Diskusmatning

Discus feeding: an infinite debate

Dr. Francesco Denitto

Introduction

Since the first living Discus was caught and reared into the aquarium, one of the main problems was… “what does it eat?”. At that time, few information came from direct observation in Nature. Discus were fished during dry season when the food availability was very scarce and fishes waited for long time before eating again their usual food, more abundant during the rain season.

Living preys were the first food that the first Discus keepers started to give them. Worms (such as Tubifex, Chironomidae, Enchitraeus) and planktonic crustaceans (Artemia, Gammaridae, Copepods, etc.) collected in the freshwater basins and rivers or bred artificially, represented the main diet for the Discus fishes bred in captivity. By the increasing of the discus passion all over the world, also the aquaristic firms producing fish foods, turned their attention towards this particular fish and the first specific foods for Discus appeared on the aquarium food market. The dry food became rapidly the main Discus food because of their easy administration and conservation.

Contemporary, thanks to its spreading in the Asian south-eastern fish farms, Asian discus breeders began to use a more economic fresh food based mainly on grinded beef hearth, highly rich in proteins which can be frozen for several months before its administration.

For a long time, our Discus have been fed with living foods, fresh meat and/or dried industrial food and each breeder, according to the personal opinion or to the suggestion coming from other breeders, followed one or another theory!

But breeding Discus is not a theory!

Then, what is about their real and effective nutritional requirement? What we really know about the biology of this - at the same time - both well-known and mysterious fish?

Today, we know few more something about this topic because, in the last years, Discus was considered subject of study of several researchers and, for this reason, they were conducted specific investigations on the effective Discus nutrient requirements.
One of the main debates which actually involve the hobbyists, is if discus need of a lot of proteins in their diet and, also if beef heart could be considered a good protein source.

It is well known that the general opinion in Germany, but also in other European countries, is not to overfeed discus with pure beef heart and other meat from warm-blood animals. By contrast, presently, Asian farmers rely heavily on live food such as Tubifex worms, blood worms (Chirinomidae) and freshly prepared feeds mostly consisting of beef heart, cockles and shrimps as protein sources. The search for suitable ingredients for a formulated feed for discus requires studies on their nutrient content and the ability of the organism to digest the nutrients them for maintenance and growth.

With the present article it is my intention to report the results of a series of investigations conducted by a team of colleagues from the University Sains Malaysia of Penang. These studies give important answers to the infinite debate about the Discus feeding. Because of my professional deformation of biologist and Discus passionate, I wanted to test on my Discus fishes the suggestions coming from this work. Although I did not use scientific procedure, my breeding experiment was enough to confirm the results obtained by Dr. Alexander Chong (main author) and his team.

Previous knowledge

One of the methods to know what a fish species eats in the wild, is to catch many specimens all over the year, dissecting them and analyzing the stomach and intestine contents throughout the use of a professional microscope. As for many other fishes, also Discus was taken in consideration by several ichtyologysts who wanted comprise its feeding ecology. Just to cite some of these researches, we can refer about the stomach content analysis conducted by Dr. Bremer (2000) on material caught and fixed during previous Amazon explorations (specimens collected by Horst Linke from December 1997 - February 1998); and the direct observation and stomach content analysis conducted on Wild Discus caught by Heiko Bleher (2006) during 40 years of Amazon explorations.
Dr. Heinz Bremer observed the presence of Particulate Organic Matter and bacteria in the intestine of Discus. Also detritus, fragments of plants and diatoms sometimes appeared in the stomach of several specimen (Bremer, 2000). He concluded that this organic compounds guarantees the survivorship of the Discus during the dry season although, as the author specifies, these results cannot give a complete answer to the feeding ecology of this species in Nature.

