By Kenneth Wingerter
Properly caring for even a small, simple aquarium system can be fairly time consuming, leaving many aquarists eager to cut a corner or two to save a little time. One common way that fish keepers accomplish this is by using prepared, store-bought foods.
To be sure, the measured use of certain high quality prepared foods is usually acceptable. The inclusion of some whole frozen items is markedly better. And a highly varied combination of prepared and frozen foods is better yet. But whatever the feeding regimen, the use of live foods has been proven time and again to greatly enhance the immunity, digestion, growth, coloration, and general health of captive species. Further, it offers the opportunity to observe the natural feeding response of one’s aquatic animals.
Certain small cladoceran crustaceans, such as Daphnia and Moina, not only serve as highly nutritious live food items, but are quite easy to culture in perpetuity.
Meet the Cladocerans: Daphnia and Moira
Daphnia spp. and Moina spp. are closely related and belong to the animalia order Cladocera.
(Editor’s note: For the sake of brevity, the author is using the species name Daphnia throughout most of this article to refer to both daphnia and moira.)
The cladocerans are a group of small, primitive, and mainly freshwater planktonic filter-feeding crustacea. They have a carapace that covers the entire body except the head. Their flattened, leaflike legs (or phyllopodia) are used for suspension feeding as well as for respiration. Cladocera are commonly referred to as water fleas due to the hopping motion they make when moving about in the water.
Daphnia are distributed throughout much of the world, though they are less abundant in the tropics where water bodies are usually nutrient poor (only six of the 50 daphnia species occur in the tropics). They prefer warm, still, or slow-moving waters with heavy organic loads. These may be ephemeral bodies of water, like rock pools, where conditions only sporadically allow for growth and reproduction.
Daphnia Reproduction and Lifecycle
Cladocera are capable of both sexual and asexual reproduction. Parthenogenesis (which loosely means “virgin birth”) is the production of offspring from eggs that have not been fertilized by a male. Individuals produced in this manner are exact clones of their mother. Consequently, males tend to be outnumbered greatly by females. Uniparent, parthenogenetic reproduction is very important among cladocerans.
Most spring and summer eggs are amictic – eggs that do not require fertilization from a male. These amictic eggs hatch parthenogenetic (developed without fertilization) females, which go on to reproduce by parthenogenesis, commonly referred to as cloning.
By autumn, when the population is subjected to crowding or environmental stress (e.g., unfavorable seasonal changes), females switch to a sexual reproductive mode. Under these circumstances, females produce two types of eggs: mictic eggs – eggs that require fertilization and contain only one set of chromosomes (haploid) – as well as male haploid eggs, which hatch parthenogenetic males. The males subsequently fertilize the mictic eggs, resulting in the production of diploid eggs (eggs containing two sets of chromosomes), which then become resting eggs. A female can produce three or four broods of resting eggs from a single fertilization event. These sexually produced offspring preserve genetic diversity within the population, thereby increasing their ability to adapt to an ever-changing environment.
Resting eggs are different from eggs that are produced during the regular growth season. Dark, roughly rectangular and a mere 1-2 mm in length, they are able to withstand dry conditions (desiccation) and cold (some species can survive freezing). Much of the durability of these resting eggs owes to a double layer of chitinous material, a tough shell-like material surrounding the eggs, referred to as the ephippium. The eggs develop in a brood pouch attached to the mother’s body and are released when her exoskeleton is molted. The encased eggs are then carried away by the movement of the water.
Ephippia can long persist in a resting state (diapause) until the return of favorable environmental conditions. Trapped in mud within or around the pond, they can remain viable for years.
As the days grow longer and water temperatures increase, resting eggs hatch and renew the cycle. Length of the embryonic maturation period depends upon temperature, ranging from 2 days at 25°C to 11 days at 10°C. All of the hatchlings are asexually reproductive females. Juvenile daphnia are more or less miniature versions of their mothers, rapidly undergoing a short series of instars (the periods between developmental stages) before reaching maturity.
During the regular growth season, a healthy, parthenogenetic female daphnia may clone itself numerous times. A new brood is produced every couple of days or so. While they produce an average of six broods in a lifetime, they may produce as many as 22. An individual can, under ideal conditions, produce over 100 eggs per brood. High summer birthrates counter losses from predation, which also peaks at this time.
Where to Buy Daphnia Eggs and Live Daphnia
Daphnia and moina starter cultures can be readily obtained by any home aquarist (primarily online). Sources for daphnia starter kits and starter cultures abound, from ebay and amazon to many aquarium and scientific supply companies. (Always check the background and/or buyer feedback of any seller before sending them money.)
You should have your culture system in place before ordering your starters (details on how to organize your first culture system follow below). When receiving and opening your shipment, don’t worry too much if the culture looks weak. With some time and good living conditions, even a few healthy individuals will eventually turn into a large, stable, healthy population.
What Size Daphnia is Best for Culturing?
Daphniids vary greatly in size. Still, even the smallest of them may be larger than newly hatched artemia (brine shrimp). Therefore, while daphnia are fine for juvenile and older fish, they are not well suited as a food source for larval fish, due to their size.
