The use of Daphnia magna as a food for tadpoles

John Skillcorn

What are they?
Daphnia are crustaceans, like crabs and lobsters and in keeping with most arthropods, have a hard, outer shell.  They are essentially fresh water animals, commonly called 'water fleas'.  However, this is only because of the manner by which they swim - a sort of hopping or jigging in the water, a movement brought about by their swimming appendages.  Having said that, one species under discussion here is called Daphnia pulex.  Pulex irritans is the human flea, but here any similarity ends.  True fleas are insects.

Daphnia pulex

Basic Biology
Daphnia magna is a large, indigenous British species often reaching several millimetres in length, naturally inhabiting hard-water ponds where it filter-feeds on phytoplankton and other micro-organisms.  Under optimum natural conditions reproduction occurs asexually by a form of parthenogenesis.  During this phase the populations are often exclusively female which produce eggs in a brood chamber and, without fertilisation, give birth directly (ovoviviparously) to young, which are clonal parthenogenetic females. In nature this usually occurs during the summer months when adequate food, high temperatures and long day-length hours stimulate and support large populations of other parthenogenetic Cladocera.

Sexual reproduction, with the subsequent production of dormant eggs, occurs during adverse environmental factors.  These factors could be high population density, food shortages, accumulation of toxic metabolites and seasonally reduced daylight and temperature cycles.  Exposed to these adverse conditions, the production of parthenogenetic eggs diminishes, while some males develop from these eggs instead of females.  At the same time, some females produce darker eggs that require fertilisation by the males, and during ecdysis these are shed with the carapace as ephippia.   

Dormant ephippia are resistant to freezing, desiccation and digestion and may remain viable for years, thus ensuring continuity of species over inclement seasons, i.e. winter and even drought.  Consequently, during cyclic fluctuations parthenogenetic females, males, sexual females and ephippia may exist simultaneously in Daphnia populations.  However, under laboratory controlled parameters of food, light and temperature, conditions can be simulated for the constant production of mainly parthenogenetic populations. 

Theory of laboratory culture of Daphnia magna
Under ideal conditions, and in contrast to most other species of Daphnia, D. magna is maintained in a hard-water medium of approximately pH 8.  Food can be almost any microscopic plant material, the one often used being the alga Scenedesmus quadricaudus, shown right.  Both predator and prey are kept between 12-22°C either under natural light, or preferably fluorescent light, it being either continuous or controlled by a timer to give a photoperiod of about Light:Dark (L:D) 14:1O hours.  Consequently large, vigorous populations of parthenogenetic Daphnia can be reared in unlimited quantities, the only dependent factors being the size of the holding receptacles (aquaria etc.) and the proficient harvesting of algal food.  Adhering to these conditions the predominant population will be parthenogenetic females.  However, usually always present are some ephippial females.  Thus, ephippia may be collected by sieving an old culture, and can be stored dry in a refrigerator where they remain viable for long periods.  They can be induced to hatch in fresh hard-water by successive freezing and thawing: 8 thaws and freezes give the maximum yield.

Scenedesmus quadricaudus

As with any aquatic organism, water quality is the main concern when setting up viable habitats.  Because it prefers slightly hard water (170 mg carbonate hardness), D. magna will do well in many parts of the UK.  Hardness can be controlled if necessary by the use of substances produced commercially to assist the keeping of some hard-water tropical fish such as Malawi cichlids.  Most other species of Daphnia can be successfully maintained in rain water, or soft tap water that has been allowed to stand long enough to release the chlorine.  The optimum here would be around 80 mg carbonate hardness.  Water hardness test kits are available at aquarist shops.  Distilled and deionised water must be avoided as this is a little too pure.  It should be remembered that Daphnia still do require small amounts of minerals to assist in the growth of their shells.

Temperature is not critical.  However, for continuous culture - especially during the winter - you must attempt to control temperature at least to some degree.  Many of the UK species do well at anything above 10° Celsius up to a limit of, say, 22°C.  The ideal for D. magna is around 20°C.

