Eating is a full-time, upside-down job
Brine shrimp don't hunt or graze in any deliberate sense — they swim upside-down through the water almost constantly, and their legs do the work of catching dinner as a side effect of swimming at all.
Eleven pairs of legs, one filter
Each of Artemia's eleven pairs of leaf-like thoracopods beats in a steady, coordinated rhythm. That motion pulls water past the body, and fine bristles (setae) along each leg strain tiny particles out of it, passing them forward along a groove toward the mouth.
1–50 micrometers
Anything in that range — mostly single-celled algae and bacteria — can be caught by the fine filtering setae on the legs.
Continuous, not selective
Filter feeding isn't picky. Whatever edible particle drifts within range of the beating legs gets swept toward the digestive tract.
Grazing the bottom
When suspended food is scarce, brine shrimp will also scrape algae directly off rocks and sediment, or feed on organic mud at the bottom.
Mostly algae, plus whatever else drifts by
Microscopic algae
Green algae — species like Dunaliella, which itself thrives in extremely salty water — form the backbone of the brine shrimp diet in most habitats. Blooms of this algae are often what turn a salt lake visibly green or pink.
Bacteria & detritus
Bacteria suspended in the water, and fine organic detritus — decaying plant and animal material — round out the diet, especially in nutrient-rich salterns.
Newly hatched nauplii can't eat at all at first — their mouths and digestive tracts aren't functional yet, so they live entirely off their internal yolk reserve until their first or second molt.
What hobbyists feed them instead
Without a natural algae bloom on tap, home and hatchery cultures are typically fed a substitute — most often baker's yeast, powdered spirulina, or a commercial microalgae paste, dosed in small amounts so the water doesn't foul. More detail on exactly how that's done is on the Raising Them page.