The explorer Heiko Bleher refers also on the importance of fruits and plants in the Discus diet, mainly during the dry season (Bleher, 2006). In fact, it is important to evidence that during the dry season the food is very limited in the water column. By the analysis of the stomach contents in Discus caught during this period of the year, it is impossible to definitely understand what Discus eat during all the seasons. Surely, these studies gave us very important information on the role of the vegetables in the Discus diet (fruits supply vitamins, carotene, fibres, etc.) but, on the other hand, they are not enough to say… discus eat just fruits!

The limit of both these investigations is that the specimens analyzed were caught only during the period of low water, the ONLY period which is possible for the explorers and fishermen to go into the forest and catching a sufficient number of fishes with the nets. The water level is not more than few meters (often less that one meter) and during the day Discus fishes love to stay in large schools under trees and underwater dry branches along the river banks or lake shores where they live (Bleher et al., 2007). This favourable situation allows the fishermen to enclose with a nets the site and caught a large number of fishes.
By contrast, during the rain season is impossible (or very difficult) to catch these fishes in the Amazon Region. The abundant precipitations make inaccessible Rivers and Lakes. The water level grow up to several ten meters (also up to 30 m). Also often the body waters overflow the banks and many high trees are completely submerged by this “continental freshwater sea”. In this condition, the fishes are able to search for a lot of submerged environments, always inaccessible to the fishermen and their nets. It is during this thriving period that Discus get the most important food source for their survivorship. Living preys, plants and fruits are abundant and Discus eat and grow much more than in the dry season. Nevertheless, as we have mentioned above, it is extremely difficult (or impossible) to catch live animals and observing their stomach contents. We just can suppose (and most probably it is true) that, in this period of the year, they prey worms (which come also from the emerged land now submerged by the waters), small planktonic crustaceans such as copepods, and also small fishes. In fact, Discus occur in the region with the highest freshwater fish species diversity anywhere in the world, the Central Amazon basin (Bleher et al., 2007). Furthermore, Bleher (2006) in his recent interesting and exhaustive monography refers about the presence of a great quantity of fruits and plants available for Discus and aquatic animals during the rain season. Probably, this vegetal component continues to be consumed by wild Discus in nature but surely it is not the only food source for them during the high water level period. Finally, both Bleher (2006) and Dr. Rolf Geisler (personal communication) referred on the presence of a benthic fauna (white worms, bloodworms, tubifex and small shrimps living in the sediments of the bottom of many Discus biotopes which they explored. They surely represent a good food source for Discus and other fishes, mainly during the rain season.

New contributes by the research: the physiological investigation

Up until 5-6 years ago, then, few scientific studies were made on the nutritional requirements of discus and each discus breeder had his own recipe using different ingredients and compositions. All animals including Discus fishes require five basic components in their diets. These are protein, fat, carbohydrates, vitamins, and minerals. The rate among these components vary with the different species requirement accordingly. So, for example, we know today that herbivorous goldfishes (Carassius auratus) and guppies (Poecilia reticulata) needs of a diet not many reach in proteins (around 30%). Goldfish and Koi Carp, for example, use the microflora in their hind gut to digest complex carbohydrates. On the contrary there are other omnivorous or carnivorous species which require food composed in great part by proteins. It is the case of the red head cichlid ‘Cichlasoma’ synspilum (and other big Central American cichlids) or the foil barb Barbodes altus which both require a protein percentage of more than 40% (for a review on this topic, see Sales & Janssens, 2003 and references herein).
Each component has a specific function in nutrition. Nevertheless, it is important to emphasize that in fish nutrition, only proteins, fats, and carbohydrates supply energy (Au et al., 2007).
The ability of the fish to utilise ingested nutrients, depends on the activities of digestive enzymes present in various locations along the digestive tract.

