Homegrown Food - Who Is This Daphnia Anyway?

What is Daphnia? Daphnia, also known as “water fleas,” are a part of the family Daphniidae. “Daphnia” itself is a genus within this family, in the same way that “Homo” is the genus of humans paired with the species name to form our full scientific name “Homo sapiens”. These small planktonic (meaning things that are unable to propel themselves against the current) crustaceans reach a size of a little more than half a centimeter, tip to tip and gain their common name from the way that they “jump” erratically from place to place as they swim through the water. Daphnia are found almost everywhere in freshwater except for truly extreme habitats such as hot springs. They can live for as long as a year in environments with limited predators but more commonly are found to live for about half of that, five to six months. They use their antennas to move through the water and their appendages help them move water for feeding. They mostly feed on single celled species of algae and also detritus with the protists and bacteria that live in or on the detritus as well.

While Daphnia are largely a freshwater species, there are some living in saline lakes. Some species of Daphnia even have recognition as threatened species. Daphnia are a great live aquarium food choice for culture; their ease of culture paired with their relatively large size also make them great for scientific studies. As a filter feeder they readily absorb anything added to the water and their internal organs can be observed readily under a microscope to easily monitor the effects. It is not an uncommon experiment to use small quantities of alcohol or caffeine to show the increase or decrease in the heart rate of daphnia under a microscope by dosing the water they are living in directly.

Daphnia reproduce in two basic ways: sexually where males fertilize eggs and parthenogenically where females essentially clone themselves and their eggs hatch into females that will grow up to either repeat the process or have eggs that are fertilized by males. The life history of Daphnia gets more interesting actually – in sexual reproduction the eggs hatch internally after one day but continue to live and grow inside the female for up to three more days. After hatching the young will go through four to six more instars, or molts over a five-to-ten-day period before they are sexually mature and ready to start reproducing themselves. (This is great news for us aquarists who want to raise them as a live food for our fishes!) But this isn’t all that Daphnia are capable of! Sometimes the habitat of Daphnia can be come unsuitable. Frequently this is because the pool that they are living in starts to dry out, and when this happens crowding occurs because a growing number of Daphnia are stuck in a shrinking volume of water. Detecting this the females will start to lay eggs with an extra thick shell that can resist higher and lower temperatures and even drying out. When conditions become more suitable the eggs hatch and a new generation of Daphnia can proliferate!

So, Daphnia are great and all, but why are we here specifically? Well, it turns out that Daphnia are fairly easy to culture, and therefore make a reasonable live food to add to a freshwater fish room. Dahnia are great for small fish – as they can contain over 80% protein when feeding on alga vs a common substitute for culture such as yeast. In all honesty, 80% protein is too high to be a regular staple food item for a fish (adult fish are looking for something more along the lines of 30-50% depending on what type of fish and other factors) but it makes a great treat and can be a great addition to young, growing fishes' diet or as a feed to encourage older fish to come into breeding condition.

Culturing them at home is simple! Daphnia do best in containers that have lots of surface area; basically, meaning that the culture container should have the largest footprint, and the shallowest depth possible. This can be an aquarium, or something as simple as a Rubbermaid container. Containers can be as small as a few gallons in volume to massive stock tanks that replicate a small pond. You can fill this container with aged water from aquariums – this will help get the system kick started and help prevent a cycle. It will also have some algae and detritus for the daphnia to feed on. That said, water from an aquarium might also be host to a variety of small Daphnia predators, and so the water can be aged or sterilized before adding it. Daphina are sensitive to different metals and additives in city water, so be sure to treat the water, or at the very least use a dechlorinator. Daphnia want to be in the high 60’s and low 70’s Fahrenheit as far as temperature goes. Once a month change out half of the water, again replacing it with aged tank water or dechlorinated water. Having light on your culture will help to grow algae and biofilm for the Daphnia to eat, and some aeration is also good especially if the culture container is slightly on the deeper end. Try to make there be an area of more and less flow in the habitat so that the Daphnia are not constantly being blown around – they come from relatively calm waters in the wild generally. Adding some floating plants like duckweed can help with keeping water quality good and also bring more beneficial life to a culture.

Daphnia can be fed whenever the water seems clear – and the easiest things to feed them are dry yeast (just mix it into some water before adding) and spirulina algae (same process as with the yeast). If you are culturing live freshwater algae, that is the best thing to feed them, and be generous. These filter feeders will clear the water rapidly, and their population will grow rapidly as well. Always start with smaller food quantities and move up but watch the clarity of the culture and you can see how fast they are eating it. When the water is clear again, it's time to feed.

Harvesting is also important because as we talked about having too many may cause them to slow reproduction as they wait for better conditions to exist. You can suck out Daphnia with a turkey baster or scoop them out with a brine shrimp net. These can be added directly to the aquarium, or they can be stored in some tank water for a few hours. You’ll be surprised by how big they can get – they may be too large even for some smaller, adult tropical fish like tetras or Endlers live bearers at adult size.

While some Daphnia can be invasive – most of the species you can find are common and can even be collected from your native waters. Acquiring a culture doesn’t need to be that hard though, ask on local fish groups or at your local fish store for a starter culture. From a protein rich live food for younger fish to a great way to provide enrichment or conditioning to breed in larger fishes, Daphnia have a place in almost every freshwater fish room.