20 Aralık 2008 Cumartesi
Introduction and Version Information
This document is aimed primarily at aquarists and anyone wishing to culture Daphnia as a food for fish or other animals (such as young newts and salamanders). It is also aimed at anyone interested in learning more about Daphnids, and in particular, the genus Daphnia. Many of the culturing aspects of this document borrow from Kai Schumann's Daphnia FAQ. Interspersed throughout are useful tips from the live_foods mailing list (email@example.com). A full list of credits can be found in the References/Credits section. I wrote and compiled this document because despite the culturing aspect being well covered by Kai and a number of others, correct detailed information on Daphnia is hard to obtain from almost all sources except scientific journals/post-graduate theses, and the internet (though you have to really dig for it). There is also a lot of conflicting advice out there, most of which I have tested and have used this experience to "filter" out what I think is good advice from the bad before putting it in here. I have explained much of the scientific phraseology and terminology used below, but if you are having problems, or if you see some incorrect information, please contact me using this form.
A note on volume: I use Imperial Gallons to measure volume. An Imperial gallon is 4.545 litres. A US gallon is 3.8 litres.
- The first version of this document was written in August 1998.
- Version 2.0 is dated December 1999.
- Version 2.5 features fixed links and many new ones for suppliers and information (January 2000).
- Version 3.0 features additions to the feeding section based on advice I've given on the Axolotl Mailing List, as well as new links and some dead links removed. If you spot any dead ones or you have one I should add, please let me know.
- Version 3.1: Removed the photos and made some minor corrections. I'll put some up of my own ASAP.
- Version 3.2: Added my own photo of Daphnia magna.
What are Daphnia?
Daphnia (or Daphnids) are members of a collection of animals that are broadly termed as "water fleas". These are predominantly small crustaceans, and Daphnia belong to a group known as the Daphniidae (which in turn is part of the Cladocera, relatives of the freshwater shrimp, Gammarus et al, and the brine shrimp, Artemia spp). They get their common name from their jerky movement through the water. Apart from the jerky movements, the resemblance to real fleas (Pulex iritans, etc), ends: real fleas are insects and share only an extremely distant common ancestry with Daphnia, since both crustaceans and insects are arthropods. In this document, the terms Daphnia, Daphnids/Daphniidae are used interchangeably and though not technically the same, they should be read as such in all cases except that of species names which are absolute. These terms will be used to describe species of the genus Daphnia, especially D. pulex and D. magna. All species of Daphnia occur in different strains - sometimes the same species can look completely different, both in terms of size and shape, depending on its origin, and environmental factors at that location. As many of their traits will apply to other related genera, we will also touch on Moina, a group of close relatives.
It is important to distinguish between Daphnia and other "water fleas" such as the various species of copepod (like Cyclops spp) and ostracod (Cypridopsis et al) which do not directly come under the scope of this document, but share many of the feeding habits of Daphnids. The jerky movement, general shape (and, to a lesser extent, colour) of daphnia are the best way to distinguish them without magnification. The morphology and anatomy of Daphnids are discussed in detail in the Anatomy Section below.
There are approximately 150 known species in North America, and a similar number in Europe (many of these species are found on both continents, either through accidental introduction by man, or nature). Many foreign species have been introduced to America and Europe from Asia and Africa (the most notorious of which is Daphnia lumholtzi, which is native to Africa). It is not uncommon to collect 20 or more species in one small area of lake bottom. A few species of Cladocera are predacious but most are herbivores or detritivores. An important link in food chains of virtually every inland body of water, cladocerans convert phytoplankton/benthic plants, bacteria, fungi and decaying organic matter into animal tissue that can be used by larger animals. In large lakes they are a major food source for many kinds of fish such as sticklebacks, minnows, the fry of larger fish and also larval amphibians. Many aquatic insect larvae and other invertebrates also feed on cladocerans.
The daphnia covered in this document feed on particles found floating in the water (phytoplankton, but also attached vegetation or decaying organic material), but the predominant foods are free-living algae (eg Chlamydomanas spp, Volvox spp, etc), bacteria and fungi. In the summer months, they can often be seen "blooming" in ponds and lakes as the concentration of algae builds up. Their prolificity is due to a great extent to their ability to replicate by parthenogenicity.
