Fairy shrimp (Anostraca) are invertebrates that include brine shrimp (Artemia (=Sea Monkeys)).
There are about 300 known fairy shrimp species. While a few fairy shrimp species live in
salty environments (hence the name brine shrimp), the majority of the fairy shrimp live in fresh water.
About 500 million years the anostracans split off from the rest of the Branchiopoda. Since
that time, evolution didn't modify them much. In the niches they live, predators are seldom, so there
was no need to change their appearance or develop measures of defense.
Fairy shrimps are medium-sized branchiopods, with an average size of 0.4 to 1.2inch (1 to 3cm), but a few species
like Branchinecta gigas may grow as long as 4inch (10cm). They have a very thin exo-sceleton - this exo-sceleton
didn't evolve into a real protection layer, since predators are seldom in or near their environment.
One peculiar feature of all anostracan species is that they swim upside down.
Fairy shrimp are filter feeders. They use their "legs" to put up edible particles up their ventral food groove, which are then
pushed towards the mouth.
Some are almost transparent and hard to spot in the water. Others may develop stripes or zones of bright color, e.g.
the ovisac (brood pouch) of females is often deep orange, red or even blue or a mixture of blue/green. The entire animal
may develop a bright red or orange color due lack of oxygen (increased haemoglobin) or carotenoids in the food.
Fairy shrimp do not live in the oceans. They occur mostly in fresh water bodies, but some live in brine, mainly Artemia ssp.
Hence the name brine shrimp for Artemia and a few other fairy shrimp species such as Branchinella spinosa in Australia
and Branchinecta ssp. Artemia originally lived on all continents except Australia - Parartemia live only in Australia.
But in the 20th century Artemia ssp. was also introduced to Australia and now there are a few spots where they exist,
mainly in solar salt works.
There are two different types of water bodies:
rain pools (filled by periodic rains); (mountain) lakes; arctic ponds
saline pools/ponds/lakes; including solar salt works (beneficial effect on water evaporation)
The fairy shrimp in Arctic or Antarctic ponds don't depend on the filling and drying up of the water body,
but by the alternation of thawing and freezing. All of these water bodies are free of vertebrate
predators (e.g. fish), since fairy shrimp cannot defend themselfs. Only birds pose a threat to fairy shrimp,
but except of Flamingos (Phoenicopterus ssp.),
most birds do not see fairy shrimp as a regular diet. The redish feathers of Flamingos are a result of feeding
by redish Artemia.
Fairy shrimp stir up sediments from the bottom of the water bodies. This serves two purposes:
First, nutritive particles from the bottom are stirred up, and second, as protection. The water body gets cloudy, so that
predators such as insects, birds or other non-aquatic animals cannot easily see the fairy shrimp.
The (sub)arctic genera Polyartemia and Polyartemiella have 17 and 19 pairs of limbs (fairy shrimp usually have 11) do not
necessarily represent the most primitive anostracans.
The sexes are separate, except in some strains of Artemia (brine shrimp), which may be parthenogenetic, which means that
females can reproduce without the interaction with males.
Fairy shrimp are bisexual and oviparous (producing eggs/cysts), with the exception of Artemia. Artemia can life birth aswell.
Males have large second antennae to grasp the females during copulation and also have two penes.
Females carry a ventral egg sac that may be either short and broad or long and thin, depending on the genus and species.
Copulation itself is a fast reflex, but the fore-play can last for hours or even days where the female and the male
swim together (the male grasping the female with the antennae).
After copulation and fertilization, the cysts are deposited in the female brood pouch. After completion of the
egg development, the cysts are shed into free water, where they either sink to the bottom or float on the surface.
The eggs (cysts; containing about 4000 cells) of the fairy shrimp are surrounded by a thick shell, that makes them hardy against drought.
The eggs of some genus even require to completely dry up to complete development and, in the case of Artemia, are then able to withstand
years (even decades) of drought. Artemia is the only genus where viviparity can occur (life birth).
Some streptocephalids eggs (about 1/3 of the eggs) can hatch shortly after being layed and so bypass the resting stage.
