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Probably a very easy question:
How can one reliably tell a

Callichthys callichthys

from a

Megalechis thoracata

?

That shouldn't be too difficult:
The colour pattern is of course very different.
The caudal is rounded in Callichthys and the same goes for the body shape, whereas Hoplo thoracata has a higher body and a spade shaped caudal.
Head of Callichthys is even more flattened which gives the impression that the eyes are pointing upwards.
H. thoracata has its eyes pointing sidewards.

The species that I have problems with in this regard is

I've got both and they look nothing like each other- honestly! As Marc says, Callichthys have a much more flattened head and slightly less deep body, small, dorsally positioned eyes, and have a much more sinuous swimming style- their head also looks like Predator (from the various films)... They're also a very different colour.

In addition, they seem to like resting in groups on the bottom.

I always find the rounded caudal a key in identifying Callichthys

The reason I askis because I've several twice been asked to identify photos of uniformly very dark brown, almost black, fish, without any visible markings at all. I had no clue whether I was looking at a Callichthys or a Megalechis thoracata. One of the fish was of unknown origin, the other from Uruguay (where both species occur).

I think both fishes were M. thoracata, but that isn't really based on anything but my gut feeling; that they had M. thoracata "vibes".

I could be wrong, but I seem to remember that Callichthys is supposed to lack the pair of triangular ventral plates which M. thoracata and the other hoplo species have.

Mike_Noren wrote:The reason I askis because I've several twice been asked to identify photos of uniformly very dark brown, almost black, fish, without any visible markings at all.

Why not post them over here?
If I'm correct, the fish you describe is shown in one of the earlier Mergus Atlas (3?) as Hoplosternum magdalenae black. But I have to look that up to be 100% sure.

Chrysichthys wrote:I could be wrong, but I seem to remember that Callichthys is supposed to lack the pair of triangular ventral plates which M. thoracata and the other hoplo species have.

I think you're right.