Do discus fish not grow due to some kind of hormone?

 

There is considerable controversy as to whether smaller Discus grow alongside larger Discus. If they don't grow, is there some hormone released by the larger fish that prevents the smaller ones from growing? My non-scientific experience is that about 90% of small fish don't grow alongside larger fish, I'm obviously talking about Discus. Based on my empirical view from 43 years of experience with Discus, and after talking with some biologists, they are adamant.

Yes, in Discus there is a hormonal and social phenomenon that may contribute to dominant Discus growing faster and "subordinate" Discus growing more slowly, as I observe. But scientifically it's not proven, so it's not correct to say that there is a single hormone that simply "prevents" the smaller ones from growing. However, there is a hormone called Cortisol, which we will discuss in another article on this blog, and it may influence the growth of smaller Discus. However, in Symphysodon, growth is due to several factors: water quality, nutrition, thyroid hormones, etc.

But before discussing cortisol, I want to dedicate an article to this hormone. When we have a group of discus, whether large (2-3 cm) or 12-13 cm, a social hierarchy is immediately established. The larger discus are usually the dominant ones and have greater access to food and territory. Many of you understand this when you observe your discus. The smaller ones may eat less or be pushed away during feeding.

 Thus, the dominant ones, having better access to food and less competition, tend to maintain a higher growth rate; that is, the decrease in growth is caused by social position. In short, my theory, "fueled" by observation and several discussions with biologists, is that social behavior, nutrition, and growth are deeply linked, which is easy to understand. If it's also linked to a hormonal issue, well...! Scientifically, it's proven that GH/IGF-1 is probably the most important factor in understanding discus growth at a scientific level. Now, what is GH/IGF-1? Here's where the scientific part comes in.

• GH (Growth Hormone) is produced by the pituitary gland. It stimulates the body to grow and, among other functions, promotes the production of IGF-1.

• IGF-1 (Insulin-like Growth Factor 1) is produced mainly in the liver in response to GH and acts directly on many tissues, stimulating cell growth, protein synthesis, and muscle and bone development.

       

Therefore, from a "scientific" perspective, GH/IGF-1 is probably the most important factor in understanding discus growth. So, through discussions with biologists on this topic, it's not that the large fish releases a hormone into the water that directly blocks the growth of the small one. It is more likely that social cues, plus stress, plus food competition, plus hormonal regulation of growth produce this difference.

If young discus have good nutrition, good environmental conditions, and low stress, the GH/IGF-1 axis tends to function favorably and they grow. When there is prolonged social stress, competition, or insufficient food, the organism can alter this regulation. The fish may then direct more energy towards maintenance and survival than towards growth. I think this conclusion is easy to understand. I usually talk about "bullying" in an aquarium when the dominant fish doesn't let the others eat, defining its territory. Several subordinate discus, or even those weaker than others, can become much smaller when kept for months with dominant specimens. Therefore, observation is crucial for those who keep discus, whether hybrids or wild-caught.