Feeding live daphnia to captive rainbowfishes results in a remarkable improvement in their colouration and health. Rainbowfishes fed an exclusive diet of daphnia grow rapidly and breed readily. Daphnia are a high bulk food; that is, a high proportion of the animal's body is covered with a chitinous exoskeleton - a sort of skeleton on the outside. This chitinous material provides food high in fibre but low in nutrients; its bulk stimulates peristalsis.
The nutritional content of daphnia varies with age, and what it has been eating. The protein content is usually around 50% of dry weight. Quite the opposite from brineshrimp, adults normally have a higher fat content than juveniles do about 20~27% for adults, and 4~6% for juveniles. Some species have been reported to have protein contents exceeding 70%. The fatty acid composition of food is important to the survival and growth of fish fry. Omega-3 highly saturated fatty acids are essential for many species of fish. Daphnia cultured on brewer's yeast (Saccharomyces cerevisiae) are high in monoenoic fatty acids. By using what is called w-yeast (yeast enriched with cuttlefish oil), daphnia will contain very high levels of omega-3 fatty acids. Commercial formulas can also be obtained from aquacultural suppliers for the enrichment of brineshrimp, rotifer, and daphnia cultures.
Daphnia are small aquatic crustaceans commonly called water fleas, due to their small size and jerky swimming motion, and are extremely abundant in suitable freshwater habitats. They are usually translucent or amber in colour and are an important component of the food chain in freshwater environments. Most species occur worldwide and are truly cosmopolitan, but on the other hand there are several that are endemic in distribution.
Daphnia are herbivores or detritivores, feeding on phytoplankton, bacteria, or decaying organic material. They are well adapted to live in algal blooms, which are high in proteins and carbohydrates. Small particles in the water are filtered out by fine setae on the thoracic legs and moved along a groove at the base of the 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.
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| Author collecting Daphnia? from a pond in Germany (1996) |
There are many species of daphnia but the most common ones cultured in the aquarium hobby are Daphnia magna and Daphnia pulex although numerous other species have been cultured successfully. The taxonomy of Daphnia pulex and related species is very confused and it can be very difficult to distinguish this species from other species with large teeth in the middle pecten of the post-abdominal claw, especially Daphnia pulicaria. The characteristics used to separate these species are extremely variable and many intermediate forms occur. Males are easier to distinguish, but are often not present for examination.
Daphnia species show a high degree of morphological variability. Head shape, length of tail spine and size of body may all vary considerably within a species. A "typical" form for some species is often hard to determine as morphological characteristics vary according to time of year, presence or absence of certain predators, geographic location etc. Some of these changes are of such magnitude that they have been described as separate species. Exposure to a predator will often delayed the onset of reproduction and increased the size of the first generation of offspring. However, different species react in different ways to the predator factor and environmental conditions.
Daphnia magna is principally a lake dweller, but can also be found in shallow ponds with muddy bottoms rich in organic matter. Daphnia pulex is principally a pond dweller where the oxygen content is higher, but is also found in lakes. It is generally considered a clean water species being dominant in nature during periods of low turbidity.
Some "Daphnia" cultures are not daphnia at all, and upon further investigation other cladoceran genera such as Ceriodaphnia, Daphniopsis, Bosmina, Bosminopsis, Moina, and Moinodaphnia can be recognised. However, as they all are the same to culture and eagerly accepted by all rainbowfishes, it will not matter which species you have; although, there is considerable size variation between the different genera.
Moina, for example, are approximately half the maximum length of daphnia. Adult moina (700 to 1,000 µm) are larger than newly hatched brine shrimp and approximately two to three times the size of adult rotifers. Young moina (less than 400 µm), however, are smaller than newly hatched brine shrimp and approximately the same size or slightly larger than adult rotifers. Therefore, moina are ideally suited for feeding rainbowfish fry, and many species can ingest newly hatched moina as their initial food.
Moina micrura Kurz, 1874 has been found one of the best species for this purpose. They are widely distributed throughout tropical and subtropical regions of the world - to north of 40° N latitude in Europe and North America, and as far south as 36° S in Australia and South Africa. A key to the families Bosminidae, Daphniidae, and Moinidae are given below.
