Artemia salina

Description

Artemia salina have three eyes and 11 pairs of legs and can grow to about 15 millimetres (0.6 in) in size. Their blood contains the pigment haemoglobin, which is also found in vertebrates. Males differ from females by having the second antennae markedly enlarged, and modified into clasping organs used in mating.

Life cycle

Males have two reproductive organs. Prior to copulation the male clasps the female with his clasping organ, assuming a dorsal position. The claspers hold the female just anterior to the ovisac. Male and female may swim clasped together for a number of days. In this state, the movements of the swimming appendages of the pair beat in a co-ordinated fashion. The females can produce eggs either as a result of mating or via parthenogenesis. There are two types of eggs: thin-shelled eggs that hatch immediately and thick-shelled eggs, which can remain in a dormant state. These cysts can last for a number of years, and will hatch when they are placed in saltwater. Thick-shelled eggs are produced when the body of water is drying out, food is scarce, and the salt concentration is rising. If the female dies, the eggs develop further. Eggs hatch into nauplii that are about 0.5 mm in length. They have one single simple eye that only senses the presence and direction of light. Nauplii swim towards the light but adult individuals swim away from it. Later, the two more capable eyes develop but the initial eye also stays, resulting in three-eyed creatures.

Ecology

In nature, they live in salt lakes. They are almost never found in an open sea, most likely because of the lack of food and relative defenselessness. However, Artemia have been observed in Elkhorn Slough, California, which is connected to the sea. Unlike most aquatic species, Artemia swims upside down.

Artemia can live in water having much more or much less salt content than normal seawater. They tolerate salt amounts as high as 50%, which is nearly a saturated solution, and can live for several days in solutions very different from the sea water, such as potassium permanganate or silver nitrate, while iodine—a frequent addition to edible salt—is harmful to them. The animal's colour depends on the salt concentration, with high concentrations giving them a slightly red appearance. In fresh water, Artemia salina dies after about an hour. It feeds mainly on green algae.

Uses

The resilience of these creatures makes them ideal test samples in experiments. Artemia is one of the standard organisms for testing the toxicity of chemicals. In addition, the eggs survive for years. Hence it is possible to buy eggs and also "Artemia growing kits" for children, containing eggs, salt, food and most necessary tools. These have been most popularly marketed under the name Sea-Monkeys. Children have the possibility to observe the life cycle of this interesting organism. Shops catering for aquarists also sell frozen Artemia as fish food. Artemia occurs in vast numbers in the Great Salt Lake where it is commercially important. However, nowadays it is believed that this lake is inhabited by a second brine shrimp species, Artemia franciscana.

Taxonomy

Artemia salina was first described (as Cancer salinus) by Carl Linnaeus in his Systema Naturae in 1758. This was based on a report by a German called Schlosser, who had found Artemia at Lymington, England. That population is now extinct, although specimens collected there are retained in zoological museums.