Daphnia are filter feeders that eat primarily planktonic algae, along with bacteria, single-celled organisms, and tiny bits of organic debris suspended in the water. They sweep particles ranging from 0.5 to 40 micrometers in size, making them remarkably efficient at clearing fine material from the water column.
Natural Diet in the Wild
Green algae form the core of the Daphnia diet. In laboratory settings, the species most commonly used as food are Scenedesmus and Chlamydomonas, both single-celled green algae that closely mimic what Daphnia graze on in ponds and lakes. But Daphnia aren’t picky. Their filtering apparatus is fine enough to capture even bacteria, though algae make up the bulk of what they consume.
You can actually tell what a Daphnia has been eating by looking at it. Daphnia feeding on green algae appear transparent with a green or yellow tint, while those feeding mostly on bacteria turn white or salmon-pink. A healthy gut filled with algae looks distinctly green under a magnifying glass. If the gut appears partially brown or completely clear, that often signals low food availability or digestive problems.
Some species, particularly Daphnia magna, will also cling to plants or other surfaces and browse for small particles, adding a bit of grazing to their otherwise filter-based feeding strategy.
How Daphnia Filter Their Food
Daphnia have five pairs of thoracic legs tucked inside their shell-like carapace, and the synchronized beating of these legs creates a steady water current. Water flows between the two halves of the carapace, and as it passes through, fine mesh-like structures on the third and fourth legs act as filters, trapping suspended algae and other particles. Comb-like structures on the second and third legs then sweep the captured food into a groove that channels it toward the mouth.
This system works on particles as small as 0.5 micrometers (roughly the size of a bacterium) and tops out around 35 to 40 micrometers. For reference, a human hair is about 70 micrometers wide, so Daphnia are filtering particles invisible to the naked eye. Their feeding rate scales with how much food is available: when particle concentrations drop, so does their filtering efficiency.
Nutritional Requirements
Algae aren’t just convenient food for Daphnia. They supply essential fatty acids that Daphnia cannot produce on their own. Only about 2% of the fatty acids in a Daphnia’s body are made internally. The rest come directly from the diet. Among the most critical is eicosapentaenoic acid (EPA), an omega-3 fat that plays a major role in reproduction. Daphnia allocate a disproportionate share of their EPA into eggs rather than body tissue, which means a diet low in this fatty acid hits reproductive output hard.
Daphnia can technically convert precursor fats like alpha-linolenic acid into EPA, but the conversion isn’t efficient enough to sustain optimal growth. Cholesterol is another essential nutrient, and it’s notably absent from cyanobacteria (blue-green algae), which is one reason cyanobacteria make poor food compared to true green algae. When essential lipids run short, both growth rate and offspring production decline measurably.
Feeding Daphnia in Home Cultures
If you’re raising Daphnia at home, typically as live food for aquarium fish, the simplest approach is green water. Green water is just water rich in suspended algae, and it replicates the natural Daphnia diet almost perfectly. You can culture it by placing aged aquarium water in a container with bright light for 14 to 24 hours a day, then adding a nutrient source to fuel algae growth.
Several methods work well. One straightforward option: keep a single goldfish in a gallon bowl with strong light. The fish waste fertilizes the water, and algae bloom naturally. Another approach is to add a crushed lettuce leaf and a small amount of liquid plant fertilizer to tank water, then let it sit in bright light until the water turns green. Water left over from boiling potatoes also works as a nutrient base, placed near a sunny window and topped off with more starchy cooking water as needed.
For supplemental feeding beyond green water, a common recipe combines two teaspoons of soy flour, one crushed multivitamin tablet, one spirulina tablet, and six ounces of water blended together. In a separate container, dissolve a pinch of active yeast in eight ounces of tank water and let it sit for 20 minutes. Mix two teaspoons of the soy blend into the yeast water and feed small amounts to your culture. Store the remainder in the refrigerator. Yeast alone, or yeast mixed with pea flour, also works as a starter food in spring cultures.
Signs of a Well-Fed Culture
Color is your best diagnostic tool. Daphnia with bright green guts are eating well. A brownish or orange tint throughout the body can indicate lipid accumulation, which is normal fat storage. Red coloration, on the other hand, signals low dissolved oxygen rather than a diet issue. Daphnia produce extra hemoglobin when oxygen drops, turning visibly red as a survival adaptation.
Clear or empty-looking guts are a warning sign. They point to insufficient food, trouble with the feeding apparatus, or digestive problems. If you notice this in a home culture, increase feeding or check that your green water is actually dense enough with algae.
Overfeeding and Water Quality
More food is not always better. Excess organic matter in a Daphnia culture breaks down into ammonia, which is toxic to all aquatic organisms. Ammonia toxicity increases as pH rises, so cultures running alkaline are especially vulnerable. When you add manure or other organic fertilizers to feed the microbial food web, the decomposition process consumes dissolved oxygen while generating ammonia, creating a double threat.
Aeration helps by keeping dissolved oxygen within the range Daphnia need for respiration, and it prevents the bacteria decomposing your food inputs from outcompeting the Daphnia for oxygen. If you’re using manure-based methods, soaking the manure for well beyond three days before adding it gives ammonia more time to break down. Monitor your culture for population crashes, which are often the first visible sign that ammonia or oxygen levels have gone wrong.