Raising Daphnia Magna as Freshwater Fish Feed

By Joshua Matthews

A Dense culture of daphnia magna in the UNL ABC lab

Introduction

Daphnia, sometimes referred to as ‘water fleas’ are a type zooplankton; teeny tiny creatures that live in most open bodies of water including ponds, lakes, and the ocean. These micro crustaceans are an important source of food for many fish and other aquatic species. Daphnia feed on micro algae, bacteria, and small organic particles that fit into their mouths. Grazing on microorganisms and feeding larger creatures such as fish, daphnia and other zooplankton are critical players in shaping food web dynamics.

Through research at the University of Nebraska Lincoln Aquatic Biodiversity and Conservation laboratory I have observed Daphnia Magna to be an ideal source of food for raising fathead minnow fry due to the large size of adults and ease of working with the organisms. When using Daphnia Magna as a live feed for fish hatchling, stable population of adult daphnia can be maintained alongside the fry. After hatching many species of fish are too tiny to eat adult daphnia however, they can eat the nutrient rich daphnia spawn. This creates a unique predator prey dynamic until the fish grows large enough to eat adult daphnia.  

Biology and Life Cycle

Daphnia Magna are among the largest zooplankton reaching around 5mm (0.2 inches) in length. Daphnia have a pair of large antennae branching out from the top of their head that is used for locomotion and capturing food. While they do have some ability to move around in a twitchy manner using these antennae, daphnia have little ability to resist the flow of water.  Daphnia have a single compound eye that is used to detect light, which the daphnia are likely attracted to due to the correlation between the presence of light and algae.

As members of the branchiopod family of crustaceans, daphnia use a reproduction strategy known as cyclical parthenogenesis and can perform both sexual and asexual reproduction. Asexual reproduction, where female daphnia reproduce female clones, is their most common form of reproduction under ideal conditions. When environmental conditions begin to turn unfavorable the females begin to produce male clones, who then fertilize haploid eggs which can withstand long periods of unfavorable conditions including drying out.  This reproductive strategy makes it possible to revive a crashed colony by collecting some of the potentially egg containing sludge from the vessel and transferring it to an environment with favorable conditions.

Daphnia reproduction cycle sourced from wiki demonstrating asexual pathogenic reproduction as well as sexual reproduction via production of male offspring and fertilization of the haploid egg.

Culturing Techniques

Daphnia grow and reproduce best when provided a constant supply of food. When culturing daphnia they can be fed live or powered algae and yeast cells. When feeding the daphnia powdered feeds, the powder should be suspended in water prior to feeding. Protein shake bottles can be very helpful for breaking up clumps of powder. Culturing live algae for daphnia is the best way to replicate their natural diet and can be a rewarding exercise. When feeding daphnia it is best to overfeed than underfeed; after feeding you should not be able to see through the water.

At the ABC lab our daphnia magna have been found to reproduce well in temperature ranging from 16℃ (60 ℉) to 25℃ (77 ℉), however they can tolerate a much wider range. Daphnia Magna are typically found in freshwater and brackish environments with salinity concentration of up to 10 parts per thousand (Zandereev, 2022).  This wide range of temperature and salinity tolerances make Daphnia Magana a very forgiving organisms to work with.

Daphnia can be cultured in vessels of all sizes and types from test tubes to ponds. Larger containers are easier to care for as they are typically slower to changes in water parameters, reducing shock and stress on the animals. Daphnia Manga do well with relatively little water disturbance. In the ABC lab we have had success using various sized aquariums with air powered sponge filters providing gentle filtration and aeration for the animals.

Harvesting daphnia is extremely easy. One option is just to dip in a container and scoop out water full of daphnia. they can also be netted or filtered out of the water. Fine grain nets with a flat surface such as those designed for brine shrimp are ideal. After catching daphnia in their culture vessel, they are easily dislodged from the net by scooping it through the tank being fed in the opposite direction from which the daphnia were caught.

Conclusions

Daphnia Magna are an easy to culture source of food for many freshwater species. They can rapidly reproduce asexually making them a sustainable source of food, and their ability to sexually produce eggs that can withstand long periods of unfavorable conditions make them an extremely forgiving species to work with. Daphnia Magna specifically is unique in its size, allowing for unique predator-prey relationships with fish hatchling and fry. These tiny creatures are fantastic options as sustainable sources of food for freshwater aquaculture and aquarium fish keeping.

References

Zadereev, E.S., Lopatina, T.S., Ovchinnikov, S.D. et al. The effect of salinity on the grazing rate and survival of Daphnia magna females adapted to different salinities. Aquatic Ecol 56, 639–652 (2022). https://doi.org/10.1007/s10452-021-09941-7