But what we know about Discus dietary requirement? Is it mainly a herbivorous or carnivorous species?
Thanks to recent studies conducted in the laboratories of the Penang University (Malaysia) we know today that discus have a pool of digestive enzymes located in the stomach and intestine able to digest easily proteins. This studies, in fact, revealed the presence of acidic protease from the stomach region with optimum activity at pH 2.0 and alkaline proteases from the intestinal section with optimum activities observed at pH ranging from 7.5 to 9.0 and 11.5 to 12.5, respectively. In particular, they showed the presence of eight distinct proteases (Table 1) (Chong et al., 2002a).

New contributes by the research: the physiological investigation

GROUP DIGESTIVE TRACT pH PROTEASES
ACIDIC  PROTEASES  STOMACH  2.0  PEPSIN-like
ALKALINE  PROTEASES  INTESTINE  7.5-9.0  TRYPSIN-type, CHYMOTRYPSIN
ALKALINE  PROTEASES  INTESTINE  11.5-12.5  SERINE-types, METALLOPROTEASE group

Table 1. Proteases found in the digestive tract of Discus. Note that the presence of trypsin-type protease in the intestinal section is typical of carnivorous fishes.

It means that the Discus organism is specialized to digest food containing protein and it is the result of a slow evolution which allows to the Symphysodon species to adapt themselves to the aquatic environment where they live. If their diet would be based just on plants and fruits (as emerged by the stomach content analysis), its digestive system would had not produced a pool of proteases as it emerged by the laboratory experiment!
Successively, thanks to the results gotten by their previous work which revealed the presence of proteases in the Discus digestive tract, the colleagues of the Malaysian University wanted to test several potential protein sources which could be used for the Discus breeding. In their new experiment, six different sources of proteins were tested: Casein, Danish fish meal, poultry offal meal, beef heart, soybean meal and wheat meal. In vivo and in vitro procedures were used for getting accurate results on the protein requirement in juveniles Discus. In particular, the “in vitro” technique involved the preparation of a stimulated optimized condition where ingredients/diets are digested through incubation with a single or mixture of enzymes (Chong et al., 2002b). It is more rapid, easier and reliable method in contrast to the conventional “in vivo” method which is based on a long time procedure that implicate the feeding of Discus in laboratory and the periodically collection of the faecal matters (final product of digestion) which have to be carefully analyzed.

The results were surprisingly. Protein digestibility - both in vitro and in vivo results - for all ingredients analysed were high, ranging from 63 to 96%. Casein showed the highest protein digestibility, followed by fish meal, beef heart and soybean meal, while both poultry offal and wheat meal gave the lowest protein digestibility values (Chong et al., 2002b).

Protein source in vivo method in vitro method
 Casein  89.68  81.06 
 Danish fish meal  78.15  70.36 
 Beef heart   75.89  64.51
 Soybean meal  66.22  52.51
 Poultry offal meal  65.34  42.00 
 Wheat meal  49.03  49.72

Table 2. Dry matter digestibility mean values for different raw materials from in vivo and in vitro analysis. Numbers correspond to a “digestibility coefficient”. Highest numbers mean high protein digestibility. From Chong et al., 2002b, modified.

The high fibrous content (4.58%) in the poultry offal meal tested as a result of substantial feather component probably explains the low digestibility coefficient values. In addition, the high lipid content (28.17%) of this meal could have also lowered the overall protein digestibility because of the formation of protein and oxidized fatty acid complexes which negatively affects protein hydrolysis rate.
Asian farmers commonly use raw beef heart as the main ingredient in preparation of moist feeds as a discus growth out diet. The good digestibility (both in vivo and in vitro procedure) showed that this ingredient can be utilized as a major dietary protein source for commercial discus farming. Another “shocking” result was that blood, a major component in this meal, has been reported to have high digestibility values (Chong et al., 2002b).

