I have been reading the many theories regarding drop eye and thought I'd put my two cents in.
My background (not that it REALLY matters, but just so you get where I'm coming from):
- aquarist since age 10 (arowanas too, but only silvers)
- Majored in Biology
- A practicing physician (the human kind, but I still know quite a bit about physiology and disease in general)
For years, I have been reading the multiple ideas people have come up with to explain the unfortunate phenomenon afflicting our beloved arowanas. The most common ideas that I'd like to discuss seem to be:
1. Fat build up. "Fat builds up behind the eyes from a diet too rich in fats."
Not likely. This theory initially sounds possible because it is loosely based in science ("if we eat too much, we get fat"). The truth is that animals store fat in different ways and in different locations. It is VERY unlikely for a "cold-blooded" animal to consume enough calories to cause it to store enough fat that would cause deformity in only one area of its body. When you combine the fact that many arowana keepers describe their fish with drop eye only on one side, this essentially (to quote Mythbusters) "Busts" this theory. Why would an animal not store fat evenly between both eyes? It goes against logic.
Another reason I tend to not give much weight to this theory is based on basic physiology. We are discussing a fish, a poikilothermic ("cold-blooded") animal. When compared with "warm-blodded " animals, fish have a much lower food and energy requirement. This is why you can feed a snake only once or twice a month. Not because snakes and fish store energy as fat for later use (this is what bears and seals and warm-blooded animals do), but because their bodies simply don't need to consume as much energy on a daily basis. The chemical process of creating large layers of fat and then breaking it down later for use is too costly and quite unnecessary for most cold-blooded animals. I just don't buy "fat" as being a cause of drop eye.
2. Genetics. "Too much inbreeding has caused a genetic trait that causes drop eye." A solid theory in my opinion, but one that should be easily disproven (by those that breed them!). Also, while silver arowana breeding is indeed beginning to increase, in no way has it caught up to the demand seen here in the United States. Most silver arowanas sold in the US are wild caught. Wild caught means genetic causes for drop eye are very unlikely.
3. Trauma. "Drop eye occurs from the fish striking the canopy/glass top/light fixtures when it jumps." Maybe. Now this is a new theory to me. I'm an Emergency Physician, so I'm VERY familiar with trauma and its effects, so this theory made me think for a while on its possibility. I will be the first to admit that while the arowana is most certainly designed to jump out of the water with great skill, it is not designed to continuously impact a glass roof every time it does so. Obviously, this can cause damage to the fish. So what could cause a fishes' eyes to protrude down after dorsal head trauma? The most likely possibility I could think of at first is that the trauma causes a small bruise/hematoma behind the eye. But bruises naturally heal and are absorbed, so the drop eye should also go away with time. Maybe the bruise causes a buildup of scar tissue to accumulate behind the eye, pushing it out over time? Maybe the trauma causes one of the muscles that controls eye movements to tear and when it heals, the eye is now permanently fixed looking down. While all of these ideas are medically possible I suppose, the odds of having them occur in so many fish approaches impossible. After much internal debate, I'm not entirely convinced that trauma is the cause of our drop eye epidemic.
So what is it then? Here's my theory:
4. Environment. "They simply look where the action's at."
I think we as aquarium keepers have overlooked one of the huge differences between our tanks and the arowana's natural environment: its literal environment.
For those that don't know, the silver arowana comes from the murky floodlands of the Amazon. This is an area where rotting wood and fallen leaves have turned the water into a dark tea color (hence the name "blackwater"). This fish cruises at the top of the water column all day long, looking for whatever it can eat either just above the water (bugs, birds), or just below its surface (more bugs, smaller fish that feed here). It rarely, if ever looks down. Pull up a picture of the blackwater amazon. The water is DARK. If you were to put your arm in up to your elbow, you would lose sight of your hand. Again, the arowana rarely looks down. There's not much to see there; it's too dark.
Compare this with the vast majority of our home aquariums: a glass aquarium (with invisible walls to bump into), filled with pristine, clear water so that we may better view our prized fish. (I realize some add blackwater extract to better simulate the water color but you and I know this is nowhere near as dark as the Amazon's waters.) Our aquariums are covered (hopefully) with a closed lid where NOTHING happens overhead other than a lightbulb turning on and off every day. No trees move overhead, no birds fly by, no clouds, no movement at all. It has no reason to ever look UP. OK floating pellets once or twice a day for 5 minutes, but most of its day is spent looking DOWN and out at its moving environment. People walk by the tank, CLEARLY VISIBLE tankmates swim below the arowana. For 99% of its life, only two thoughts ever enter an arowana's mind: "is it a threat?" or "is it food?". The arowana is clearly a predator but is far from the top of the Amazonian food chain (this is painfully obvious to all of us that know how easily spooked our fish are).
Now add to this the level of your aquarium on its stand. Probably waist/stomach level when you stand next to it? So think about how you observe your fish on a daily basis. Do you watch it from the top? Of course not, its covered (or had better be!) and arowanas swim too close to the top of the water column anyways! So you squat down or sit in a chair and view him from BELOW. What does the fish do? Makes eye contact with you (so cool!). He looks down...again.
Remember all the stories about the scarcity of drop eye among wild or pond kept arowanas? This would further support my theory. In a pond and in most of the wild, there's nowhere to look but up!
= no drop eye
Again, this is just my theory. Tell me what you think. I'm up for a good debate. A good start would be "then why doesn't drop eye occur in asian arowanas too?" A good reply: "because they're not originally from the Amazon blackwater."
Who really knows? Let's all talk about it and input our experiences.
This forum is absolutely one of THE BEST places to put an end to the drop eye mystery.