Parthenogenicity is the ability to self-replicate without fertilisation of any form (a type of asexual reproduction) - the offspring are exact genetic replicas of the parent (clones), and any differences in the physical state of the clones is due to environmental conditions. Parthenogenesis seems to have evolved to allow daphnia to take advantage of good conditions (food, temperature, etc) as soon as they arise. In the wild, during the late spring, summer and early autumn (depending on temperature, food availability and presence of waste products of their metabolism), daphnia reproduce by parthenogenicity, bearing, on average, ten live young per individual (the entire race is made up of females during this period). Developing embryos are often visible in the mother's body without the aid of a microscope. Generation after generation of females can be born in this way, with new females reproducing as early as four days old at intervals as often as every three days, for up to twenty five times in their lifetime (though this number is usually far smaller, and females tend to produce a lot less than one hundred offspring). You don't need to be a mathematician to imagine the magnitude of a healthy daphnia population.
When food is scarce some eggs develop into males and the females produce eggs that must be fertilised (the sexes reproduce via haploid means, i.e. half the number of chromosomes to procreate from each sex, as opposed to parthenogenic reproduction which is diploid). These eggs develop into small embryos which then go into suspended animation, and are shed with the carapace as dark brown/black saddle-shaped cases known as ephippia (ephippium is Latin for saddle). These can survive harsh conditions and are quite capable of withstanding a dry spell if their pond dries up for a while, and they can sometimes even survive freezing. The ephippial females of most Daphnids are easy to tell from their live-bearing counterparts because the developing ephippium is visible as a black spot towards the rear end of the animal. When conditions improve again, the egg producing generations begin producing live young once again (all females), and the male sex dies out completely until it is needed when conditions worsen once again.
There are often pulses of population growth, when numbers increase almost logarithmically by parthenogensis, using up a lot of food and causing overcrowding, and then the numbers fall sharply and ephippia are produced. There are usually two of these pulses every year, though in a good year there can be many pulses. Even in an aquarist's culture, pulses will be noticeable. See the Maintenance section for ways to counter population fall-off.
Daphnia are represented worldwide as a number of different species. This document uses D. pulex and its variants as its model (found throughout the northern hemisphere), but its characteristics can be applied equally well to most other species, be they from the same genus or related genera. Once thought of as an animal of polluted waters, Daphnia have been proven to be very sensitive to poor water conditions and a number of research and industrial groups use Daphnia to test water quality. For example, they are very sensitive to halide concentration, like the chloride or fluoride in tap water, which are extremely toxic to daphnia, even more so than to fish. They are also sensitive to metal ion concentration, like sodium, potassium, magnesium and calcium, which in increased concentrations can cause immobility and death, and daphnia are extremely sensitive to copper, zinc and most dissolved toxins (e.g. dichromate ions). They are often used to monitor water quality so that only safe water is released into the environment by industry and water treatment plants.
In terms of nutritional information, Daphnia have a protein content of around 50% dry weight and a fat content of 20-27% for adults (4-6% for juveniles). Some species have been reported to have a higher protein content, and Moina are quite often found to be 70% protein. As with most live foods, "they are what they eat", and so vitamin and other formulas are available as food for filter feeders like Daphnia and Artemia which will give them certain food values or an increase in a given fatty acid, for example (H.U.F.A. or Highly Unsaturated Fatty Acids are often sold for this purpose).
Figure 1: Anatomy of female Daphnia pulex (De Geer) (greatly magnified); diagrammatic; (muscles not shown in fig 1). B, brain; BC, brood chamber; C, digestive caecum; CE, compound eye; F, fornix; FA, first antenna (antennule); H, heart; INT, intestine; O, ocellus; OV, ovary; R, rostrum or beak; SG, shell gland. (Fig 1 was greatly modified from Storch, 1925.)
Figure 2: Photograph of a female Daphnia magna
Anatomy of Daphnia
Daphnia tend to be almost kidney shaped, posessing only a single compound eye (though they have an ocellus, a simple eye), two doubly-branched antennae (frequently half the length of the body or more), and leaf-like limbs inside the carapace that produce a current of water which carries food and oxygen to the mouth and gills. Their bodies are almost transparent and with a microscope you can see the heart beating, and sometimes even their last meal (the gut may appear green if the individual has been feeding on algae).
A carapace covers the body, including the 4 to 6 pairs of thoracic appendages, and is used as a brood chamber. The abdomen and post-abdomen (distal to the anus) is generally bent forward under the thorax. The post-abdomen bears two large claws used primarily for cleaning debris out of the carapace. Swimming is accomplished by downward strokes of the large second antennae.
