Black Skirt Tetra Pair?

[shsinha]

I have two tetra shown below. I notice that one of them has small circular fin near the tail (by arrow) but other does not have. Are they male and female? If yes, which one is what?

 

[shsinha]

I have two tetra shown below. I notice that one of them has small circular fin near the tail (by arrow) but other does not have. Are they male and female? If yes, which one is what?

missed the image..

 

[The-Wolf]

all of my black widows have the adipose fin, I'm not sure why you have one without.
the way to sex them is Males may show white spots on the tail fin during "season" (for want of a better term). Also Females are slightly larger with a rounder body (when viewed head on).

 

[shsinha]

all of my black widows have the adipose fin, I'm not sure why you have one without.
the way to sex them is Males may show white spots on the tail fin during "season" (for want of a better term). Also Females are slightly larger with a rounder body (when viewed head on).

Their rear half changes color from dark to light after light is put on. Looks interesting...