The first written record of the existence of the brine shrimp only dates back to 1755 (Schlosser in Kuenen and Baas-Becking, 1938). Nonetheless this "filtering animal" was already known since much longer times by different ethnic groups who attributed a better salt production in the brine pools to the presence of Artemia ; hence its popular names such as brineworm, Salztierchen, verme de sale, sóféreg, Bahar el dud, Fezzanwurm, etc. Despite the primitive optical equipment available at that time Schlosser's drawings were very detailed and rightly give the adult animal 11 pairs of thoracopods. Several other scientists, including Linnaeus (1758), later described 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 0f this Anostracan Crustacea. Soon Artemia was used as a most suitable test-object in the most diverse disciplines of the biological sciences: histology, genetics, radiobiology, toxicology, biochemistry, molecular biology, ecology, etc. (cf. Sorgeloos, 1976). In this regard it is striking to note that many scientists utilizing Artemia for their fundamental research, even nowadays, are not familiar with the life cycle of the brine shrimp in nature. Salt lakes and brine ponds with Artemia populations are found allover the world (Claus et al., 1977). The ecological conditions in these biotopes are extreme (e.g. up to 250 g of salts per liter water), and as a result only a small number of bacterial and algal species can survive. As a consequence of the 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 salina. 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 (200-300µm 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 or "Dauereiern" 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 metabolism of the embryo is activated. A number of hours later, the outer membranes of the cyst burst (= "breaking" 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. This 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 antennullae and the rudimentary mandibles. An unpaired red ocellus is situated in the head region between the antennullae. The ventral side of the animal is covered by a large labrum. The larva grows and differentiates through about 15 molts: the trunk and abdomen are elongating; the digestive tract becomes functional; 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 trunk-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 lose their primitive locomotory function; i.e. they lose their long setae, bend forward to the head and undergo a 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 oar-like endopodites having a locomotory activity and the membranous exopodites functioning as gills. The adult animal 8-10mm long, is characterized by the stalked lateral (complex) eyes, the sensorial antennullae, 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 sensorial papilla 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 forward 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). |