The daphnia's body is shaped rather like a clam with a slit along the belly side. A single eye capable of distinguishing light from darkness is present in the head. If a shadow is cast across a daphnia pond, the animals quickly swim out of the shadow into the light. Two large appendages behind the head (actually modified antennae) are used like oars to propel the animal in a jerky fashion through the water. Other appendages are inside the shell, and they are used to filter suspended particles on which the daphnia feed.
Most daphnia are cyclical parthenogens, so are well adapted to short seasons of reproduction such as in seasonal environments. Natural populations are composed of females who clonally reproduce until conditions within their environment deteriorate. One clutch of eggs is normally released into the brood pouch during each adult instar (an instar is the period between moults). Since these animals are crustaceans, the only way they can grow in size is to moult their exoskeleton. As long as environmental conditions remain favourable females will continue to reproduce in this manner, producing only female offspring capable of asexual reproduction.
When unfavourable conditions occur, e.g., severe temperature changes, drying, or crowding which leads to competition for food or a decrease in the quality or size of food, then the production of parthenogenetic eggs declines. Some eggs develop into males, and females capable of sexual reproduction. These females have modified carapaces, which are thicker and darker dorsally than a regular carapace and produce haploid eggs that must be fertilised by the males. The fertilised egg goes through several cell divisions, the zygote enters a resting stage and cell division stops. Males are slightly smaller and different in form compared to females.
The mechanisms underlying the production of males and haploid eggs are not clear. Male production seems to be correlated with crowding and a rapid reduction in food supply (a constant low food supply simply inhibits reproduction). Short-day photoperiod seems to increase the production of ephippia in Daphnia pulex in contrast to the longer light periods of midsummer, ie as autumn approaches and days get shorter, and the number of ephippia produced seems to increase.
After these haploid eggs are fertilised by the males, the wall of the brood pouch thickens and encloses the eggs in a semi-elliptical saddle-shaped ephippium. These ephippia contain embryos in a state of arrested development. When the female moults the ephippium is cast off. It does not disintegrate, but remains intact protecting the eggs, which will not complete their development and hatch until favourable conditions return.
Daphnia can suspend their growth and development for years or even centuries during periods of unfavourable conditions. In habitats, which dry completely, annual recruitment is entirely from the ephippia. Ephippia sink or float and are able to withstand extremes of temperature and moisture. When environmental conditions are again favourable the embryos develop into parthenogenetic females and break free of the ephippium. The ephippia are also used as a means of dispersal for many species, being carried by the wind or in the fur, feathers or digestive tracts of animals to new habitats.
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. Generally the life span increases as temperature decreases, due to lowered metabolic activity. The average life span of Daphnia magna is about 40 days at 25°C, and about 56 days at 20°C. The average life span of Daphnia pulex at 20°C is approximately 50 days. The time required to reach maturity (produce their first offspring) in Daphnia pulex varies from six to 10 days (mean = 7.78 days) and appears to be dependent on body size. The growth rate of the organism is greatest during its juvenile stages (early instars), and the body size may double during each of these stages. Daphnia pulex has three to four juvenile instars, whereas Daphnia magna has three to five instars. Each instar stage is terminated by a moult. Growth occurs immediately after each moult while the new carapace is still elastic.
1. Thorax, abdomen, and thoracic legs covered by the carapace. Brood chamber is a space between the body and carapace; legs foliaceous and not clearly segmented - 2
2. Carapace not gelatinous. Second antennae branched - 3
3. Second antennae with 10 or fewer swimming setae (counting setae on both branches of antenna) - 4
4. One branch of the second antennae with three segments the other with four (basal segment short); base of first antennae not covered by a flange on the side of the head - 5
5a. First antennae less than three times as long as wide and attached to head near the carapace margin: Daphniidae
5b. First antennae more than five times as long as wide; not attached to the head near the carapace margin - 6
6a. First antennae rigidly fixed to head, tapered to a point and curved, resembling elephant tusks; first antennae with several sensory hairs about halfway between tip and base, none at tip: Bosminidae
6b. First antennae blunt and flexible; sensory hairs at the tip of the first antennae - 7
7. First antennae attached posterior to compound eye; post-abdomen with a row of lateral feathered setae, and a distal bident tooth on post-abdominal margin near base of post-abdominal claw: Moinidae
© Copyright Adrian R. Tappin
Created July, 2005
Updated December, 2008
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