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The Amazing Brine Shrimp

Brine shrimp are small animals that live in saltwater. They are related to the kind of shrimp we eat, but they are not edible to humans. Brine shrimp do, however, serve as a major source of food for fish and birds. They live in saltwater lakes and seas all over the world, but not in oceans. They are also easy to grow in a small aquarium at home, and have been sold as pets since 1957 under the name Sea-Monkeys, though of course they aren’t related to monkeys at all.

One way that brine shrimp avoid being eaten is by living in water that is too salty for other creatures to tolerate. They can survive in water with a wide range of salinity (saltiness), but the amount of salt in the water affects how fast they can grow and reproduce. Brine shrimp survive best in cold water, since cold water holds more oxygen. They keep the water clean by eating algae and bacteria that grow in the water, and by removing excess phosphorus and nitrogen from the water.

Brine shrimp are so small that they appear to be transparent (clear) when seen under a microscope. They have many appendages, similar to arms, legs, or fins, that help them move through the water and gather food. They have three eyes, including two compound eyes, to help them locate food, find mates, and avoid being eaten. The arrow in the picture shows which way the brine shrimp swims through the water.

Brine shrimp can reproduce in several ways. The normal method of reproduction is for the female to lay eggs, which must be fertilized by the male. These eggs can be encased in cysts if winter is approaching. A cyst is a hard, protective coating on the egg that enables it to survive being dried out and frozen. Eggs encased in cysts can remain dormant for many years. Scientists have hatched brine shrimp eggs up to 100 years old! In fact, some cysts will not open until after they have been soaked and dried repeatedly. This ensures that some brine shrimp will survive even if the lake they live in goes completely dry for a while.

If a brine shrimp is living in a lake that is drying out, it may get separated from other brine shrimp. If a female cannot find a mate, she can’t produce cysts, but she can produce eggs that will be clones of her. These babies may survive long enough for the lake to be refilled, so they can find mates and continue their population. Sexual reproduction (mating) creates babies that have different genes from their parents, so they don’t all have the same weaknesses. This genetic diversity makes it harder for the entire population to be wiped out. Having several ways to reproduce makes it easier for brine shrimp to survive in changing environments.

If you observe brine shrimp under a microscope, you will see that some of the females are pregnant. The eggs look similar to the compound eyes, except that there are many of them, and they are on the other end of the brine shrimp. Eggs can hatch after being laid, or they can hatch while they are still inside the mother. If you watch carefully, you can sometime see the eggs being laid. When they hatch, the newborn brine shrimp are less than half a millimeter (0.5mm) long, which is smaller than the period at the end of this sentence. They will grow to a maximum length of about one centimeter.

You may also see brine shrimp eating if you give them food. In addition to bacteria and algae, brine shrimp will eat yeast, egg yolk, and wheat flour. If you feed them yeast that has been dyed red, you can see the yeast moving past their appendages and through their food groove.

Questions:

  1. How do brine shrimp avoid being eaten?
  2. What are two ways that brine shrimp reproduce?
  3. What will you see if you look at brine shrimp under a microscope?
  4. Look at brine shrimp with a micro-ruler underneath them. Draw what you see in the circle, including the ruler, to show how big they are at low power.
  5. What else do you notice as you look at the brine shrimp?