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The aquarium breeding and propagation of rainbowfishes requires not only specific aquarium conditions, but in most cases also the cultivation and use of live food organisms for feeding newly hatched larvae. The larvae require culture techniques which are normally different than juveniles or adults, especially with respect to feeding. The main reason for this is that the developing larvae are usually very small, extremely fragile, and generally not physiologically fully developed. For example, their small size (i.e., small mouth size), the uncompleted development of their perception organs (i.e., eyes, chemoreceptors) and digestive system, are limiting factors in proper feed selection and use during the early first-feeding period. High mortality rates can often occur, especially during the early feeding stages. It is perhaps not surprising therefore that larval nutrition, and in particular that of the sensitive first-feeding larvae, has become one of the major obstacles preventing the successful breeding of many freshwater fish species. However, mortality can be the result of several factors including inbreeding, inferior water conditions and improper hatching conditions.
Rainbowfish larvae in the wild feed on a wide range of live foods, but under aquarium conditions they are usually fed on a limited number of foods (two or three) which frequently are not part of their natural food and hence their nutritional composition is not always the most suitable for maximum growth, development and survival of the larvae. Moreover, larval development of the fish will undertake several morphological and physiological changes which in nature are simultaneous with changes in behaviour and even habitat and type of food consumed. All these changes will affect the nutrient availability and feed utilisation by the larvae in order to match their growing requirements. In theory, most of these problems can be simplified by the proper development of diets which are able to cover nutritional requirements at different times of larval development. In order to achieve those diets we need, among many other important things, is to have a complete knowledge of the nutrient requirements for rainbowfish species.
As the fish larvae grow, the amount of food they need increases and they prefer to eat larger-sized prey. It has been generally accepted that the optimal prey size for fish larvae is determined by their mouth size. Commercial fish larvae feeds have been developed primarily based on mouth size data. This has resulted in successful improvement of larval survival and growth in many fish species. Adjustment of the amount of feed depending on larval developmental stage has also resulted in better growth and survival.
The mouth size of first-feeding larvae restricts the size of the food particles which can be ingested. In general, mouth size is correlated with body size, which in turn is influenced by egg diameter and the period of endogenous feeding (i.e., yolk sac consumption period). Rainbowfishes are lecithotropic; they live off the yolk sac provided within the egg for a certain period of time after hatching. This means that there is a certain period of time where it is not necessary for a larval fish to obtain their nutrition from external sources. Rainbowfish larvae can survive for as long as 8~10 days without external food sources providing that the yolk sac contained enough nutrients. However, if an egg does not hold adequate nutrients, larvae hatching from that egg would have a shorter period of unfed larval life than a larvae hatching from an egg with a good supply of nutrients. Both protein and lipid are very important constituents of fish eggs. They are the primary components of vitellogenin, the lipoprotein that is the main energy source in the yolk of eggs.
Live feeds are a convenient food source for the larvae of all rainbowfish species. In their natural environment rainbowfish larvae feed primarily on zooplankton. However, these organisms are not normally available in captivity. Zooplankton is a broad categorisation spanning a range of organism sizes that includes both small protozoans and large metazoans. It includes holoplanktonic organisms whose complete life cycle lies within the plankton, and meroplanktonic organisms that spend part of their life cycle in the plankton before graduating to either the nekton or a sessile, benthic existence. An abundance of zooplankton is particularly important for larvae to develop into juveniles and for juveniles to develop into sub-adults.
Natural foods may also include phytoplankton and detritus. As plants decompose they become broken down into tiny fragments. The fragments become colonised by bacteria and fungi which feed off the decomposing material. These tiny fragments and the microscopic plants, animals, bacteria and fungi associated with them are known as 'detritus'. Detritus is a major component of a rainbowfishes diet at all stages of its life cycle. The tiny plant fragments themselves are not very nutritious but the microorganisms associated with them are a readily digestible, nutritious, protein rich food source.
Most rainbowfish larvae are not particular about the types of zooplankton they will eat, but the animals must be small enough for the larvae to ingest. Rainbowfish larvae are usually small in size (2~5 mm, total length), have poorly developed eyes, do not swim well but are mostly present in the water column, and require easily digestible food. Having such tiny mouths, the size of the food is crucial to their surviving the most difficult period in their lives. Zooplankton swim slowly and stay suspended in the water column, thus being available for capture and consumption by larval rainbowfishes. Larger juveniles fed very tiny zooplankton may grow slowly because of the energy expended in catching small prey, but usually they will not starve if enough zooplankton is available. However, feeding tiny larvae with zooplankton that are mostly too large for them to eat usually results in starvation.
In general, if you want your rainbowfishes to achieve maximum growth and low mortality rates then you will need to provide them with at least some live foods. They should also be fed several times a day. This is not always possibly as we all have other things in our life to do each day. However, there are several kinds of live foods that can be fed to rainbowfish larvae that will remain alive in the aquarium until consumed. This often means that you will have to culture or collect live foods. Live foods such as zooplankton, phytoplankton, paramecia, freshwater rotifers, copepods, or other micro-organisms all fall into this category. As the larvae develop they can be fed other live foods such as brine shrimp nauplii and microworms that will remain alive in the aquarium from 2 to 4 hours, providing food until you can feed them again. Brine shrimp nauplii are used primarily because of their ease of culture.
The reason why live food is better for larval rainbowfish growth has not yet been clearly defined. Perhaps proteins present in phytoplankton and zooplankton but not synthesised by the physiological system of the larvae are important. In such cases, live food organisms provide digestive enzymes that breakdown the food ingested by larvae. However, other evidence has led to contradictory views regarding the role of the live food contribution in the digestion process of fish larvae. Live food organisms contain a 'package' of enzymes, gut neuropeptides and nutritional 'growth' factors that enhance digestion. These substances are frequently omitted in formulated diets. Moreover, particulate diets for larvae contain proteins and other ingredients that can be difficult to digest (especially since formulated diets contain 60~90% dry matter while zooplankton has only 10%). Inclusion of digestive enzymes, especially proteases, in the diets for fish larvae has been reported to significantly improve nutrient utilisation and performance of larvae, but still not as much as larvae fed on live food. Also of importance are several essential biochemical compounds such as poly-unsaturated fatty acids. Primary producers of these fatty acids are phytoplankton and zooplankton.
From the practical viewpoint of the aquarist, a good diet should be readily available, simple to produce as well as versatile in application. The consistent availability of sufficient quantities of food organisms is of the utmost importance in successfully breeding and raising rainbowfishes. In this respect, the collection and feeding of wild plankton has proven unreliable and not always practical. In response, there has been increasing interest in developing alternative feeds for fish larvae, including alternative zooplankton species and formulated diets.
The major difficulty for the aquarist is providing organisms appropriate to the size of the larvae at the first feeding stage and then supplying the large numbers necessary to maintain them. The preferred size of prey for larval fish increases as mouth size and feeding competency increase and different types of live foods have to be cultured for the different stages in the larval rainbowfishes development. For example, different species of microalgae (phytoplankton) range from 2 to 100 µm; rotifers from 50 to 200 µm, copepods from 100 to 300 µm and brine shrimp nauplii 400 to 800 µm. Apart from these main groups, a few other live feeds are used on a more limited scale including microworm (Panagrellus redivivus), vinegar eels (Turbatrix aceti), and cladoceran crustaceans. This group includes many species (Daphnia, Moina etc.). They can reach up to 4~6 mm but typically are much smaller than this; the smallest species is around 250 µm. Larger juveniles and even adults of some species often selectively prey on these crustaceans.
© Copyright Adrian R. Tappin
Updated December, 2008.
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