Larger daphniid species seem to have much lower carrying capacities; that is, they reach their population tolerance sooner than smaller species, limiting the number that can be kept in an enclosed population. Egg production of the behemoth D. magna plummets as population density reaches 25-30/L. Daphnia can rarely be maintained in continuous culture at densities over 500/L, whereas moina can easily be kept at densities as high as 5,000/L. Moina have been shown to be 3-4 times more productive than daphnia.
Actual productivity, of course, will vary quite a bit with differences in culture method. Whatever the method, the primary aim is to steadily maintain those conditions that favor uninterrupted parthenogenetic reproduction. This requires some monitoring of water quality, temperature, aeration, photoperiod and feeding.
Starting Your Daphnia Tank
This very simple culture method can provide more than enough live feed to meet the needs of most home aquarists. This method combines aspects of batch culture and continuous culture for relatively trouble-free operation that can be utilized for extended periods of time. All that is required is a couple of containers, an air pump, a light with a timer, and a few square feet of floor space.
The culture vessel can be any small, clean container: a standard 5-20 gallon fish tank, plastic storage bin, or large bucket (e.g., a 5-gallon Homer bucket). Vessels should be kept away from areas that are breezy, in direct sunlight, or in any area that is subject to large temperature fluctuations.
Culture water should be 18-20°C for daphnia and 24-31°C for moina. Position the light over the vessels and set on a 12- to 20-hour photoperiod. Maintain a pH of 6.5 to 9.5. Keep ammonia concentrations below 0.2 mg/L.
Only purified water should be used, as daphnia are highly sensitive to contaminants such as metal ions. Air may be delivered via an open-ended section of rigid tubing. Air flow should be moderate. Diffusers should not be used, as small bubbles can get trapped within the animal’s carapace (shell).
It should come as no great surprise that daphnia are an excellent source of nutrition for small fishes; countless species the world over have heavily relied upon this abundant resource for eons. Fish keepers have been touting the benefits of feeding live daphnia for about as long as people have been keeping fish.
The nutritional value of live daphnia depends greatly upon what it eats. Direct enrichment of daphnia is easy and effective. Choose a smaller alga with a well-rounded fatty acid profile. Tetraselmis green alga and Spirulina alga are excellent choices of feed and can be found in aquatic supply stores, along with other algae options. One may supplement B-vitamins with the use of active baker’s yeast, though only very sparingly due to its ability to quickly foul the culture water. Daphnia are typically about 50 percent protein by dry weight, and moina even a bit more, making them especially useful for growing out juvenile fish.
The use of an automatic feeder is best, though daily manual feeding will suffice. Store-bought frozen algal pastes are a nutritious and cost-effective source of food. Enough food should be added to produce a green tint to the water (approximately 105 to 106 cells/ml). Culture water should not be allowed to clear for long, if at all; at the same time, one must always be careful to avoid overfeeding.
Harvesting Daphnia to Feed to Fish
Only one of the two vessels are harvested at a time. Using a rotating schedule, one vessel can be harvested every one or two days.
Harvest is best undertaken a few hours after re-greening the culture water, as to allow for maximal enrichment of the daphnia.
To collect the animals, it is best to use an appropriately sized (something on the order of 50- to 150-µm) plankton screen. A short length of tubing can be used to direct the water through the drain to the screen, which should sit in some water to avoid leaving small air bubbles on the animal. The harvested batch can be temporarily transferred to a bottle before being fed out to the fish, but should be used as soon as possible.
Harvests will generally alternate between the two vessels; however, always harvest and restart any culture that appears to be declining. Take care to avoid feeding spoiled food; all thawed, unused portions of algal paste must be refrigerated and used within a few days or discarded.
Preparing the Tank for Each New Daphnia Population
The emptied vessel should be cleaned thoroughly; no organic film should be allowed to grow on its inner walls. Be sure to reclose the valve at the bottom before refilling. Next, approximately 25 percent of the content of the other vessel is removed and added to the clean, empty vessel. Each vessel is then filled to the surface with purified water and re-greened as needed. Be sure to immediately restore the air supply.
Culturing Daphnia: If at First You Don't Succeed…
Once, a colleague of mine at a fish repository was having difficulty raising a small population of moina (to his credit, the culture vessel he was attempting to raise them in was very small and only appropriate, maybe, for something like small cultures of Paramecia). At that time, I was managing the grow-out operation for an affiliated research center and was eager to try them out. Having found my colleague’s culture in pretty bad condition, I was able to rescue no more than a dozen individual moina. But, after transferring them to a more suitable vessel, the few survivors rapidly proliferated.
In short time I was able to start another such vessel, and then I was able to use these two vessels to repeatedly seed a battery of much larger vessels from which I consistently produced an enormous quantity of live feed for over several years.
But, towards the end of my time there, it all came crashing down. After one long, busy weekend during which I neglected to perform the routine maintenance on the cultures, they declined to the point of no return. While I regretted losing such a valuable resource after such a good run, it was a truly enlightening experience.
Practice Makes Perfect
By exercising some control over the culture environment and keeping reasonably sanitary conditions, a couple of vessels (or a bunch, for that matter) can generate a good amount of high quality live feed for months or years. The routine tasks of maintaining daphnia cultures become easy (and maybe even fun) with some practice. The greatest reward for taking on the extra work, though, is the obvious positive impact it will have on the health and appearance of the aquatic animals in your keep.