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Feeding: Daphnia

Daphnia are small freshwater crustaceans that may also be known as water fleas. They are called this because of their short jerky hopping movement through the water. There are many species of Daphniidae and their distribution is world wide. Of all the species, the genera of Daphnia and Moina are the most diverse, and are a major food source for both young and adult freshwater fish. In the orient Moina is the species of Daphniidae most used in fish culture.

There is a big size difference in the Daphniidae, depending on the species. Newly hatched Moina are slightly larger than newly hatched brine shrimp, and twice as big as average adult rotifers, but newly hatched Daphnia are twice as big as Moina, and may not be suitable for some of the smaller fish fry because of their size.

The daphnia has both sexual and asexual phases. In most environments, the population consists entirely of females that reproduce asexually. Under optimum conditions, a female may produce more than 100 eggs per brood, repeating every 3 days. A female may have as many as 25 broods in its lifetime, but the average is about 6. The female will start to reproduce at about 4 days old with a brood size of 4 to 22 eggs. Under adverse conditions, males are produced, and sexual reproduction begins. The result is the laying of resting eggs, just like the brine shrimp. Factors that can trigger this are a lack of food, low oxygen supply, a high population density, or low temperatures.

The nutritional content of Daphnia varies with age, and what its been eating. The protein content is usually around 50% of dry weight. Quite the opposite from Artemia, adults normally have a higher fat content than juveniles, about 20-27% for adults, and 4-6% for juveniles. Some species have been reported to have protein contents exceeding 70%. Live Moina are about 95% water, 4% protein, 0.54% fat, 0.67% carbohydrates, and 0.15% Ash. The fatty acid composition of food is important to the survival and growth of fish fry. Omega-3 highly saturated fatty acids are essential for many species of fish. Moina cultured on bakers yeast are high in monoenoic fatty acids. By using what is called w-yeast (yeast enriched with cuttlefish oil), Moina will contain very high levels of Omega-3 fatty acids. Moina can take up lipids very easily from the emulsion, but there is a side effect to this, apparently it also slows productivity, so this emulsion should only be fed to a batch separate from the main growout colony. Commercial formulas are available in pet supply houses for the enrichment of Artemia, Rotifer, and Daphnia cultures.

Written by Kai Schumann