In most species complex movements of the thoracic appendages produce a constant current of water between the valves. Small particles (less than 50 microns in diameter) in the water are filtered out by fine setae on the thoracic legs and moved along a groove at the base of legs to the mouth. Although there is some evidence that certain types of food, such as particular types of algae, Protozoa, or bacteria may be selected by some species, it is generally believed that all organic particles of suitable size are ingested without any selective mechanism. When undesirable material or large tangled masses are introduced between the mandibles, they may be removed by spines on the first legs and then kicked out of the carapace by the post-abdomen.
Males are distinguished from females by their smaller size, larger antennules, modified post-abdomen, and first legs, which are armed with a hook used in clasping.
Adult daphnia range in size from half a millimetre to almost a centimetre, depending on the species, though within a given species, size can vary greatly (female Daphnia magna can be between 3 mm and 5 mm).
Colour is sometimes quoted in literature as a means of identifying species, but this can be a very vague indicator of species because within a species (and even a strain), the colour of genetically-identical individuals can be quite different due to the adaptive physiology of Daphnids - in water which has low oxygen levels, Daphnia tend to develop more hemoglobin to increase their oxygen uptake from the water. Oxy-hemoglobin, i.e. that which has coordinatively bound oxygen, is red in colour and this gives the see-through body of daphnia a red pigmentation. Individuals of the same strain in oxygen-rich environments tend to be yellow or almost unpigmented. An example of a species that seems to exist with very little hemoglobin in comparison to other members of its genus, is Daphnia hyalina. It is usually found in the open water of lakes where dissolved oxygen is plentiful. The colour is also moderated by what food is predominating in the diet. Daphnia fed on green algae will be transparent-green in colour, while those feeding on bacteria will be salmon-pink.
The life span of Daphnia, from the release of the egg into the brood chamber until the death of the adult, is highly variable depending on the species and environmental conditions (Pennak, 1978). Generally the life span increases as temperature decreases, due to lowered metabolic activity. The average life span of D. magna is about 40 days at 25oC and about 56 days at 20oC. The average life span of Daphnia pulex at 20oC is approximately 50 days. Four distinct periods may be recognized in the life history of Daphnia: (1) egg, (2) juvenile, (3) adolescent and (4) adult (Pennak, 1978). Typically, a clutch of 6 or 10 eggs is released into the brood chamber. The eggs hatch in the brood chamber and the juveniles, which are already similar in form to the adults, are released in approximately two days when the female molts (casts off her exoskeleton). The time required to reach maturity varies from 6 to 10 days. Daphnia typically invest most of their energy in reproduction (D. magna 69%, D. pulex 67%), while they invest comparitively little in growth (23%). This serves to highlight the heavy emphasis on fast reproduction to take advantage of good conditions.
What kinds are there?
There are many different species of Daphnia and closely-related genera. However, those of main concern to the aquarist will probably be Daphnia pulex, Daphnia magna, and the various Moina species. Moina are very tiny Daphnids and are often used as an easier-to-culture substitute for Artemia (brine shrimp) as fry food, because young Moina are only slightly larger than newly-hatched brine shrimp, and first stage daphnia are not much bigger. Identifying Daphnia species accurately is best achieved using their size, the shape of the ephippium, the shape of the rostrum and whether the post-abdomenal extension is long/short and/or whether smaller appendages close to it are setose (covered with small hair-like structures). It should be noted that within each species there are usually different strains, some of which may have different traits to the "norm".
Daphnia pulex - Although a species in its own right, D. pulex is very hard to distinguish from a number of closely related forms (some consider them seperate species, others sub-species, since hybridisation is common. One example of this is D. galeata). Generally, it is like a smaller replica of D. magna, and is usually fed to the same size of fish. The strain which I culture and know as D. pulex (from the F.B.A.'s 1966 Key to British Cladocera), tends to be larger than that described by many texts (such as that by the British Columbian Resources Inventory Committee), but it is this latter variety which I have found in the wild and identified as D. pulex. I obtained my Daphnia pulex strain from a local aquarist shop only because it happened to come in on some plants. The males are usually 1.5 mm and females 2.5 - 3.5 mm, but the generic D. pulex is taken to have males of about 1.3 mm, and females up to 2.2 mm. Other than the size difference, the two species are almost exactly the same in appearance. D. pulex's colour ranges from yellow to almost red, and in this respect can be very similar to D. magna. The are a number of ways to distinguish D. pulex from D. magna. These include D. pulex's smaller size, one of the appendages of the carapace near the post-abdomenal is setose, the ephippium is more triangular than D. magna's and the embyos in it are not parrallel, and there is only one extension from the top edge of the ephippium. D. pulex produces ephippia quite readily in bad conditions.
