the metabolism of the dormant cyst starts.
At salinities higher then 70 ppt cysts can not hatch because of the too high osmotic gradient. In salinities lower than 5 ppt cysts will hatch but resulting naupliae will die quickly.
Light triggering is needed at the beginning of the hatching to start metabolism.
In freshwater and seawater, dehydrated and hydrated cysts sink. In brine (saturated salt solution) they float.
Growth is optimal at 28°C and 35 ppt and drops below pH 7.
Lethal temperature limits are 0°C and 37–38°C.
Salinity changes can be administered very abruptly without harm. For instance from 30 to 90–100 ppt.
Sudden transfer from 30 to 0°C can also be done without killing them. At 0°C, activity will stop but can be reactivated by increasing the temperature.
Mostly salinity tolerance is up to 200–250 ppt. Limitation is more caused by oxygen depletion than by salinity itself.
Below pH 7 general appearance of Artemia deteriorates. pH 8–8.5 is optimal.
Lethal temperature limits are the same as for naupliae.
Just as naupliae, adults can stand sudden temperature fluctuations.
Low oxygen concentrations are more harmful for young naupliae than for older larvae and adults, since during larval development the exopodites become functional as respiratory structures.
The colour of adult Artemia is correlated with the mode of reproduction, because hemoglobin synthesis is activated by low oxygen concentration in the water. Red Artemia indicate oviparous reproduction, pale whitish Artemia indicate ovoviviparous reproduction.
Artemia is an obligatory non-selective particle filter feeder and removes suspended particles smaller than 40–60 μm down to a few μm from the water with great effectiveness.
Food particles may consist of algae cells (non-filamentous), Protozoa, organic detritus particles, etc.
Only from instar II and III larval stages food is taken up. Instar I through III larvae have a yolk reserve.
In most waters where Artemia occurs naturally, no other plankton feeder is present. Therefore the only food competition will be among the Artemia themselves.
Young naupliae are positively phototactic.
Adults are negatively phototactic.
Artemia tends to gather in clouds along the shores. Sometimes long Artemia trails can be seen in the water.
Vertical and horizontal distribution patterns are completely different during day and night time.
