Do corydoras change colour?

[seangee]

I have noticed in the past that my peppers (c. paleatus) are not all the same colour, ranging form shades of brown to grey and blue green. Most in my tank had gone to shades of brown with the difference only being apparent when I added new fish.

Just over a week ago I switched my substrate from a buff / sand colour to a dark grey/black. Nothing else has changed so I would like to think my fish were not stressed. I have a group of 18, the lighting has always been quite subdued and the water is soft and acidic. But in the last week they have definitely become noticeably darker and more vibrant, almost as if they are taking on the colour of the substrate as a camoflage mechanism.

This makes sense but I was not aware that corys do this - I assume its not a conscious thing on their part.

 

[Byron]

You are correct, the substrate does affect the colour of Corydoras, and many other upper fish react similarly. A darker substrate will result in more pigmentation to the external cells, thus darkening the fish.

Several years ago I had my panda cories in a tank with a buff-tone substrate; when I moved them into a tank with a black-tone substrate, they became very noticeably darker and remained that way--until a few years later they were moved into a different tank with a slightly lighter substrate (dark grey play sand).

I cannot emphatically say this is a conscious or subconscious attempt to blend in with the substrate, but I would suspect it is more likely the latter. All the cells of fish are highly sensitive to light; the internal circadian rhythm (which is present in all animals) is governed by light. Species of pencilfish in the genus Nannostomus have a diurnal pattern shift; the horizontal lines on the species breaks up into dashes at night. Observations of this occurring in blind fish prove that it is not a conscious reaction to darkness perceived visually but the cells themselves reacting to the external light/dark.

There are of course other factors affecting "automatic" changes in pigmentation, changes that may be temporary or permanent depending upon the factor involved. Fish with bright overhead lighting will always be paler, because they are being stressed. This overhead light also reflects off the substrate, which is why white should never be used. Look to the species natural habitat and provide as close as possible to that, and the fish will be less stressed and healthier.