Daphnia magna is the species normally associated with the name Daphnia by most aquarists. It has been a favourite live food source since the hobby began. D. magna males are 2 mm in length and females are 3-5 mm. They range in colour from a pale yellow to salmon-pink, often this is affected by their food source. D. magna has a wide temperature tolerance, but its optimum temperature is between 18 and 22oC (64-72F). In the wild, Daphnia magna is quite rare, but where it does occur, it is usually very abundant. Like all Daphnia spp ephippia, the ephippium of D. magna is shaped like a saddle. However, the embryos inside the ephippium (of which there are usually two in Daphnia species) are parallel, and it is like a curve-edged rectangle, with the top long side having two extensions beyond the rectangle at either end. Although magna does produce ephippia, it is less inclined to do so than D. pulex. D. magna is usually considered more "delicate" than D. pulex and as a result it is not as tolerant of culture fouling as D. pulex.
Moina spp are, although similar to daphnia and other members of the Daphniidae, is quite distinct from the genus Daphnia. They are very small in comparison to most Daphniidae (less than 0.5mm in length), and their young are only about the size of newly-hatched brine shrimp. However, their culture is almost identical to that of Daphnia, and although they are harder to acquire, their use as a cheap alternative to brine shrimp as a first fry food has made them popular with many fish breeders.
There are many other species of Daphnia/Cladoceran to be found throughout the world. Some of the less well-known are described below, but it should be noted that these are much harder to come by unless you happen to live in a country which has the species in the wild.
D. hyalina is more often found in the open water of lakes. In comparison to D. magna, D. hyalina is very transparent and as a result it is sometimes hard to see individuals in the water. D. hyalina is generally 3 mm in length, the body shape slightly less rotund than that of D. magna, and the "head" is less distinct as it doesn't extend as far from the body as it does in D. magna. At this time the I'm not aware of any large-scale culturing of this species, despite the fact that is quite common in Europe.
Bosmina coregoni is a rather small species (about 1 mm in length) and is found in weedy ponds and canals. Due to its small size it is more suitable for use as a fry food than other Daphnia. It is a very active species and can often be confused with other small daphnids without the aid of a microscope or high power magnifying glass. Its body shape is shaped rather like a pea which has started to sprout. It is commonly found in Northern Europe. Interestingly, many "Moina" cultures are actually Bosmina cultures.
Simocephalus vetulus is a large daphnid which seems prone to being trapped in the surface film of water. It tends to be more tolerant of acidic water than other Daphnids, and often exceeds 6 mm in length. The author has encountered this species on a number of occasions in the wild, and it is almost comical to watch an individual wheeling about in the surface film attempting to break free. Again this species is commonly found in Northern Europe.
In places, this section borrows heavily from Kai Schumann's FAQ and Franc Gorenc's article, with additions and changes by myself.
Salinity - Daphnia are typically freshwater organisms and there are no marine species of the Daphnia genus. 99% of Cladocerans are found in freshwater, and the remaining few species are mostly found in brackish, not sea water. Some species have been observed in salinities up to 4 ppt, and salinities of 1.5 to 3.0 ppt are common in pond cultures in the Orient.
Oxygen - Daphnia are generally tolerant of poor water quality, and dissolved oxygen varies from almost zero to supersaturation. Like the Brine Shrimp, their ability to survive in an oxygen poor environment is in their ability to synthesize hemoglobin. The production of hemoglobin may be promoted by high temperatures, and a high population. Also, like brine shrimp, Daphnia are not tolerant of fine air bubbles. A slow aeration is needed with Daphnia as a large bubble column will strip the Daphnia out and kill them. I (and a number of others), have found that Bio-foam filters (designed for fry-tanks) are ideal for aerating Daphnia culture, and the removal of larger particles from the water is an added bonus. However care should be taken to use them at minimal air flow to avoid over-agitating the water.
pH and ammonia - A pH between 6.5 and 9.5 is acceptable, with the optimum being between 7.2 and 8.5. Ammonia is generally highly toxic to all organisms, even in small amounts, but in alkaline conditions, the toxicity is radically increased, and this will drastically impare Daphnia reproduction, but will not affect the actual health of the animals themselves. So it seems that on the small scale that we require, monitoring of pH and ammonia is not critical to success. NOTE: in general, the more extreme the pH, the higher the toxicity of dissolved minerals and gases. Also, remember that pH is a logarithmic scale - a pH of 5 is ten times more acidic than a pH of 6, and likewise, a pH of 9 is ten times more alkaline than a pH of 8.
