Discus feeding: new contributes by the research

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Discus feeding: new contributes by the research

Post by Poddan » 14 Sep 2008, 10:08

Discus feeding: new contributes by the research
Dr. Francesco Denitto*

*Biology researcher at the Salento University (Lecce, Italy)

Up until 5-6 years ago, 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 (see also, “Trophy Discus” by Au, Seng & Denitto, 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., 2002).

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., 2002). 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.
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 (see the recent book “Bleher’s Discus” by 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) Protein source have to be in order of 450-500 g/Kg fresh 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. In other words, 1 Kg of fresh (frozen) mixture composed by 50% beef heart and 50% other components (vegetables, vitamins, etc.) supplies about 84 grams of protein of 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.
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Re: Discus feeding: new contributes by the research

Post by stanchung » 04 Oct 2008, 18:02

Good read, I would think the vegetable matter would be eaten as a source of vitamins. Carotene, vitamin C etc.
http://www.discus.com.my > timbalan administrasi dxk :D

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