It is also important to consider that Discus fishes start to digest proteins since 3-4 weeks old because the digestive tract it is fully formed and it begins to synthesize all the enzyme pool, as demonstrate by a third experiment conduced by the Malaysian team (Chong et al., 2002c). In particular, this study shows the late development of acidic protease (mainly pepsin) in discus larvae with significant activities not detected until 20-25 days after hatching. In fact, although a stomach is completely formed as early as 10 days after hatching in discus, just some alkaline protease activity (mainly trypsin) is already present during the first few days of the discus larval stage.
Based on this study, the use of a dry (commercial food) or fresh diet (mixture) could start not before 3-4 weeks after hatching to ensure full capacity of digestion and adsorption. Living Artemia nauplii would represent the best food for Discus larvae up to the third-fourth week of their life.

After it was scientifically demonstrated that discus are omnivorous fishes able to digest protein, it remains still just to answer to another important question: what is the protein contribute to the Discus diet? The answer,come from another experiment planned and conducted by the Asian colleagues. Briefly, they used a group of juveniles (mean initial weight of about 4.50 grams each) which were fed with five isoenergetic, semipurified diets (casein, gelatine and Danish fish meal as protein sources) twice a day to satiation for 12 weeks. Five levels of protein were evaluated (350, 400, 450, 500 and 550 g/kg diet). The results were very clear: growth rate increased significantly with protein level up to 500 g/kg diet and then decreased (Chong et al., 2000). In detail, the analysis suggested an optimal requirement of 450-500 g/kg. Besides, numerous studies reported growth depression in response to protein levels higher than the optimum, possibly owing to the energy costs of deaminating excess amino acid. In other words, it is not suggested to overfeed Discus with a diet excessively reach in protein (more than 450-500 g/Kg diet) because of the Discus organism would be induced to use a lot of energy to “destroy” the exceeded amino acids not necessary for their metabolic necessities. The energy involved to deaminate the exceeding amino acids is detracted by the total amount of the produced energy and then it may cause a decreasing of the growth rate in juveniles.

Conclusions

In conclusion, the results of these scientific works give an important contribution to the comprehension of the feeding biology of Discus. Particularly, they give us several information and below shortly summarized:

(1) Discus fish is a cichlid which it has evolved a digestive system able to digest also a complex protein compound as demonstrated by the presence of a pool of acidic and alkaline proteases in their stomach and intestine. It means that, in nature, this fishes prey living organisms (invertebrates and small vertebrates too) and other food which furnish them an important part of their nutrient requirement, mainly during the rain season.

(2) As demonstrated by previous investigation (Bremer, 2000; Bleher, 2006), fruits and plants represent another important part of their food and this must be always taken in consideration by Discus breeders too.

(3) Regarding the Discus breeding, although in Nature Discus fishes… do not eat cows, beef heart resulted an alternative and valid protein source for Discus bred in captivity. It can be easily digested and then it can be used for preparing fresh or dried mixtures, but without exceeding in quantity. At the light of these facts, we highly recommended to not use pure beef heart as exclusive fresh food. In this case it would become indigestible and it could cause intestinal occlusions and flagellate proliferations in the gut.

(4) Fresh protein source have to be in order of 450-500 g/Kg diet for getting an optimal growth. Note that this value has been estimated on the juveniles. In particular, beef heart trimmed of fat and tissues, supplies 16.8% protein (see Table 3). In other words, 1 Kg of fresh (frozen) mixture composed by 50% beef heart and 50% other components (vegetables, vitamins, etc.) supplies about 84 gr. of protein from animal origin. Still we have not detailed data regarding dietary protein requirements in adult specimen. We can just suppose that, for adults, this value is lowest because of they have a low metabolism if compared to the juvenile requirements.

(5) Finally I would also suggest to vary Discus diet in respecting to the seasonal changes in nature. Discus physiology has evolved to tackle two different periods - dry season (June-October) and rain season (November-May) - in which they have two different diets. During the dry season, Discus have mainly a herbivorous diet poor in protein, while in the other period, they presumably have a diet more reach in protein. For this reason, regarding the adults rearing, I suggest to alternate diet phases: one more reach in protein and another which its diet would be based mainly on vegetal compound. On the contrary, juveniles have to be feed with foods (dry or fresh) more reach in protein, but always in accordance to the results above reported.