Dissolved minerals - In contrast to their tolerance of low oxygen, Daphnia are very sensitive to disturbances of the ionic composition of their environment. They become immobile and eventually die with the addition of salts like sodium, potassium, magnesium, and calcium. Low concentrations of phosphorus (less than 0.5 ppm) will stimulate reproduction, but concentrations higher than 1.0 are lethal to the young. Daphnia magna are quite resistant to phosphorus and can withstand concentrations as high as 5-7 ppm. Daphnia are not affected by the addition of nitrogen in fertilizers for the promotion of algae growth. As with any aquarium venture, the water used should be treated with aeration or de-chlorinator to remove chlorine before the culture is started. Concentrations of only 0.01 ppm copper will result in reduced movement in Daphnia. They are extremely sensitive to metal ions like copper and zinc, pesticides, detergents, bleaches and other dissolved toxins. For this reason, they are often used to test waste-water from industry. Municipal and well water may be contaminated enough to kill the culture. The best source of water is from your aquarium water changes, but you could also use filtered stream or lake water (fish-free water bodies are recommended due to possible disease introduction to your fish aquarium by way of the daphnia), or rain water collected from areas of low/no air pollution (and rain water collected in cities or industrial areas is usually safe if left to stand for a week and only the top-most 3/4 used). Never use distilled or deionised water, as it does not have the minerals needed for growth.
A small degree of temporary and permanent hardness in the water usually encourages growth and reproduction because Daphnia make use of calcium and other minerals in their chitinous carapaces. D. magna tends to prefer harder water (170 mg carbonate hardness) and D. pulex a little less hard (90 mg carbonate hardness). I usually add a very tiny pinch of powdered Tufa rock (often used in Marine aquaria) to my water because there is very little temporary hardness in the water I use and no permanent hardness whatsoever.
Temperature - Daphnia have a wide tolerance to temperature. The optimum temperature for Daphnia magna is 18-22oC (64-72F). D. pulex seems to do well at well at almost any temperature above 10oC. Moina withstand extremes even more, resisting daily variations of 5-31oC (41-88 F); their optimum being 24-31oC (75-88 F). The higher temperature tolerance of Moina make this species a better choice where temperatures may rise above the comfort levels for D. magna at certain times of the year.
What do I feed them?
This is a general section on feeding Daphnia. I have summarised my own experiences at the end of this section. Daphnia have similar feeding habits to other tiny crustaceans (especially to the likes of Artemia). The best foods for culturing are algae (typically free-living green algae species which tend to turn water to "pea soup"), yeasts (Sacromyces spp, and similar fungi), and bacteria. Combinations of the above seem to have the most success (i.e. yeast and algae seem to compliment one another). Each food type will be discussed in turn, together with its advantages and disadvantages, and means of attaining/growing it.
Micro algae is consumed in great quantities by Daphnia, and the abundance of daphnia is usually proportional to the density of algal blooms. There are a number of ways to grow algae, all of which are very basic and require little effort.
Placing a container of water outside in good sunlight will usually guarantee a good growth of algae within two weeks, usually a lot less. Algal spores are carried on the wind and will colonise the water, but it usually speeds up the process if you "seed" the water with some algae from a container that has already has a bloom.
Miracle grow, an organic plant fertiliser, can be used to grow algae (after all they're just plants). One method is to use 1 gallon containers, 1 for each day of the week. These sit on a window sill which gets good sunlight for as much of the day as possible. By bubbling air through the containers (an airpump with 7 side lines will do, because there really only needs to be a small movement in the water), algae won't grow on the sides of the containers where it can block the sunlight. Add 1 tea spoon of Miracle grow per gallon. This system is then seeded with green water in tank #1 - two days later tank #2 - two days later tank #3, etc. When this has turned bright green (within 2 weeks), pour it into a Daphnia tank. Refill the container with water mixture and seed with tank #2 which should be about to turn bright green. This is repeated with each container as they turn bright green. As you might be able to tell, this will provide about 1 gallon of fresh green water every two days.
The advantages of algae as a food are that it is very easy to culture and it is excellent for growing daphnia. There are no disadvantages really, other than the fact that it requires bi-daily maintenance / renewal.
