Feeding Larvae
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. High mortality rates can often occur, especially during the early feeding stages. The main reason for this is that the developing larvae are usually very small, extremely fragile, and generally not physiologically fully developed. For example, larval rainbowfishes have a small mouth size and are limited in the size of prey they can consume. Moreover, their ability to forage for prey is limited due to their small size and limited fin development.
These are limiting factors in proper feed selection and use during the early first-feeding period. It is perhaps not surprising therefore that larval nutrition can be a major obstacle in the successful raising of rainbowfishes in captivity. 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 (one or two), which frequently are not part of their natural food, and hence their nutritional composition may not always be the most suitable for maximum growth, development and survival. Moreover, larvae undergo 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 of rainbowfish species.
It is generally accepted that the optimal prey size for rainbowfish larvae is determined by their mouth size. The mouth size of first-feeding larvae restricts the size of the food particles which can be ingested. Commercial larvae feeds in the aquaculture industry have been developed primarily based on mouth size data. This has resulted in successful improvement of larval survival and growth in many aquacultured fish species. Adjustment of the amount of feed depending on larval developmental stage has also resulted in better growth and survival. However, there are few, if any, published information on the mouth size of rainbowfish larvae. 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 (the yolk will be absorbed slower in cooler water and faster in warmer water). 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.
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 subadults.
Most rainbowfish larvae are not particular about the types of zooplankton they eat, but the animals must be small enough for the larvae to ingest. Rainbowfish larvae are usually small in size (3−6 mm, total length), have poorly developed eyes, do not swim all that well but are mostly present in the water column, and require easily digested 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 the 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.
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. Phytoplankton is a particularly important food source for larval rainbowfishes. Filamentous and unicellular algae, especially epiphytic diatoms and desmids, also provide an important food source (> 10% of diet) for rainbowfish larvae.
In general, if you want your rainbowfishes to achieve maximum growth and have low mortality rates then you will need to provide them with at least some live foods. This often means that you will have to culture or collect 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.
There are however, several kinds of live foods that can be fed to rainbowfish larvae that will remain alive in the aquarium until consumed. Live foods such as infusoria, zooplankton (rotifers, cladocerans, copepods, etc.), microalgae (greenwater), or other micro-organisms all fall into this category. Microalgal species can vary significantly in their nutritional value, and this may change under different culture conditions. Nevertheless, a carefully selected mixture of microalgae can offer an excellent nutritional food for larval rainbowfishes.
Adjustment of the amount of feed depending on larval developmental stage has also resulted in better growth and survival. As the larvae grow, the amount of food they need increases and they prefer to eat larger-sized prey. It is well accepted that fish nutritional requirements vary with some factors such as the relationship between the diet and life stage and therefore, the dietary content for feeding larvae is different to that needed for the growth of juveniles. As the larvae develop they can be fed other live foods such as brine shrimp nauplii and microworms. Brine shrimp nauplii and microworms are used primarily because of their ease of culture. A major drawback in feeding brine shrimp nauplii to freshwater larvae is that the brine shrimp nauplii only survive for about 1–2 hours when placed into fresh water.
Although it is known that rainbowfishes will accept artificial diets, lower growth and survival rates are commonly obtained when such diets are used as the sole food source, particularly during the larvae and juvenile stages. Live feed items actively swim and are more inclined to orient themselves in mid-water column or even close to the surface where most rainbowfish larvae and juveniles feed. Commercial dry-feed items may only remain suspended momentarily in the water column after which they fall to the tank bottom where they may remain uneaten by the fish. Numerous attempts to replace live food by artificial diets has had limited success so far.
The reason why live food is better for 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 the larvae. Live food organisms contain enzymes, gut neuropeptides and nutritional growth factors that enhance digestion. These substances are frequently omitted in formulated diets.
Furthermore, particulate diets for fish larvae contain proteins and other ingredients that can be difficult to digest (especially since formulated diets contain 60−90% dry matter while natural zooplankton has only 10%). Inclusion of digestive enzymes, especially proteases, in the diets for fish larvae has been reported to significantly improve nutrient utilisation by the larvae, but still not as much as larvae fed on live food.
Additional support for this hypothesis is found in the different growth results when fish larvae are fed with either decapsulated cysts or live nauplii of Artemia. On an individual weight basis, the decapsulated cysts and nauplii of Artemia have similar biochemical composition in all the major nutrients. Thus, with regard to the amount of nutrients, there is no difference in feeding Artemia cysts or nauplii to fish larvae. However, in some species higher growth rates have been achieved with live nauplii. However, other evidence has led to contradictory views regarding the role of the live food contribution in the digestion process of fish larvae. 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 rainbowfish 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 rainbowfishes larval 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. Live feeds are a convenient food source for all rainbowfishes. Larger juveniles and even adults will selectively prey on these live foods.