My background (not that it REALLY matters, but just so you get where I'm coming from):
- aquarist since age 10 (arowanas too, but only silvers)
- Majored in Biology
- A practicing physician (the human kind, but I still know quite a bit about physiology and disease in general)
For years, I have been reading the multiple ideas people have come up with to explain the unfortunate phenomenon afflicting our beloved arowanas. The most common ideas that I'd like to discuss seem to be:
1. Fat build up. "Fat builds up behind the eyes from a diet too rich in fats."
Not likely. This theory initially sounds possible because it is loosely based in science ("if we eat too much, we get fat"). The truth is that animals store fat in different ways and in different locations. It is VERY unlikely for a "cold-blooded" animal to consume enough calories to cause it to store enough fat that would cause deformity in only one area of its body. When you combine the fact that many arowana keepers describe their fish with drop eye only on one side, this essentially (to quote Mythbusters) "Busts" this theory. Why would an animal not store fat evenly between both eyes? It goes against logic.
Another reason I tend to not give much weight to this theory is based on basic physiology. We are discussing a fish, a poikilothermic ("cold-blooded") animal. When compared with "warm-blodded " animals, fish have a much lower food and energy requirement. This is why you can feed a snake only once or twice a month. Not because snakes and fish store energy as fat for later use (this is what bears and seals and warm-blooded animals do), but because their bodies simply don't need to consume as much energy on a daily basis. The chemical process of creating large layers of fat and then breaking it down later for use is too costly and quite unnecessary for most cold-blooded animals. I just don't buy "fat" as being a cause of drop eye.
2. Genetics. "Too much inbreeding has caused a genetic trait that causes drop eye." A solid theory in my opinion, but one that should be easily disproven (by those that breed them!). Also, while silver arowana breeding is indeed beginning to increase, in no way has it caught up to the demand seen here in the United States. Most silver arowanas sold in the US are wild caught. Wild caught means genetic causes for drop eye are very unlikely.
3. Trauma. "Drop eye occurs from the fish striking the canopy/glass top/light fixtures when it jumps." Maybe. Now this is a new theory to me. I'm an Emergency Physician, so I'm VERY familiar with trauma and its effects, so this theory made me think for a while on its possibility. I will be the first to admit that while the arowana is most certainly designed to jump out of the water with great skill, it is not designed to continuously impact a glass roof every time it does so. Obviously, this can cause damage to the fish. So what could cause a fishes' eyes to protrude down after dorsal head trauma? The most likely possibility I could think of at first is that the trauma causes a small bruise/hematoma behind the eye. But bruises naturally heal and are absorbed, so the drop eye should also go away with time. Maybe the bruise causes a buildup of scar tissue to accumulate behind the eye, pushing it out over time? Maybe the trauma causes one of the muscles that controls eye movements to tear and when it heals, the eye is now permanently fixed looking down. While all of these ideas are medically possible I suppose, the odds of having them occur in so many fish approaches impossible. After much internal debate, I'm not entirely convinced that trauma is the cause of our drop eye epidemic.
So what is it then? Here's my theory:
4. Environment. "They simply look where the action's at."
I think we as aquarium keepers have overlooked one of the huge differences between our tanks and the arowana's natural environment: its literal environment.
For those that don't know, the silver arowana comes from the murky floodlands of the Amazon. This is an area where rotting wood and fallen leaves have turned the water into a dark tea color (hence the name "blackwater"). This fish cruises at the top of the water column all day long, looking for whatever it can eat either just above the water (bugs, birds), or just below its surface (more bugs, smaller fish that feed here). It rarely, if ever looks down. Pull up a picture of the blackwater amazon. The water is DARK. If you were to put your arm in up to your elbow, you would lose sight of your hand. Again, the arowana rarely looks down. There's not much to see there; it's too dark.
Compare this with the vast majority of our home aquariums: a glass aquarium (with invisible walls to bump into), filled with pristine, clear water so that we may better view our prized fish. (I realize some add blackwater extract to better simulate the water color but you and I know this is nowhere near as dark as the Amazon's waters.) Our aquariums are covered (hopefully) with a closed lid where NOTHING happens overhead other than a lightbulb turning on and off every day. No trees move overhead, no birds fly by, no clouds, no movement at all. It has no reason to ever look UP. OK floating pellets once or twice a day for 5 minutes, but most of its day is spent looking DOWN and out at its moving environment. People walk by the tank, CLEARLY VISIBLE tankmates swim below the arowana. For 99% of its life, only two thoughts ever enter an arowana's mind: "is it a threat?" or "is it food?". The arowana is clearly a predator but is far from the top of the Amazonian food chain (this is painfully obvious to all of us that know how easily spooked our fish are).
Now add to this the level of your aquarium on its stand. Probably waist/stomach level when you stand next to it? So think about how you observe your fish on a daily basis. Do you watch it from the top? Of course not, its covered (or had better be!) and arowanas swim too close to the top of the water column anyways! So you squat down or sit in a chair and view him from BELOW. What does the fish do? Makes eye contact with you (so cool!). He looks down...again.
Remember all the stories about the scarcity of drop eye among wild or pond kept arowanas? This would further support my theory. In a pond and in most of the wild, there's nowhere to look but up!
= no drop eye
Again, this is just my theory. Tell me what you think. I'm up for a good debate. A good start would be "then why doesn't drop eye occur in asian arowanas too?" A good reply: "because they're not originally from the Amazon blackwater."
Who really knows? Let's all talk about it and input our experiences.
This forum is absolutely one of THE BEST places to put an end to the drop eye mystery.
