Brine Shrimp: Biology, Ecology, and Aquarium Use

Introduction to Brine Shrimp

Perhaps no other aquatic species contributes to oceanic ecology quite like the brine shrimp. Brine shrimp are a form of zooplankton, versatile in their physical and chemical characteristics. Their hardiness and survivability make them especially well suited for preservation, and they are commonly sold for home aquariums. They have a fantastic ability to adapt to variations in temperature, dissolved oxygen content, and salinity. Their appeal to home aquarium enthusiasts includes an almost chameleonic range of colors, which they adopt based on their food intake. Brine shrimp have an elongated body and eleven pairs of legs — a body plan that easily explains why they are commonly confused with true shrimp and how they acquired the name.

Brine shrimp are also known as Artemia salina, though this is more of a generic name, as several distinct varieties exist. They are arthropods belonging to the class Crustacea. As zooplankton, they serve as a major food source for other oceanic life, including Daphnia and copepods. Brine shrimp live in salt swamps, man-made evaporation ponds used to extract salt from seawater, and in salt lakes such as those found in the intermountain desert region of the western United States.

Habitat, Salinity Tolerance, and Behavior

Because of their high salt tolerance, brine shrimp are not subject to predation by many species. Adult Artemia can tolerate salinities of up to 50%. This means they are literally at home in the salt ponds where salinity ranges from approximately 2.9% to 3.5%, and they also thrive in environments like the Great Salt Lake in northern Utah, where salinity ranges between 25% and 35%. Interestingly, brine shrimp can also tolerate and even thrive in brackish water environments. While they are largely free from predation, they have limited options when it comes to their own nourishment — a constraint that, paradoxically, contributes to their hardiness.

Like many primitive aquatic organisms, brine shrimp are attracted to light. They rise toward the surface during the day and sink at night. This positive phototaxis in Artemia keeps them at the same depth as their prey. Phototaxis is defined as locomotion in response to light, and the phototactic behavior of brine shrimp is not yet fully understood. Adult brine shrimp swim away from light, while larval forms swim toward it. In all stages, the shrimp orient themselves so that their ventral surface faces the light source. A pH ranging from 8 to 9 is optimal for brine shrimp — a range that corresponds to the pH of most salt lakes and solar evaporation ponds in which they naturally occur.

Physical Characteristics and Color Variation

Environmental factors greatly influence the physical characteristics of brine shrimp. Under normal circumstances, they feed almost entirely on the photosynthetic green alga Dunaliella. The oxygen content of the water determines their physical appearance, which is in turn a direct consequence of what they are able to consume. With a good oxygen supply, Artemia appear pale pink or yellow. When green algae are abundant, they take on a greenish hue. If oxygen levels in the water are low, with large amounts of organic matter present, or if salinity has increased due to evaporation, the Artemia will feed on bacteria, detritus, and yeast cells rather than algae. Under these conditions, they produce hemoglobin and appear red or orange in color.

Life Cycle and Reproduction

If conditions are ideal for growth, reproduction is rapid and a self-sustaining Artemia population is possible. Many aquarists and hobbyists purchase Artemia from stores as a brownish powder — a testament to the organism's remarkable resilience. Each particle of this powder is known as a cyst: a dormant, encased embryo that is metabolically inactive. The Artemia life cycle begins with the hatching of these dormant cysts. The cysts can remain dormant for many years as long as they are kept dry. When placed back into salt water, they rehydrate and resume development. Ideal hatching conditions involve soaking in salt water for approximately 15 to 20 hours at 25°C.

Development proceeds as the cyst wall breaks open, releasing the embryo. Initially, the embryo remains attached to the cyst wall, suspended in the water in a manner resembling a floating parachute — a stage therefore called the Umbrella stage. At this point, the embryo enters the characteristic crustacean larval form known as a nauplius. The nauplius is the primitive crustacean larva: the first larval stage to emerge from the egg and the earliest free-swimming phase in crustacean development. It generally has an oval-shaped body, three pairs of appendages, and a single median eye near the front. In terms of sheer numbers, the crustacean nauplius is considered by some to be the most abundant type of multicellular animal on Earth and serves as an important food source for fish and predatory invertebrates.

In the first larval stage, the brine shrimp nauplii are brownish-orange in color due to their yolk reserves. They cannot yet feed because their feeding apparatus is underdeveloped. Approximately 12 hours after hatching, the second larval stage begins, and the nauplii start filter-feeding on particles of microalgae, bacteria, and detritus. Growth continues through 15 distinct molting stages before adulthood is reached, a process taking approximately eight days. Adult Artemia average about 8 mm in length but can grow up to 20 mm under ideal environmental conditions.

The proliferation of brine shrimp naturally depends on temperature and other environmental conditions. In low salinity and at optimal food levels, fertilized females produce free-swimming nauplii at a rate of up to 75 per day. Females have a brood pouch from which active young are liberated under favorable conditions, producing 10 to 11 broods over an average life cycle of 50 days. Under exceptionally ideal conditions, an adult Artemia can live as long as three months and produce up to 300 nauplii or cysts every four days. Cyst production is induced by high salinity, chronic food shortages, and large fluctuations in oxygen levels between day and night.