Characterization of Discus Mucus During Parenting
By Fred Goodall
date December 2005
 

Results of studies conducted in 2004 and 2005 have finally answered the question of what is in discus "slime coat" which the fry feed upon.  Mass spectrometry, protein analysis by Bradford assay, free amino acid analysis by chemical testing and finally Electrophoresis was used to identify the proteins, amino acids and peptides in parent discus mucus during fry feeding and cross compared those findings to juvenile discus mucus.  All samples were taken by "skin scraping" from the bodies of the test subjects and the ventral area was avoided in all test subjects to eliminate contamination of results by urine.

Fry were in three groups: fry fed only parent fish mucus, those fed with Artemia napulii and tested 1 hour later, those fed Artemia and tested 3 hours later.  The interesting thing is that it was found that all fry reduced feeding ( bites of mucus were monitored in a 30 second period ) by day 15 of free swimming.  For the fry fed Artemia bite / feeding rates decreased during the 1 hour after eating Artemia... but by 3 hours after Artemia feeding... bites of mucus slowly increased but by day 15 free swimming the bite rate ( number of bites of parent mucus in 30 second periods ) for all 3 groups stayed very low until day 30 of free swimming when the bite rate dropped under 1 bite every 30 seconds.

Parent discus epidermal mucus from females feeding fry of day 10 to 15 free swimming age was compared to non parenting discus of 5 to 6 months age.  Parent fish were between 600 grams and 700 grams body weight and juvenile discus were 350 g to 400 g body weight.  Individual comparisons might vary due to body mass and production of mucus but as a group compared to another group the body weight "control" allowed uniform group results.1

These results were that parent discus slime coat ( epidermal mucus ) contains a higher amount of overall protein and specifically a much higher amount of Phenylalanine, an "essential amino acid" while juvenile discus mucus contains higher levels of Alanine, Aspartic acid, Glycine, Proline, Serine, and Leucine.  Both parent and juvenile discus had high levels of the essential amino acids Isoleucine and Lysine in their mucus ( Chong, et all, 2005 )1.  Spectrometry analysis showed that the parent discus mucus showed matching peptides to a trout ( Oncorhynchus mykiss ) mucus protein named type II epidermal keratin.  This is a "structure building" protein and regulates skin growth.1

Fish epidermal mucus in general contains water, lipids, amino acids, secretory proteins, glycoproteins, sloughed skin cells and bacteria ( Shephard, 1994 )2.  Parental discus mucus contained  point 74 mg of protein per ml of mucus while juvenile discus mucus contained point 63 mg protein per ml of mucus1. The list of "free" amino acids ( amino acids not part of larger protein molecules )  found in discus mucus : ( both juvenile and parent fish ) Alanine, Aspartic acid, Cystine, Glutamic acid, Glycine, Proline, Serine, Tyrosine, and the Essential amino acids; Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan and Valine.1  Free amino acids are building blocks for more complex "proteins", but the important part for us discus fanatics is that "free" amino acids are thought to be a food source for discus fry for the first 15 days the fry are free swimming while the digestive enzymes of fry are developing from day 5 free swimming.3  The main findings of this earlier study was that discus fry should be fed Artemia as the non parent mucus food until 20 - 25 days after hatching as they can not digest foods other than parent mucus and Artemia until that time and most probably need the free amino acids in parent mucus as food until their digestive enzymes develop more fully by day 10 free swimming3 After 20 - 25 days free swimming the fry have digestive enzymes that are acid based and are thus able to digest foods other than Artemia and parent mucus.3  This would match up with "hobby lore" regarding discus fry eating and surviving on meat mixes and crushed flake and pellet foods usually fed to juvenile and adult discus.

Using the list of free amino acids some experimentation with unsweetened, unflavored gelatin and amino acids might offer us an alternative to egg yolk feeding of fry raised away from the parents.  On going studies on parent mucus and discus mucus in general should give us detailed information on what is included in discus mucus beside the protein and protein precursors.3

Acknowledgments

Dr. Alexander Chong for the materials used for this article and for his most kind permission to paraphrase passages of his published studies. Nick Polyzos for raising questions to be answered and proof reading.

References

Footnoted

1. Kenny Chong, Tham Sock Yim, John Foo, Lam Toong Jin, Alexander Chong,  2005  Aquaculture  "Characterisation of proteins in epidermal mucus of discus fish ( Symphysodon spp. )  during parental phase".

2. Shephard, K.I. , 1994 Functions for Fish Mucus. Rev. Fish Biol. Fish. 4,  401 - 429.

3.  Alexander Chong, Roshada Hashim, Leng-Choy Lee and Ahyaudin bin Ali,  2002, Aquaculture Research, 33, 663 - 672  "Characterization of protease activity in developing discus Symphysodon aequifasciata larva" 

Not footnoted

Govoni  J.J., Boehlert G. W., Yatanabe, Y. ( 1986 ) The physiology of digestion in fish larvae. Environmental Biology of Fish 16.  59 - 77

Warner A H & Matheson C. ( 1998 ) Release of proteases from larvae of the brine shrimp Artemia francissoma and their potential role during the molting process.  Comparative Biochemistry and Physiology  119B. 255 - 263.