What makes a fish tropical?

[Terry Ellacott]

Can anyone tell me what stops working in tropical fish when the temperature drops ?

Why do neon tetras die but goldfish survive ?

Do all tropical species share a common metabolic pathway which is different to cold water fish?

I've tried the Google search method but failed to find anything.

Thanks Terry

[Daniel]

From observation when my tropical fish are kept at temperatures in the 60°F - 65°F range they become lethargic and have a markedly decreased appetite. But so do the honey bees in my observation hive when kept at suboptimal temperatures. I know with my honey bees suboptimal temperatures cause the rate of enzyme-catalyzed reactions to decrease as the temperature decreases eventually impeding muscle movement. 

I suspect what stops working is the rate of enzyme-catalyzed reactions needed for metabolism and evolved to be optimized for certain higher temperature ranges. This rate becomes insufficient for tropical fish when the temperature drops, while other cool water fish like goldfish have ancestors that evolved over time with the enzymes optimized for a slightly cooler temperature range and therefore do just fine.

Since this effect occurs across all living things and not just fish, it is unlikely it is a specific metabolic pathway so much as it is just the effect of temperature on the rate of enzyme-catalyzed reactions that are critical for all life.

[Terry Ellacott]

Thanks for your reply. 

I had similar ideas about enzyme efficiency at different temperatures. I wondered if there is any information about  enzymes that are common to tropical fish from different parts of the world and if there are different enzymes controlling the same pathways in cold water fish. 

[Daniel]

This paper is the just the tip of the Google iceberg but is representative of what's out there.

Analyzing Cold Tolerance Mechanism in Transgenic Zebrafish (Danio rerio)

In this paper the authors examine the effect of energy homeostasis during cold stress on zebra fish that had been given the promoter for creatine kinase from carp. The paper found that this form of creatine kinase enhanced the biological processes related to cold tolerance including circadian rhythm, energy metabolism, lipid transport, and metabolism.

However, there doesn't appear be a single common mechanism so much as just different forms of the same gene in warm water fish versus a cold water fish.

 

[Terry Ellacott]

Thanks again. 

I'm afraid the paper went a bit over my head.  I started thinking about this because I wanted to know if it is possible to select for greater tolerance to cold. I have had a colony of Corydoras metae for 20 years and I am sure the fish in the colony now have a greater tolerance to high nitrates and pH crashes compared with the original fish I bought. If there are a range of enzymes in each species of fish it might be possible to select for greater cold tolerance. In twenty years time I might not have to buy that replacement heater.