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Life history of the brine shrimp Artemia

by Patrick Sorgeloos

In 1938, Keunen and Baas-Becking reported that the first written record of the existence of the brine shrimp dates back to 1755. Nonetheless this "filtering animal" was known since much longer times by different ethnic groups who attributed a better salt production in brine pools to the presence of Artemia; hence its popular names as brineworm, Salztierchen, verme de sale, sofereg, Bahar el dud, Fezzanwurm, etc.

Schlosser's drawings of a male(A) and a female (B) brine shrimp (from Keunen and Baas-Becking, 1938).

Despite the primitive optical equipment available at that time, Schlosser's drawings were very detailed and rightly gave the adult animal 11 pairs of thoracopods. Several other scientists, including Linnaeus (1758), later discribed adult Artemia with only 10 thoracopods. This controversy lasted until 1836 when finally Audouin confirmed the observations of Schlosser.

From the second half of the 19th century on, several studies were published dealing with the morphology and taxonomy of this Anostracan crustacean. Soon Artemia was used as a most suitable test-object in the most diverse disciplines of biological sciences : histology, genetics, radiobiology, toxicology, biochemistry, molecular biology, ecology, etc.

Salt lakes and brine ponds with Artemia populations are found worldwide. The ecological conditions in these biotopes are extreme (e.g. the salinity can exceed 300 g salts/l water), and as a result only a small number of bacterial and algal species can survive. As a consequence of this often occurring blooms of monocultures of specific algal species, these waters are colored red, blue or green. One of the very few invertebrates that could adapt to such an extreme habitat is the brine shrimp Artemia. Favored by the absence of predators and food competitors, Artemia mostly develops into very dense populations in the salinas.

At certain moments of the year, enormous quantities of minuscule brown particles (200-300 micrometer in diameter) are floating at the lake's surface and are finally thrown ashore by wind and waves.

These apparently inert particles are in fact the inactive dry cysts of the brine shrimp which remain in diapause as long as they are kept dry or under anaerobic conditions. Upon immersion in seawater, the cysts hydrate, become spherical and within their shell the embryo resumes its reversibly interrupted metabolism. After about 24 hours the outer membranes of the cyst burst (="breaking" or E-1 stage) and the embryo appears, surrounded by the hatching membrane.

The only structural feature which can be observed is the nauplius eye. During the following hours the embryo leaves the cyst's shell. Inside the hatching membrane, the newly differentiated antennae and mandibles start moving ; within a short period of time the hatching membrane is ruptured and the free-swimming nauplius is born. The first instar larva which is colored brownish-orange due to the presence of yolk, has three pairs of appendages : the antennae which have a locomotory function, the sensorial antennulae and the rudimentary mandibles. An unpaired red ocellus is situated in the head region between the antennulae. The ventral side of the animal is covered by a large labrum.

As of the second instar stage, the metanauplius has a functional digestive system.

The larvae further grows and differentiates through about 15 molts : the trunk and abdomen are elongating ; food particles are collected from the medium by the setae of the antennae ; paired lobular appendages which will differentiate into the thoracopods are budding in the thrunk-region ; lateral complex eyes are developing on both sides of the ocellus ; etc.

From the 10th instar on, important morphological changes are taking place : the antennae loose their primitive locomotory function ; i.e. they loose their long setae and undergo sexual differentiation. In the future males they develop into hooked graspers, while in the females the antennae degenerate into sensorial appendages. The thoracopods are now differentiated into three functional parts : the telopodites acting as a filter, the oarlike endopodites having a locomotory activity, and the membranous exopodites functioning as gills.

The adult animal 8-10mm long, is characterised by the stalked lateral (complex) eye, the sensorial antennulae, the linear digestive tract, and the 11 pairs of functional thoracopods. In the male Artemia the antennae are transformed into muscular graspers which have a frontal knob at their inner side. In the posterior part of the trunk region a paired penis can be observed.

Female Artemia have very primitive antennae with sensorial function ; their paired ovaries are situated on both sides of the digestive tract behind the thoracopods. The ripe oocytes are transported from the ovaries into the unpaired brood pouch or uterus via two oviducts.

Precopulation in adult brine shrimp is initiated by the male in grasping the female with its antennae between the uterus and the last pair of thoracopods. In this "riding position" the couples can swim around for long periods.

Copulation itself is a very fast reflex : the male abdomen is bent foreward and one penis is introduced into the uterus aperture. The fertilized eggs develop into either free-swimming nauplii (ovoviviparous reproduction) which are set free by the mother, or when reaching the gastrula stage, they are surrounded by a thick shell and are deposited as cysts, which are in diapause (oviparous reproduction).


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