There are two general kinds of yeast that we need be concerned about - activated and inactive. Activated yeast is generally a better food to feed because it will not foul the water as quickly/as much as the inactive kind. Bakers, brewers, and almost any kind of yeast are suitable for daphnia cultures, but it is recommended that no more than half an ounce of yeast per five gallons of water be fed every five days. If you're using yeast, especially inactive yeast, consider adding some algae to the water as this will counter any fouling which may result from adding the inactive yeast (this isn't so important with activated yeasts). Do take care not to overfeed inactive yeast as it will foul the culture and therefore kill your daphnia.
Some bakers yeasts come with added ingredients like Calcium Sulphate and Ascorbic acid (vitamin C) to aid fast activation of the yeast. These are harmless to daphnia cultures, but care should be taken when adding this kind of yeast because Ascorbic acid can give pHs less than 6, which are far from ideal with Daphnia. However, I have never had any pH changes when using such "mixes" in moderation, and the calcium sulphate gives vital calcium for the daphnia's carapaces.
The advantages of yeast as a food are that it's easy to acquire, and there is a minimum of fuss when preparing it for the culture. The only slight disadvantage is that it's not quite as good a food as algae (the daphnia need to consume more weight of yeast than algae to get the same food value). However, yeast is far better than any other food except some bacteria, which have almost as high a food value.
Bacteria have a similar food value to fungi, but they generally reproduce faster than fungi and algae, although the food value doesn't tend to be as high. Bacteria are "cultured" by taking 5-6 ounces of dried horse, cow or sheep dung (dried for two reasons: it's easier to deal with, and most antibiotics or growth promoters which were fed to the animal will break down if the dung is left to dry for a while) and tying it in a nylon bag (such as tights/pantyhose), and hanging this in the water with the daphnia. Animal dung (including human dung, though don't use human faeces unless you want typhoid or worse...) contains copious quantities of bacteria from the digestive system, and these will leech out of the dung into the water and reproduce. Typically, the water will go cloudy after a time, indicating that the bacteria are starting to multiply. This should be changed once a week for maximum effect. Another method is to soak the dung for weeks until it decomposes into a nutrient slurry, then drip the liquid into the tank at a rate of 16 fluid ounces per five to eight days.
Another way to culture bacteria in a hurry is to throw a handful of salmon (or trout pellets), dog biscuits or other meat-based food into a few gallons of water with some added aquarium water. Within a few days it is usually cloudy with bacteria.
Bacteria are a good food source, and easily acquired/cultivated. The only downside is the smell of the decaying matter (which can be pretty bad at times). An important thing to remember is that horse dung usually contains tetanus (also a bacterium), so care should be taken when handling it (make sure you have no open cuts/sores on your hands or arms).
These include bran, wheat flour, and dried blood. These should be considered similar to inactive yeast, and the same amounts and care should be taken when administering them. The only real difference is that the food value isn't as high as the corresponding weight of yeast.
Some of My Own Experience
Unless you have a very large container, like an outdoor pond, I don't think "green water" is worth the effort. I fertilise the water with salmon pellets (the Indiana University Axolotl Colony's at the moment - May 2000). The amount depends on your container size and current daphnia population. Too much and you foul the water and everything dies. As a guide, I would say for a 4 foot long aquarium one or even two handfuls is enough to fertilise the water if there is an already healthy population of daphnia. If you have less, then don't use as many pellets or the bacteria population will go out of control. this is reliant on temperature, ideally in the early 20s celsius / ~70F.
People recommend green water as the best food for daphnia. I would have to say that I mainly agree with this, but I think that bacteria are just as nutritious. I haven't bothered feeding green water to daphnia since 1998 so draw your own conclusions. I've been maintaining two populations since June 1998 and they have never completely died out. They do pulse though. If you want a recommendation for a quick fix substitute for green
water, get yourself a bag of frozen peas and one of carrots. Mix about 80% peas and 20% carrots together and then stick them in a food blender. Blend these until you have a mulch. You're looking for the "juices", so take any liquid and squeeze the mulch to get all of the liquid from it. This contains particles of a size small enough for daphnia to sieve from the water (less than 50 microns). It's far more concentrated than water with algae in it, so use it sparingly. It gives just as good a result. I've used this a few times, but I'm just too lazy most of the time to bother with anything except rotting pellets.