Finally, this article would show that hobbyist experiences, scientific approaches and in situ explorations can be used in synergy to give a more completed vision about the Discus ecology and it is same for many other aquarium species for which our knowledge are still scant.
Nevertheless, although today we have much more information about it, Discus remains an intriguing fish and surely many other still have to be discovered about its mysterious life. Personally, I am conducting other studies in the laboratory of Zoology of the “Salento University” of Lecce (Italy) which would have the goal to add other information on the Discus feeding ecology. I hope to refer soon on the pages of the Diskus Brief about the new results.

References

Au D., Seng S.S., Denitto F., 2007 – Trophy Discus. The art of selecting, grooming, and showing discus. Cichlid Press (Ad Konings ed.), El Paso, Texas: 128 pgg.

Bleher H., 2006 – Bleher’s Discus. Vol. 1. Aquapress, Miradolo Terme (PV), Italy: 671 pgg.

Bleher H., Stölting K.N., Salzburger W., Meyer A., 2007 – Revision of the Genus Symphysodon Heckel, 1840 (Teleostei: Periciformes: Cichlidae) based on molecular and morphological characters. aqua, International Journal of Ichtyology, 12 (4): 133-174.

Bremer H., 2000 – Discus e Scalari. Nutrimento e abitudini alimentari nei biotopi naturali. In: Discus Annuario 2000, Primaris Ed., Rozzano (MI), Italy: 22-28.

Chong A.S.C., Hashim R., Ali A.B., 2000 – Dietary protein requirements for discus (Symphysodon spp.). Aquaculture Nutrition, 6: 275-278.

Chong A.S.C., Hashim R., Chow-Yang L., Ali A.B., 2002a - Partial characterization and activities of proteases from the digestive tract of discus fish (Symphysodon aequifasciata). Aquaculture, 203: 321-333.

Chong A.S.C., Hashim R., Ali A.B., 2002b – Assessment of dry matter and protein digestibilities of selected raw ingredients by discus fish (Symphysodon aequifasciata) using in vivo and in vitro methods. Aquaculture Nutrition, 8: 229-238.

Chong A., Hashim R., Lee L.C., Ali A.B., 2002c – Characterization of protease activity in developing discus Symphysodon aequifasciata larvae. Aquaculture Research, 33: 663-672.

Sales J., Janssens G.P.J., 2003 - Nutrient requirements of ornamental fish. Aquatic Living Resources, 16: 533-540.

* Dr. Francesco Denitto, is a marine biology researcher at the Salento University of Lecce (Italy) where he obtained his PhD in Fundamental Ecology. During his long time experience with the aquarium hobby, Discus have been the top of his fish breeding skills. He regularly lectures about the “world underwater” and has published hundred of articles on Discus for Italian and international aquarium magazines. It is also co-author of the recent “Trophy Discus” book and international Discus judge.

Table 3.

Protein contribution (%) from several food sources commonly used in Discus feeding.

Note the highest percentage in dried food (without the water component) because of protein are concentrated in few grams of product.

* Dried red mosquito larvae:  54.9%
* Dried Artemia adult:  51.0%
* Discus granulat:  45.0-55.0%
* Dried Artemia nauplii:  42.0%
* Dried Daphnien:  37.7%
Soybean meal:  36.8%
Fresh (frozen) Beef liver:  20.0%
Fresh (frozen) Salmon:  18.4%
Fresh (frozen) Cod:  17.0%
Fresh (frozen) Beef Heart:  16.8%
Fresh (frozen) herring:  16.5%
Wheat meal:  12.9%
Fresh (frozen) squid:  12.6%
Fresh (frozen) Red mosquito larvae:  6.5% 
Fresh Carrot:  1.1%

* Information taken from labels of commercial products

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