Lucky you to have got hold of these. This is one of my favourite species of cory. We see them in North America rarely, for some reason, but I've had four since 2009, and this summer managed to get three more (all they had). I last had them back in the late 1990's.
I've found that this species is less inclined to remain together; my corys are presently all together in my largest tank, and there are 45 of them representing some dozen or so species. These Corydoras similis tend to be on their own most of the time, even though there are seven of them.
There is some variability in the pattern and colour with this species. There is a blueish colour to the blotch at the base of the caudal fin and the anterior part of the fish has a beautiful mauve or sometimes golden sheen that can be quite reflective of light.
The species epithet similis is the Latin for "looks like," a reference to the similar colouration and pattern of this species with Corydoras ourastigma; this latter species has a long snout whereas C. similis is rounded. But beyond this, there are four other species, Corydoras guapore, C. caudimaculatus, C. spectabilis and C. ourastigma, which have similar basic patterns, and except for the latter, these all occur in the Madeira/Guapore basin. Only C. similis and C. ourastigma have a "smudged" blotch at the base of the caudal fin rather than a distinct spot as in other spotted-head species. But, there can be quite a difference in the extent of this smudge.
The only way to sex most cory species is to view them from directly above. Females will appear considerably plumper around than males, with adult fish (as those in the photos are).
Byron.