One final note on pellets - don't crush them much first. If you do you'll release all of the nutrients at once instead of over a few days and you can get the bacteria going out of control. I should mention that I also keep water slaters (the European fresh water louse) in my daphnia cultures because they break down solid waste and prevent the pellets from being covered in fungus and floating at the surface. The fresh water louse is a crustacean
that looks like a wood louse and it is not a fish louse (louse is just the name). I don't recommend snails in the culture either because for one thing, some species can act as a parasite vector, and also because they use up calcium and that's reserved for the daphnia!
I've heard of ground-up liver in water being recommended. In that case it's mainly the blood that is the fertiliser. I've tried animal's blood and found it ok, but the pellets win in my opinion, followed by algae and the mixture I mentioned above.
How do I maximise the yield?
Daphnia production is relatively simple, if not an exact science. However, there are proven measures that increase the productivity of a culture.
Good aeration (good in so far as the manner in which the water is aerated, not good as in quantity) is probably the largest contributing factor for good production. Some species prefer no aeration, but Daphnia magna seems to do well with it. It allows you to keep more daphnia in the same container. It also circulates the water, (which counters stagnancy and fouling). It minimises the possibility of algae growing on the walls of the container, and it also keeps inert food in suspension which is more conducive to most daphnia feeding habits. The only problem with aeration is that fine bubbles can lodge underneath the daphnia's carapace and float it to the surface and preventing it from feeding. Therefore airstones should be avoided (unless used in a bio-foam filter in which case the air bubbles combine together), or coarse airstones (or better yet, no airstones) should be used instead.
A good method (tried and trusted) of aerating the water which I have mentioned previously is the bio-foam filter (there are a number of models available, any will do). These are commonly used in fry tanks, but are ideal for daphnia. They trap larger particles in the water (they don't trap algae), and help break them down, releasing nutrient for the algae to feed on. The outlets are usually very good at aerating the water, but care should be taken to use them at low to minimum flow to avoid over-agitating the water (we want some aeration, not a torrent). I haven't had a fouled culture yet while using a bio-foam.
Carry out regular maintenance as described in the Maintenance section, especially water changes.
Cull/Harvest the culture regularly (again, see the Harvesting section). This encourages constant growth and also keeps the daphnia from exhausting both the oxygen and the food in the water too fast for it to be replaced.
Some people like to keep a light on 24 hours a day for their daphnia tank as this can encourage faster growth and reproduction. I haven't tried this myself. However, you probably won't need to do this once your culture gets going. One important note though - continuous 24 hour periods in which the daylight is less than 12 hours and with a temperature significantly below 18oC appears to trigger ephippium production and a general fall off in population growth. This would tend to suggest that these conditions are emulating those of late autumn/winter in the wild.
Water changes: some people recommend you change up to 75% of the water in a culture every day. This is really dependent on what rate you're "running" the culture at. The more food you feed, the faster the water will be fouled, and therefore the more frequent the water changes. This is really up to the individual, but be aware that output levels will drop if there is a build up of metabolites/toxins/etc in the water. This could lead to crashing.
When it comes to culturing Daphnia, harvesting may be the aim of the exercise, but it's also essential. Overcrowding is a serious danger otherwise. Even if you have to wash the culled ones down the sink because you have too many, still cull them, otherwise the culture could become unstable. If you're culturing below about 25oC, then it's generally a good idea to begin culling only midway through the second week unless you started out with a large number of daphnia (or your culture is growing like crazy already). This is because a lot of cultures take a few days to settle and start reproducing. When culling, try to use a net which has a large enough mesh to let young daphnia through, but just small enough to catch the adults (some people prefer to drain 1/4 of the tank into a net, and replace the water with new fertilised water, and in this way you do two jobs at once). Not more than 1/4 of the population should be harvested daily, but the harvest may vary according to the quality of the population. But remember to cull regularly. In daylight/roomlight, when you stop the aeration, and let the tank settle, the Daphnia will concentrate on the surface where they're easier to harvest.
Harvested Daphnia can be kept alive for several days in the refrigerator in clean water. They will resume normal activity when the water warms up. The nutritional quality will not be as good because they have been starving for several days, so a supplemental feeding is required for best effect. Daphnia can be stored for long periods by freezing them in a low salinity water (7ppt, 1.0046 density). Of course this kills the Daphnia, so adequate circulation is required to keep them in suspension during feeding. They also will not be as nutritious as the nutrients rapidly leach out in the aquarium. Nearly all the enzyme activity is lost in ten minutes, and in an hour all free amino acids, and most bound amino acids are lost. Fish will not feed on frozen Daphnia as readily either.
