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The purpose of this study was to examine the effects of three experimental variables on the stability and fecundity of a brine-shrimp community in a closed, micro algal-based microcosm. Xenic microalgal cultures and brine shrimp (Artemia franciscana Kellogg) in 300-cc acrylic spheres (referred to as biospheres or microcosms) were subjected to three experimental variables: (1) the culture volume (150 ml or 250 ml), (2) presence or absence of daily culture mixing (sphere rotation), and (3) a microcosm atmosphere that was either closed or open to the surrounding atmosphere. Each sphere started with 200 brine shrimp cysts and the microalga Nannochloris at a density of 5-7 x 107 cells/ml in a culture medium of 35 ppt total salts. The biospheres were monitored twice a week with closeup color photography, noting culture color and transparency, oxygen bubble accumulation, settling of planktonic algae, and the density, size, and maturity of the brine shrimp over time. The brine shrimp copulated by 21/2 weeks; a second generation of nauplii appeared in ca 30 days, temporarily increasing the density. These observations constitute the first report of brine shrimp sexual reproduction (ovoviviparity) in closed microcosms. The density of the brine shrimp appeared to stabilize during days 50-82, at an approximated 0.08 individuals/ml. Because all biospheres were yellow in the final weeks, i.e. showed no signs of Nannochloris, the brine shrimp were likely sustained by bacteria generated from the brine-shrimp feces. Blue-green algal filaments, a culture contaminant, at the air-water interface, out of the reach of brine shrimp, appeared to have provided oxygen in the absence of Nannochloris.