As has already been suggested, the L:D ratio is important, particularly so if you are attempting to culture algae as a food source for the Daphnia.  This is recommended due to the fact that it is virtually impossible to pollute your Daphnia cultures with decaying organisms as might be the case if using, for example, yeast.  And after all, algae and diatoms are the natural food sources of these crustaceans.   Although Scenedesmus has been recommended as the ideal food, it is by no means essential.  Any container of water left in bright light e.g. sunlight for a week or two will become colonised by a variety of species of microscopic green plants, many of which would be ideal food organisms.  They develop naturally from spore-like capsules carried about on the wind.  The culture of Daphnia hinges, therefore, on the successful culture of algae.

Practical culture of algae
Being green plants, algae such as Scenedesmus need various minerals, and these can be provided by adding one of the proprietary organic fertilisers available from garden centres.  Miracle-Gro has been found suitable and should be used at the rate of around a teaspoon per gallon of water.  Using an aquarium air pump, air is gently bubbled through the solution which has previously been seeded (if possible) from a viable source of diatoms and other algae.  This is not critical, however, but will give faster results.  The container can be something as simple as a plastic lemonade bottle.  Two days later an exact copy can be set up, and two days later another and yet another, and so on until a two-week supply of seven bottles is set up.  The bottles are put in a light position - either in sunlight or stood around a fluorescent light source.  Within these two weeks, light and temperature permitting, the water in the bottles will turn bright green.  In the meantime, the Daphnia containers can be set up.

Sources of Daphnia  
These could be your local aquarist dealer (for D. magna among other species), your local pond or, indeed, any body of fresh, clean, still water.  It is indeed amazing where these animals may be encountered.  For example, I have come across dense clouds of them in drinking troughs for cattle and other large livestock.  These were in fields quite a distance from any pond or body of permanent water, and were accompanied by various aquatic insects and copepods.  They are also found in ditches with either still or very slow moving water.  One is left wondering how they actually get there.  They can be fished out easily using a fine-mesh net, but you should make sure that you are not capturing other organisms likely to either injure tadpoles or deplete the Daphnia cultures themselves.

Practical culture of Daphnia
Plastic containers are best for the Daphnia cultures.  These can be obtained from a variety of sources, but in the UK DIY supermarkets often have large, plastic storage boxes on special offer in groups of threes or fives at very low prices.  These would be ideal, although it must be said that some plastics are unsuitable.  Trial and error need to be observed here.  Of course, glass aquaria would serve very well, and while being inert would not be quite as tough, portable or convenient as plastic.

Suitable water from a reliable source is placed into the boxes and allowed to stand (especially if it's tap water straight from the tap) for twenty four hours.  After this period of time, and if the algal cultures are bearing fruit, the Daphnia can be added.  It is preferable, although not essential, to very gently agitate the water with an air line without a diffuser stone.  You must avoid fine bubbles as these can lodge in the carapace of Daphnia and hence be transferred to tadpoles.

Next, empty the oldest algal culture in its entirety into the Daphnia container(s).  Refill the bottle with fresh culture solution, reseed with algae from bottle number two, and replace the airline.  Repeat this process at two-day intervals with the remaining algal culture bottles.  Reseed each new culture from the next oldest in the line.

If all this seems too much of a bother, with very careful management, Daphnia can be fed on yeast, or the bacteria and other unicellular organisms that result when decaying vegetable matter e.g. lettuce is placed in water.  However, because these types of organisms rely to a greater or lesser extent on the decay of organic matter, it is comparatively easy to pollute a Daphnia culture by feeding to excess, with disastrous results.  The use of living diatoms and algae is therefore the recommended procedure.  Incidentally, the effects of decomposing matter can be counterbalanced to some extent if freshwater snails are kept in the same containers.  They are useful as 'vacuum cleaners' and will eat large quantities of decaying material.

By following the above instructions faithfully, it should be possible to maintain cultures of these organisms almost indefinitely.  Good aeration, water changes, stable temperatures, regular thinning i.e. harvesting and feeding should ensure success.  Finally, if your cultures die off for some reason, transfer the mulm at the bottom of the containers to other containers with fresh water.  The cultures should spring to life within a week or so.