A daphnia culture requires very little maintenance other than partial water changes (the amount really depends on the volume of water and the number of daphnia in the culture - more water usually needs less changing, more daphnia usually means more water needs to be changed, to a maximum of 50% per week). Do feed your daphnia on a regular basis.
The key to avoiding population fall-off/crashing is to have constantly good conditions, and to avoid sudden changes, such as large temperature drops, culture fouling, or the addition of dangerous chemicals to the water. Remember that if you're keeping your colonies outside, the population will naturally decrease in winter, but should increase again in the following season.
Should your culture die off, don't despair. Either change most of the water or take the mulm and put it in a new container - if conditions are good, the ephippia should hatch within 4 to 8 days (if you accidentally poisoned the culture with chemicals, you may need to obtain a new starter culture because the daphnia may not have had time to produce ephippia, and even if they did, even ephippia will not survive for very long in strong chemicals like bleach or even mild acid).
If you go on holiday for a few weeks, don't be surprised if your daphnia have produced ephippia and the population has decreased while you were away!
Dos and Don'ts
This section is intended as a reminder of things to remember and a precaution against simple mistakes people often make without realising it.
Do make sure you age your tap water and have a good idea of what's in it:
If it contains chlorine (chloramine will also put chloride into the water), it must be left to stand or aerate the water for 24 hours to drive off the chlorine gas (fluoride is usually very low in concentration and isn't really something to worry about).
If your water contains lots of ammonia, this will inhibit the population growth of your culture - again, aerating the water vigourously helps drive off ammonia (although it takes a lot longer than with chlorine).
Any metals in the water can often be toxic to daphnia. Some can inhibit population growth (see the section on Physical Requirements).
Do carry out regular partial water changes.
Do feed regular modest amounts of food (this will depend on the culture size).
Do make sure you have adequate water hardness for your daphnia, otherwise they will not reproduce at a high rate, and will probably just produce ephippia.
Do try to maintain a constant temperature (about 20oC is ideal for most species).
Do cull/harvest your cultures at least once a week to avoid overcrowding.
Don't wash your hands with soap/detergent just before you put your hands in a daphnia culture unless you've _thoroughly_ rinsed your hands because soap and detergents are toxic to daphnia.
Don't overfeed - if anything, underfeed your daphnia to avoid fouling and toxic build-up of ammonia.
Don't put your daphnia in a container of dense algae (and don't change too much water from the tank for algae water at one time) because algal blooms tend to raise pH to very high levels (over pH 9), and coupled with even a low ammonia concentration, this could be disastrous for the daphnia, killing them in short order. Ammonia toxicity increases with higher pH.
Don't keep all your eggs in one basket (or daphnia in one container). Even the best of us will have accidents on occasion or have an emergency, etc, and our cultures may be poisoned or die off for no apparent reason. To counter this, it is best to seed as many water vessels, aquariums, and even flower window boxes with daphnia when the culture is first obtained. This will ensure you will be able to "restart" without too much fuss.
Don't use insecticides near your daphnia containers, and don't leave your daphnia container in a room that was just painted/varnished/etc, because the fumes/gas can be toxic, and even if it doesn't kill the daphnia, the fish you feed them to may be slowly poisoned.
Don't use airstones in a daphnia culture. Use an open airline tube or a bio-foam filter (the latter contains an airstone inside in the apparatus, but the bubbles are not fine enough to harm the daphnia when the bubbles emerge into the tank. Note however, there should only be a weak flow of air through the filter to avoid too much water flow).
Don't add miracle grow, etc, directly to the culture. It won't be much use, and it could also poison the culture, in high concentrations.
Don't add fresh tap water to a daphnia culture unless you want to kill them all with the chlorine.
Don't keep your daphnia in the dark for days at a time as this can stimulate them to produce ephippia.
Pennak, Robert - Freshwater Invertebrates of the US (1978).
Scourfield, DJ & Harding, JP - A key the the British Species of Freshwater Cladocera with notes (1957-1966).
Schumann, Kai - Daphnia FAQ.
The subscribers to the Live Foods mailing list (see the Links Section).
Many of the sites mentioned in the links section
Sterry, Paul - Pond Watching (198x).
Clegg, John - The Observers book of pondlife.
Many snippets of information from a variety of Zoology and Limnology books, too numerous to mention. I have been lucky in my life to have attended Trinity College Dublin for my Bachelor's degree. Legal Deposition meant I was able to find books in the library there that are very rare.